S
Steve Summit
Archive-name: C-faq/diff
Comp-lang-c-archive-name: C-FAQ-list.diff
URL: http://www.eskimo.com/~scs/C-faq/top.html
It's been a long time. Far too long, but that's water under the bridge.
Thanks, all, for your patience.
This article lists the many, many differences between the previous
version of the comp.lang.c FAQ list (version 3.5, last modified
February 7, 1999, last posted June 1, 2004) and this finally-new one.
I have scoured my archives for every unacknowledged suggestion and
promised improvement, so for once I can say: this update is mostly
complete. At this point, if you've sent me a suggestion which I
haven't acknowledged or which isn't reflected here, I guess it
wouldn't hurt if you sent it again.
This update has been so long delayed that some of its improvements
(such as the "new" question 19.17c) are already mildly obsolete;
they were made to my master manuscripts years ago but have, alas,
never seen the light of day until now.
Because of some overly ambitious formatting changes I've embarked on,
the updated HTML version is *still* not quite ready. But the light
at the end of the tunnel is in sight: that grandish unveiling will
happen in the next month or so.
I've categorized the diffs that follow into brand-new questions,
significant changes, minor adjustments, and (sob!) retired questions.
The notation is based on that of the standard `diff' program: `<'
indicates old, changed, or deleted text; `>' indicates new text.
(For my fellow packrats out there: don't worry, the deleted
questions, and most of the other deleted text, will live on
in the web version...)
First, the new questions. (Some of these are migrated from the
book-length version, but most of them are brand new.)
==========
====================
Next, here are the significant changes.
==========
[Q1.1 How should I decide which integer type to use?]
If for some reason you need to declare something with an
*exact* size... be sure to encapsulate the choice behind
< an appropriate typedef.
---
==========
[Q1.4 What should the 64-bit type be on a machine that can support it?]
< A: The forthcoming revision to the C Standard (C9X) specifies type
< long long as effectively being at least 64 bits, and this type
< has been implemented by a number of compilers for some time.
< (Others have implemented extensions such as __longlong.)
< On the other hand, there's no theoretical reason why a compiler
< couldn't implement type short int as 16, int as 32, and long int
< as 64 bits, and some compilers do indeed choose this
< arrangement.
---
==========
[Q1.7 What's the best way to declare and define global variables...]
A: First, though there can be many "declarations" (and in many
< translation units) of a single "global" (strictly speaking,
< "external") variable or function, there must be exactly one
< "definition". (The definition is the declaration that actually
< allocates space, and provides an initialization value, if any.)
---
==========
(Unix compilers and linkers typically use a "common model" which
allows multiple definitions, as long as at most one is
initialized; this behavior is mentioned as a "common extension"
< by the ANSI Standard, no pun intended. A few very odd systems
< may require an explicit initializer to distinguish a definition
< from an external declaration.)
---
==========
[Q1.25 My compiler is complaining about an invalid redeclaration...]
< A: Functions which are called without a declaration in scope
< (perhaps because the first call precedes the function's
< definition) are assumed to be declared as returning int (and
without any argument type information), leading to discrepancies
< if the function is later declared or defined otherwise. Non-int
< functions must be declared before they are called.
---
==========
[Q1.30 What am I allowed to assume about the initial values...]
==========
[Q1.31 This code, straight out of a book, isn't compiling]
< A: Perhaps you have a pre-ANSI compiler, which doesn't allow
< initialization of "automatic aggregates" (i.e. non-static
< local arrays, structures, and unions). (As a workaround, and
< depending on how the variable a is used, you may be able to make
< it global or static, or replace it with a pointer, or initialize
< it by hand with strcpy() when f() is called.)
---
==========
[Q1.34 I finally figured out the syntax for... pointers to functions...]
< An explicit declaration for the function is normally needed,
< since implicit external function declaration does not happen in
< this case (because the function name in the initialization is
< not part of a function call).
---
==========
[Q2.4 How can I implement opaque (abstract) data types in C?]
A: One good way is for clients to use structure pointers (perhaps
additionally hidden behind typedefs) which point to structure
==========
[Q2.6 I came across some code that declared a structure like this...]
these "chummy" structures must be used with care, since the
programmer knows more about their size than the compiler does.
< (In particular, they can generally only be manipulated via
< pointers.)
< C9X will introduce the concept of a "flexible array member",
< which will allow the size of an array to be omitted if it is
< the last member in a structure, thus providing a well-defined
< solution.
---
==========
[Q2.10 How can I pass constant... structure arguments?]
< A: As of this writing, C has no way of generating anonymous
< structure values. You will have to use a temporary structure
< variable or a little structure-building function.
---
< The C9X Standard will introduce "compound literals"; one form of
< compound literal will allow structure constants. For example,
< to pass a constant coordinate pair to a plotpoint() function
< which expects a struct point, you will be able to call
---
< Combined with "designated initializers" (another C9X feature),
< it will also be possible to specify member values by name:
---
==========
2.12: My compiler is leaving holes in structures, which is wasting
< space and preventing "binary" I/O to external data files. Can I
< turn off the padding, or otherwise control the alignment of
---
< A: Your compiler may provide an extension to give you this control
< (perhaps a #pragma; see question 11.20), but there is no
< standard method.
---
==========
[Q2.14 How can I determine the byte offset of a field within a structure?]
< A: ANSI C defines the offsetof() macro, which should be used if
< available; see <stddef.h>. If you don't have it, one possible
< implementation is
---
==========
[2.15 How can I access structure fields by name at run time?]
< A: Build a table of names and offsets, using the offsetof() macro.
< The offset of field b in struct a is
<
< offsetb = offsetof(struct a, b)
<
< If structp is a pointer to an instance of this structure, and
< field b is an int (with offset as computed above), b's value can
< be set indirectly with
---
==========
[Q2.20 Can I initialize unions?]
< A: The current C Standard allows an initializer for the first-named
< member of a union. C9X will introduce "designated initializers"
< which can be used to initialize any member.
---
==========
[Q3.2 Under my compiler... Regardless of the order of evaluation...]
example, the compiler chose to multiply the previous value by
< itself and to perform both increments afterwards.
---
[Q3.8 How can I understand these complex expressions?]
The second sentence can be difficult to understand. It says
that if an object is written to within a full expression, any
< and all accesses to it within the same expression must be for
< the purposes of computing the value to be written. This rule
effectively constrains legal expressions to those in which the
accesses demonstrably precede the modification.
---
==========
[3.9 So... we don't know which..., but i does get incremented..., right?]
< A: *No*. Once an expression or program becomes undefined, *all*
---
==========
[Q4.2 I'm trying to declare a pointer and allocate some space for it...]
< A: The pointer you declared is p, not *p. To make a pointer point
< somewhere, you just use the name of the pointer:
---
==========
[4.3 Does *p++ increment p, or what it points to?]
< A: Postfix ++ essentially has higher precedence than the prefix
---
==========
[Q4.10 ...How can I pass a constant by reference?]
< A: You can't do this directly. You will have to declare
---
==========
[Q5.6 If NULL were defined as follows...]
==========
5.20: What does a run-time "null pointer assignment" error mean?
< How can I track it down?
< A debugger may let you set a data watchpoint on location 0.
< Alternatively, you could write a bit of code to stash away a
< copy of 20 or so bytes from location 0, and periodically check
< that the memory at location 0 hasn't changed. See also question
< 16.8.
==========
[Q6.15 How can I declare local arrays of a size matching a passed-in array?]
< A: Until recently, you couldn't. Array dimensions in C
< traditionally had to be compile-time constants. C9X will
< introduce variable-length arrays (VLA's) which will solve this
---
expressions, perhaps involving function parameters.
==========
[Q6.16 How can I dynamically allocate a multidimensional array?]
==========
[Q6.19 How do I write functions which accept two-dimensional arrays...]
< C9X will allow variable-length arrays, and once compilers which
< accept C9X's extensions become widespread, this will probably
---
==========
[Q7.27 So can I query the malloc package to find out how big...]
==========
[Q7.32 What is alloca() and why is its use discouraged?]
==========
[Q8.6 How can I get the numeric (character set) value corresponding to...]
==========
[Q10.7 Is it acceptable for one header file to #include another?]
the prestigious Indian Hill Style Guide (see question 17.9)
disparages them; they can make it harder to find relevant
definitions; they can lead to multiple-definition errors if a file
< is #included twice;
---
==========
< 10.11: I seem to be missing the system header file <sgtty.h>.
< Can someone send me a copy?
---
Standard headers exist in part so that definitions appropriate
to your compiler, operating system, and processor can be
supplied. You cannot just pick up a copy of someone else's
header file and expect it to work, unless that person is using
< exactly the same environment. Ask your compiler vendor why the
< file was not provided (or to send a replacement copy).
---
==========
[Q10.26 How can I write a macro which takes a variable number of arguments?]
< C9X will introduce formal support for function-like macros with
< variable-length argument lists. The notation ... will appear at
---
< definition will be replaced by the variable arguments during
---
==========
[Q11.1 What is the "ANSI C Standard?"]
< More recently, the Standard has been adopted as an international
< standard, ISO/IEC 9899:1990, and this ISO Standard replaces the
< earlier X3.159 even within the United States (where it is known
---
< As of this writing, a complete revision of the Standard is in
< its final stages. The new Standard is nicknamed "C9X" on the
< assumption that it will be finished by the end of 1999. (Many
< of this article's answers have been updated to reflect new C9X
< features.)
---
< The original ANSI Standard included a "Rationale," explaining
< many of its decisions, and discussing a number of subtle points,
< including several of those covered here. (The Rationale was
< "not part of ANSI Standard X3.159-1989, but... included for
< information only," and is not included with the ISO Standard.
< A new one is being prepared for C9X.)
---
==========
[Q11.2 How can I get a copy of the Standard?]
< Note that ANSI derives revenues to support its operations
< from the sale of printed standards, so electronic copies
< are *not* available.
---
< The last time I checked, the cost was $130.00 from ANSI or
< $400.50 from Global. Copies of the original X3.159 (including
< the Rationale) may still be available at $205.00 from ANSI or
< $162.50 from Global.
< In the U.S., it may be possible to get a copy of the original
< ANSI X3.159 (including the Rationale) as "FIPS PUB 160" from
<
< National Technical Information Service (NTIS)
< U.S. Department of Commerce
< Springfield, VA 22161
< 703 487 4650
==========
[Q11.10 Why can't I pass a char ** to a function which expects...]
< You must use explicit casts (e.g. (const char **) in this case)
< when assigning (or passing) pointers which have qualifier
< mismatches at other than the first level of indirection.
---
==========
< 11.27: Why does the ANSI Standard not guarantee more than six case-
< insensitive characters of external identifier significance?
---
The limitation is only that identifiers be *significant*
< in the first six characters, not that they be restricted to
< six characters in length. This limitation is marked in the
< Standard as "obsolescent", and will be removed in C9X.
---
==========
[Q11.33 ...implementation-defined, unspecified, and undefined behavior.]
==========
[Q12.2 Why does the code... copy the last line twice?]
Usually, you should just check the return value of
< the input routine (in this case, fgets() will return NULL on end-
< of-file); often, you don't need to use feof() at all.
---
==========
[Q12.21 How can I tell how much... buffer space... for a... sprintf call?]
< The "obvious" solution to the overflow problem is a length-
< limited version of sprintf(), namely snprintf(). It would be
< used like this:
---
< It will be standardized in C9X.
---
< When the C9X snprintf() arrives, it will also be possible to use
< it to predict the size required for an arbitrary sprintf() call.
< C9X snprintf() will return the number of characters it would
< have placed in the buffer, not just how many it did place.
< Furthermore, it may be called with a buffer size of 0 and a
< null pointer as the destination buffer. Therefore, the call
---
< will compute the number of characters required for the fully-
---
==========
[Q12.23 Why does everyone say not to use gets()?]
A: Unlike fgets(), gets() cannot be told the size of the buffer
it's to read into, so it cannot be prevented from overflowing
< that buffer. As a general rule, always use fgets().
---
==========
< 12.26: How can I flush pending input so that a user's typeahead isn't
---
There is no standard way to discard unread characters from a
stdio input stream, nor would such a way necessarily be
sufficient, since unread characters can also accumulate in
< other, OS-level input buffers. You may be able to read and
< discard characters until \n, or use the curses flushinp()
< function, or use some system-specific technique. See also
< questions 19.1 and 19.2.
---
==========
[Q12.30 I'm trying to update a file in place...]
writing in the read/write "+" modes. Also, remember that you
can only overwrite characters with the same number of
replacement characters, and that overwriting in text mode may
< truncate the file at that point. See also question 19.14.
---
==========
[Q12.34 Once I've used freopen(), how can I get the original stdout... back?]
< It is barely possible to save away information about a stream
< before calling freopen(), such that the original stream can
< later be restored, but the methods involve system-specific calls
< such as dup(), or copying or inspecting the contents of a FILE
< structure, which is exceedingly nonportable and unreliable.
---
==========
12.36b: How can I arrange to have output go two places at once,
e.g. to the screen and to a file?
==========
[Q13.2 Why does strncpy() not always place a '\0' terminator...]
You can get around the problem by using strncat() instead
of strncpy(): if the destination string starts out empty
==========
[Q13.12 How can I get the current date or time of day in a C program?]
< printf("It's %.24s.\n", ctime(&now));
---
==========
[Q13.14 How can I add N days to a date?]
A: The ANSI/ISO Standard C mktime() and difftime() functions
< provide some support for both problems.
---
< These solutions are only guaranteed to work correctly
---
Another approach to both problems,
==========
[Q13.15 I need a random number generator.]
If you do find yourself needing to implement your own random
number generator, there is plenty of literature out there; see
< the References. There are also any number of packages on the
< net: look for r250, RANLIB, and FSULTRA (see question 18.16).
---
==========
[Q13.17 Each time I run my program, I get the same sequence of numbers...]
A: You can call srand() to seed the pseudo-random number generator
< with a truly random initial value. Popular seed values are the
< time of day, or the elapsed time before the user presses a key
< (although keypress times are hard to determine portably;
---
==========
[Q14.1 When I set a float... 3.1, why is printf printing it as 3.0999999?]
A: Most computers use base 2 for floating-point numbers as well as
< for integers. In base 2, one divided by ten is an infinitely-
< repeating fraction (0.0001100110011...), so fractions such as
< 3.1 (which look like they can be exactly represented in decimal)
---
[Q14.3 ...I keep getting "undefined: sin" compilation errors.]
A: Make sure you're actually linking with the math library. For
< instance, under Unix, you usually need to use the -lm option, at
---
==========
[Q14.5 What's a good way to check for "close enough" floating-point equality?]
use something like
#include <math.h>
if(fabs(a - b) <= epsilon * fabs(a))
< for some suitably-chosen degree of closeness epsilon (as long as
< a is nonzero!).
---
==========
[Q14.8 The predefined constant M_PI seems to be missing...]
machine's copy of <math.h>.
If you need pi, you'll have to define it yourself, or compute it
< with 4*atan(1.0).
---
==========
[Q14.9 How do I test for IEEE NaN and other special values?]
< C9X will provide isnan(), fpclassify(), and several other
---
==========
[Q14.11 What's a good way to implement complex numbers in C?]
< C9X will support complex as a standard type.
---
==========
[Q15.6 How can I write a function analogous to scanf()...]
< A: C9X will support vscanf(), vfscanf(), and vsscanf().
< (Until then, you may be on your own.)
---
==========
[Q16.1b I'm getting baffling syntax errors which make no sense at all...]
< A: Check for unclosed comments or mismatched #if/#ifdef/#ifndef/
< #else/#endif directives; remember to check header files, too.
---
[Section 18. Tools and Resources]
==========
[Q18.1 I need...]
tools to compute code ccount, Metre, lcount, or csize,
< metrics or see URL http://www.qucis.queensu.ca/
< Software-Engineering/Cmetrics.html ;
---
McCabe and Associates
==========
[Q18.2 How can I track down these pesky malloc problems?]
< MEMDEBUG from ftp.crpht.lu in pub/sources/memdebug .
---
< Bounds-Checker for DOS, from Nu-Mega Technologies,
< P.O. Box 7780, Nashua, NH 03060-7780, USA, 603-889-2386.
CodeCenter (formerly Saber-C) from Centerline Software,
< 10 Fawcett Street, Cambridge, MA 02138, USA, 617-498-3000.
---
< Insight, from ParaSoft Corporation, 2500 E. Foothill
< Blvd., Pasadena, CA 91107, USA, 818-792-9941,
< (e-mail address removed) .
---
< Purify, from Pure Software, 1309 S. Mary Ave., Sunnyvale,
< CA 94087, USA, 800-224-7873, http://www.pure.com ,
< (e-mail address removed) .
< (I believe Pure was recently acquired by Rational.)
---
< Final Exam Memory Advisor, from PLATINUM Technology
< (formerly Sentinel from AIB Software), 1815 South Meyers
< Rd., Oakbrook Terrace, IL 60181, USA, 630-620-5000,
< 800-442-6861, (e-mail address removed), www.platinum.com .
< ZeroFault, from The Kernel Group, 1250 Capital of Texas
< Highway South, Building Three, Suite 601, Austin,
< TX 78746, 512-433-3333, http://www.tkg.com, (e-mail address removed) .
---
==========
[Q18.3 What's a free or cheap C compiler I can use?]
< compiler, or gcc. It is available by anonymous ftp from
< prep.ai.mit.edu in directory pub/gnu, or at several other FSF
< archive sites.
---
< There are currently no viable shareware compilers for the
< Macintosh.
---
< There is a shareware compiler called PCC, available as
< PCC12C.ZIP .
< Another recently-developed compiler is lcc, available for
< anonymous ftp from ftp.cs.princeton.edu in pub/lcc/.
---
< Archives associated with comp.compilers contain a great deal of
< information about available compilers, interpreters, grammars,
< etc. (for many languages). The comp.compilers archives
< (including an FAQ list), maintained by the moderator, John R.
< Levine, are at iecc.com . A list of available compilers and
< related resources, maintained by Mark Hopkins, Steven Robenalt,
< and David Muir Sharnoff, is at ftp.idiom.com in pub/compilers-
< list/. (See also the comp.compilers directory in the
< news.answers archives at rtfm.mit.edu and ftp.uu.net; see
< question 20.40.)
---
==========
[Q18.7 Where can I get an ANSI-compatible lint?]
< A: Products called PC-Lint and FlexeLint (in "shrouded source
< form," for compilation on 'most any system) are available from
<
< Gimpel Software
< 3207 Hogarth Lane
< Collegeville, PA 19426 USA
< (+1) 610 584 4261
< (e-mail address removed)
---
Another ANSI-compatible lint (which can also perform higher-
< level formal verification) is LCLint, available via anonymous
< ftp from larch.lcs.mit.edu in pub/Larch/lclint/.
---
==========
[Q18.8 Don't ANSI function prototypes render lint obsolete?]
< A: No.
---
==========
[Q18.9 Are there any C tutorials or other resources on the net?]
< Tom Torfs has a nice tutorial at
< http://members.xoom.com/tomtorfs/cintro.html .
---
"Notes for C programmers," by Christopher Sawtell, are
< available from svr-ftp.eng.cam.ac.uk in misc/sawtell_C.shar and
< garbo.uwasa.fi in /pc/c-lang/c-lesson.zip .
---
The Coronado Enterprises C tutorials are available on Simtel
< mirrors in pub/msdos/c or on the web at http://www.swcp.com/~dodrill .
---
< Rick Rowe has a tutorial which is available from ftp.netcom.com
< as pub/rowe/tutorde.zip or ftp.wustl.edu as
< pub/MSDOS_UPLOADS/programming/c_language/ctutorde.zip .
< There is evidently a web-based course at
< http://www.strath.ac.uk/CC/Courses/CCourse/CCourse.html .
---
==========
[Q18.10 What's a good book for learning C?]
Several sets of annotations and errata are available on the net, see
e.g. http://www.csd.uwo.ca/~jamie/.Refs/.Footnotes/C-annotes.html ,
http://www.eskimo.com/~scs/cclass/cclass.html , and
< http://www.lysator.liu.se/c/c-errata.html#main .
---
< Mitch Wright maintains an annotated bibliography of C and Unix
< books; it is available for anonymous ftp from ftp.rahul.net in
< directory pub/mitch/YABL/.
< Scott McMahon has a nice set of reviews at
< http://www.skwc.com/essent/cyberreviews.html .
The Association of C and C++ Users (ACCU) maintains a
< comprehensive set of bibliographic reviews of C/C++ titles, at
< http://bach.cis.temple.edu/accu/bookcase or
< http://www.accu.org/accu .
<
---
< This FAQ list's editor has a large collection of assorted
< old recommendations which various people have posted; it
< is available upon request.
==========
[Q18.13 Where can I find the sources of the standard C libraries?]
< A: One source (though not public domain) is _The Standard C
< Library_, by P.J. Plauger (see the Bibliography).
< Implementations of all or part of the C library have been
< written and are readily available as part of the NetBSD and GNU
< (also Linux) projects.
---
==========
[Q18.15 Where can I get a BNF or YACC grammar for C?]
A: The definitive grammar is of course the one in the ANSI
< standard; see question 11.2. Another grammar (along with
< one for C++) by Jim Roskind is in pub/c++grammar1.1.tar.Z
< at ics.uci.edu (or perhaps ftp.ics.uci.edu, or perhaps
< OLD/pub/c++grammar1.1.tar.Z), or at ftp.eskimo.com in
< u/s/scs/roskind_grammar.Z .
---
< A fleshed-out, working instance of the ANSI grammar
---
==========
[Q18.15d I need code for performing multiple precision arithmetic.]
A: Some popular packages are the "quad" functions within the BSD
Unix libc sources (ftp.uu.net, /systems/unix/bsd-sources/.../
< /src/lib/libc/quad/*), the GNU MP library, the MIRACL package
< (see http://indigo.ie/~mscott/ ), and the old Unix libmp.a.
---
==========
[Q19.1 How can I read a single character from the keyboard...]
< Note that the answers are often not unique even across different
< variants of a system; bear in mind when answering system-
---
everyone else's.
==========
[Q19.3 How can I display a percentage-done indication...]
current line. The character '\b' is a backspace, and will
< usually move the cursor one position to the left.
---
==========
[Q19.8 How can I direct output to the printer?]
==========
[Q19.10 How can I do graphics?]
==========
[Q19.12 How can I find out the size of a file, prior to reading it in?]
You can fseek() to the end and then use
ftell(), or maybe try fstat(), but these tend to have the same
sorts of problems: fstat() is not portable, and generally tells
you the same thing stat() tells you; ftell() is not guaranteed
< to return a byte count except for binary files. Some systems
---
portable, either.
==========
[Q19.20 How can I read a directory in a C program?]
(MS-DOS also has FINDFIRST and FINDNEXT routines which
< do essentially the same thing.)
---
==========
[Q19.23 How can I allocate arrays or structures bigger than 64K?]
to allocate huge amounts of it contiguously. (The C Standard
does not guarantee that single objects can be 32K or larger,
< or 64K for C9X.) Often it's a good idea to use data
---
==========
[Q19.25 How can I access memory (a memory-mapped device...]
< Then, *magicloc refers to the location you want.
---
==========
[Q19.27 How can I invoke another program...]
==========
[Q19.37 How can I implement a delay... with sub-second resolution?]
< A: Unfortunately, there is no portable way. V7 Unix, and derived
< systems, provided a fairly useful ftime() function with
< resolution up to a millisecond, but it has disappeared from
< System V and POSIX. Other routines you might look for on your
---
==========
[Q19.40 How do I... Use sockets? Do networking?]
and W. R. Stevens's _UNIX Network Programming_. There is also
plenty of information out on the net itself, including the
< "Unix Socket FAQ" at http://kipper.york.ac.uk/~vic/sock-faq/
---
==========
[Q20.14 Are pointers really faster than arrays?]
< It is "usually" faster to march through large arrays with
---
==========
[Q20.20 Why don't C comments nest? ...]
< Note also that // comments, as in C++, are not yet legal in C,
< so it's not a good idea to use them in C programs (even if your
< compiler supports them as an extension).
---
==========
< 20.20b: Is C a great language, or what? Where else could you write
---
==========
[Q20.27 ... Can I use a C++ compiler to compile C code?]
< compilation modes. See also questions 8.9 and 20.20.
---
==========
< 20.32: Will 2000 be a leap year? Is (year % 4 == 0) an accurate test
< for leap years?
---
< A: Yes and no, respectively. The full expression for the present
< Gregorian calendar is
---
==========
[Q20.34 ...how do you write a program which produces its own source code...?]
< (This program, like many of the genre, neglects to #include
==========
[Q20.35 What is "Duff's Device"?]
< A: It's a devastatingly deviously unrolled byte-copying loop,
< devised by Tom Duff while he was at Lucasfilm. In its "classic"
< form, it looks like:
---
==========
[Q20.38 Where does the name "C" come from, anyway?]
A: C was derived from Ken Thompson's experimental language B, which
was inspired by Martin Richards's BCPL (Basic Combined
Programming Language), which was a simplification of CPL
< (Cambridge Programming Language).
---
==========
[Q20.40 Where can I get extra copies of this list?]
< What about back issues?
< This list is an evolving document containing questions which
< have been Frequent since before the Great Renaming; it is not
< just a collection of this month's interesting questions. Older
< copies are obsolete and don't contain much, except the
< occasional typo, that the current list doesn't.
==========
[Bibliography]
Samuel P. Harbison and Guy L. Steele, Jr., _C: A Reference Manual_,
< Fourth Edition, Prentice-Hall, 1995, ISBN 0-13-326224-3. [H&S]
---
Donald E. Knuth, _The Art of Computer Programming_. Volume 1:
< _Fundamental Algorithms_, Second Edition, Addison-Wesley, 1973, ISBN
< 0-201-03809-9. Volume 2: _Seminumerical Algorithms_, Second Edition,
< Addison-Wesley, 1981, ISBN 0-201-03822-6. Volume 3: _Sorting and
< Searching_, Addison-Wesley, 1973, ISBN 0-201-03803-X. (New editions
< are coming out!) [Knuth]
---
Robert Sedgewick, _Algorithms in C_, Addison-Wesley, 1990,
< ISBN 0-201-51425-7. (A new edition is being prepared;
< the first half is ISBN 0-201-31452-5.)
---
Brown, John R. Buchan, Joe Buehler, Kimberley Burchett, Gordon Burditt,
Carter, Mike Chambers, Billy Chambless, C. Ron Charlton, Franklin Chen,
Jonathan Chen, Raymond Chen, Richard Cheung, Avinash Chopde, Steve
Clamage, Ken Corbin, Dann Corbit, Ian Cottam, Russ Cox, Jonathan
Bob Makowski, Evan Manning, Barry Margolin, George Marsaglia, George
Matas, Brad Mears, Wayne Mery, De Mickey, Rich Miller, Roger Miller,
Bill Mitchell, Mark Moraes, Darren Morby, Bernhard Muenzer, David Murphy,
Walter Murray, Ralf Muschall, Ken Nakata, Todd Nathan, Taed Nelson,
DaviD W. Sanderson, Frank Sandy, Christopher Sawtell, Jonas Schlein,
Paul Schlyter, Doug Schmidt, Rene Schmit, Russell Schulz, Dean Schulze,
Watson, Kurt Watzka, Larry Weiss, Martin Weitzel, Howard West, Tom
indirectly, to this article.
====================
Next, here are all sorts of minor wording changes and cosmetic
differences:
==========
< [Last modified February 7, 1999 by scs.]
---
Certain topics come up again and again on this newsgroup. They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
< responses, and on tedious corrections to the incorrect answers which are
< inevitably posted.
---
If you have a question about C which is not answered in this article,
< first try to answer it by checking a few of the referenced books, or by
< asking knowledgeable colleagues, before posing your question to the net
---
==========
(However, this is a large and heavy document, so don't
< assume that everyone on the newsgroup has managed to read all of it in
---
because they missed their answer in it.)
==========
< be able to obtain the most up-to-date copy on the web at
---
==========
< or from one of the ftp sites mentioned in question 20.40.
---
Since this list is modified from time to time, its question numbers
may not match those in older or newer copies which are in circulation;
< be careful when referring to FAQ list entries by number alone.
---
< Other versions of this document are also available. Posted along
---
==========
< is available on the web at the aforementioned URL. Finally, for those
< who might prefer a bound, hardcopy version (and even longer answers to
< even more questions!), a book-length version has been published by
< Addison-Wesley (ISBN 0-201-84519-9).
---
< This article is always being improved. Your input is welcomed. Send
---
==========
< 1.1: How do you decide which integer type to use?
---
< float and double. None of the above rules apply if the address
< of a variable is taken and must have a particular type.
---
< 1.4: What should the 64-bit type on a machine that can support it?
---
The .c file containing the definition should also #include the
< same header file, so that the compiler can check that the definition
---
==========
clear. The problem with the NODEPTR example is that the typedef
< has not been defined at the point where the "next" field is
---
==========
< 1.21: How do I declare an array of N pointers to functions returning
< pointers to functions returning pointers to characters?
---
< A: The first part of this question can be answered in at least
< three ways:
---
cdecl can also explain complicated declarations, help with
< casts, and indicate which set of parentheses the arguments
---
==========
< Any good book on C should explain how to read these complicated C
---
mimics use").
==========
1.30: What am I allowed to assume about the initial values
< of variables which are not explicitly initialized?
---
A: Uninitialized variables with "static" duration (that is, those
declared outside of functions, and those declared with the
< storage class static), are guaranteed to start out as zero,
---
==========
A: Is the declaration of a static or non-local variable? Function
< calls are allowed only in initializers for automatic variables
---
A: A string literal can be used in two slightly different ways. As
< an array initializer (as in the declaration of char a[]), it
---
==========
Anywhere else, it turns into an unnamed, static array of
characters, which may be stored in read-only memory,
< which is why you can't safely modify it.
---
(For compiling old code, some compilers have a switch
< controlling whether strings are writable or not.)
---
< When the name of a function appears in an expression like this,
---
implicitly taken), much as an array name does.
==========
< 2.4: What's the best way of implementing opaque (abstract) data types
< in C?
---
< A: No. There is no single, good way for a compiler to implement
< implicit structure comparison (i.e. to support the == operator
---
==========
A simple byte-by-byte comparison could founder on random bits
< present in unused "holes" in the structure (such padding is used
< to keep the alignment of later fields correct; see question 2.12).
---
< Note also that if the structure contains any pointers, only
---
to be valid when read back in.
==========
Finally, note that for widespread portability you must use the
< "b" flag when fopening the files; see question 12.38.
---
< A: Structures may have this padding (as well as internal padding),
< if necessary, to ensure that alignment properties will be
< preserved when an array of contiguous structures is allocated.
< Even when the structure is not part of an array, the end padding
---
==========
< A: At the present time, there is little difference. The C Standard
---
integral types, without errors.
==========
that they obey block scope. (A compiler may also generate
< nonfatal warnings when enumerations and integers are
< indiscriminately mixed, since doing so can still be considered
< bad style even though it is not strictly illegal.)
---
(Loosely speaking, by "multiple, ambiguous side effects" we mean
< any combination of ++, --, =, +=, -=, etc. in a single expression
---
modified twice or modified and then inspected.
==========
< Note that (long int)(a * b) would *not* have the desired effect.
---
< or a delibrate but nonstandard extension if a particular
---
< Whenever possible, you should choose appropriate pointer types
< in the first place, instead of trying to treat one type
---
==========
< void * acts as a generic pointer only because conversions are
---
==========
< 4.12: I've seen different methods used for calling functions via
---
==========
pointers, and that "real" function names always decay implicitly
into pointers (in expressions, as they do in initializations;
< see question 1.34). This reasoning (which is in fact used in
< the ANSI standard) means that
---
function pointer followed by an argument list except call the
< function pointed to.) An explicit * is still allowed.
---
< 5.4: What is NULL and how is it #defined?
---
< preprocessor macro NULL is #defined (by <stdio.h> and several
< other headers) with the value 0, possibly cast to (void *)
---
< NULL should *only* be used for pointers; see question 5.9.
---
< A: Not in general. The complication is that there are machines
---
pointers to different types of data.
==========
< pointer arguments of other types would still be problematical,
---
This article uses the phrase "null pointer" (in lower case) for
< sense 1, the character "0" or the phrase "null pointer constant"
---
==========
< A: C programmers traditionally like to know more than they might
< need to about the underlying machine implementation.
---
integer zero instead of an error message, and if that uncast 0
< was supposed to be a null pointer constant, the code may not
< work.
---
Some 64-bit Cray machines represent int * in the lower 48 bits
< of a word; char * additionally uses the upper 16 bits to
---
==========
A: This message, which typically occurs with MS-DOS compilers, means
< that you've written, via a null (perhaps because uninitialized)
< pointer, to an invalid location (probably offset 0 in the
< default data segment).
---
< A: In one source file you defind an array of characters and in the
---
< It is important to realize that a reference like x[3] generates
---
==========
< If you can't use C9X or gcc, you'll have to use malloc(), and
---
==========
< Finally, in C9X you can use a variable-length array.
---
A: The rule (see question 6.3) by which arrays decay into pointers
< is not applied recursively.
---
not provide an automatically-managed string type. C compilers
< only allocate memory for objects explicitly mentioned in the
---
==========
< A: You got lucky, I guess. The memory pointed to by the
< unitialized pointer p happened to be writable by you,
---
A pointer value which has been freed is, strictly speaking,
< invalid, and *any* use of it, even if is not dereferenced,
---
==========
A: In C, characters are represented by small integers corresponding
< to their values (in the machine's character set), so you
---
have its value.
==========
< DEBUG("i = %d" _ i)
---
controversial trigraph sequences). The ANSI C standard also
< formalizes the C run-time library support routines.
---
< The text of the Rationale (not the full Standard) can be
< obtained by anonymous ftp from ftp.uu.net (see question 18.16)
< in directory doc/standards/ansi/X3.159-1989, and is also
< available on the web at http://www.lysator.liu.se/c/rat/title.html .
< The Rationale has also been printed by Silicon Press,
< ISBN 0-929306-07-4.
---
< Public review drafts of C9X are available from ISO/IEC
---
11.5: Why does the declaration
extern int f(struct x *p);
< give me an obscure warning message about "struct x introduced in
< prototype scope"?
---
A: "const char *p" (which can also be written "char const *p")
declares a pointer to a constant character (you can't change
< the character); "char * const p" declares a constant pointer
---
pointer).
==========
< A: The problem is older linkers which are under the control of
---
on the systems which have them.
==========
If you're interested in writing portable code, you can ignore
< the distinctions, as you'll want to avoid code that depends
---
==========
A: The functions in <locale.h> begin to provide some support for
< these operations, but there is no standard routine for doing
---
==========
< If you're worried about using floating point, you could use
---
==========
< A: In general, a header file contains only declarations.
---
how carefully your compiler's binary/decimal conversion routines
(such as those used by printf) have been written, you may see
< discrepancies when numbers (especially low-precision floats) not
< exactly representable in base 2 are assigned or read in and then
---
base 10 to base 2 and back again).
==========
< Another possibility is to to format the value in question using
---
==========
< A: Some compilers for small machines, including Borland's
---
==========
< char *vstrcat(char *first, ...)
---
< void error(char *fmt, ...)
---
< examples in questions 5.2 and 15.4). Finally, if their types
---
< local arrays. Many systems have fixed-size stacks, and
---
(e.g. Unix) can be confused when the stack tries to grow by a
huge chunk all at once.
==========
< 16.8: What do "Segmentation violation" and "Bus error" mean?
---
< Finally, the author of this FAQ list teaches a C class
< and has placed its notes on the web; they are at
---
< 18.10: What's a good book for learning C?
---
< The GNU libplot package maintains the same spirit and supports
< many modern plot devices;
< see http://www.gnu.org/software/plotutils/plotutils.html .
---
A: If the "size of a file" is the number of characters you'll be
< able to read from it in C, it is difficult or impossible to
---
==========
< readdir() only returns file names; if you need more
---
==========
< (Also, remember to call pclose().)
---
busy-wait, but this is only an option on a single-user, single-
< tasking machine as it is terribly antisocial to any other
---
==========
It is possible, and desirable, for *most* of a program to be
ANSI-compatible, deferring the system-dependent functionality to
< a few routines in a few files which are rewritten for each
< system ported to.
---
or have the function return a structure containing the
< desired values, or (in a pinch) consider global variables.
---
==========
is not only clearer to the human reader, it is more likely to be
recognized by the compiler and turned into the most-efficient
< code (e.g. using a swap instruction, if available).
---
< More information may be found in FORT.gz by Glenn Geers, available
< via anonymous ftp from suphys.physics.su.oz.au in the src
< directory.
<
< cfortran.h, a C header file, simplifies C/FORTRAN interfacing on
< many popular machines. It is available via anonymous ftp from
< zebra.desy.de or at http://www-zeus.desy.de/~burow .
---
This FAQ list's maintainer also has available a list of a few
< other commercial translation products, and some for more obscure
< languages.
---
< A: The contest is in a state of flux; see
---
Finally, here are a few questions which don't really need to be in
the posted-to-Usenet list every month. (As mentioned, though, they'll
live on in the web-based version.)
==========
< 1.22: How can I declare a function that can return a pointer to a
< function of the same type? I'm building a state machine with
< one function for each state, each of which returns a pointer to
< the function for the next state. But I can't find a way to
< declare the functions.
<
< A: You can't quite do it directly. Either have the function return
< a generic function pointer, with some judicious casts to adjust
< the types as the pointers are passed around; or have it return a
< structure containing only a pointer to a function returning that
< structure.
==========
< 2.7: I heard that structures could be assigned to variables and
< passed to and from functions, but K&R1 says not.
<
< A: What K&R1 said (though this was quite some time ago by now) was
< that the restrictions on structure operations would be lifted
< in a forthcoming version of the compiler, and in fact structure
< assignment and passing were fully functional in Ritchie's
< compiler even as K&R1 was being published. A few ancient C
< compilers may have lacked these operations, but all modern
< compilers support them, and they are part of the ANSI C
< standard, so there should be no reluctance to use them.
<
< (Note that when a structure is assigned, passed, or returned,
< the copying is done monolithically; the data pointed to by any
< pointer fields is *not* copied.)
==========
< 13.14b: Does C have any Year 2000 problems?
<
< A: No, although poorly-written C programs do.
<
< The tm_year field of struct tm holds the value of the year minus
< 1900; this field will therefore contain the value 100 for the
< year 2000. Code that uses tm_year correctly (by adding or
< subtracting 1900 when converting to or from human-readable
< 4-digit year representations) will have no problems at the turn
< of the millennium. Any code that uses tm_year incorrectly,
< however, such as by using it directly as a human-readable
< 2-digit year, or setting it from a 4-digit year with code like
<
< tm.tm_year = yyyy % 100; /* WRONG */
<
< or printing it as an allegedly human-readable 4-digit year with
< code like
<
< printf("19%d", tm.tm_year); /* WRONG */
<
< will have grave y2k problems indeed. See also question 20.32.
==========
< 13.24: I'm trying to port this A: Those functions are variously
< old program. Why do I obsolete; you should
< get "undefined external" instead:
< errors for:
<
< index? use strchr.
< rindex? use strrchr.
< bcopy? use memmove, after
< interchanging the first and
< second arguments (see also
< question 11.25).
< bcmp? use memcmp.
< bzero? use memset, with a second
< argument of 0.
==========
< 15.7: I have a pre-ANSI compiler, without <stdarg.h>. What can I do?
<
< A: There's an older header, <varargs.h>, which offers about the
< same functionality.
==========
< 18.5: How can I shut off the "warning: possible pointer alignment
< problem" message which lint gives me for each call to malloc()?
<
< A: The problem is that traditional versions of lint do not know,
< and cannot be told, that malloc() "returns a pointer to space
< suitably aligned for storage of any type of object." It is
< possible to provide a pseudoimplementation of malloc(), using a
< #define inside of #ifdef lint, which effectively shuts this
< warning off, but a simpleminded definition will also suppress
< meaningful messages about truly incorrect invocations. It may
< be easier simply to ignore the message, perhaps in an automated
< way with grep -v. (But don't get in the habit of ignoring too
< many lint messages, otherwise one day you'll overlook a
< significant one.)
====================
Steve Summit
(e-mail address removed)
Comp-lang-c-archive-name: C-FAQ-list.diff
URL: http://www.eskimo.com/~scs/C-faq/top.html
It's been a long time. Far too long, but that's water under the bridge.
Thanks, all, for your patience.
This article lists the many, many differences between the previous
version of the comp.lang.c FAQ list (version 3.5, last modified
February 7, 1999, last posted June 1, 2004) and this finally-new one.
I have scoured my archives for every unacknowledged suggestion and
promised improvement, so for once I can say: this update is mostly
complete. At this point, if you've sent me a suggestion which I
haven't acknowledged or which isn't reflected here, I guess it
wouldn't hurt if you sent it again.
This update has been so long delayed that some of its improvements
(such as the "new" question 19.17c) are already mildly obsolete;
they were made to my master manuscripts years ago but have, alas,
never seen the light of day until now.
Because of some overly ambitious formatting changes I've embarked on,
the updated HTML version is *still* not quite ready. But the light
at the end of the tunnel is in sight: that grandish unveiling will
happen in the next month or so.
I've categorized the diffs that follow into brand-new questions,
significant changes, minor adjustments, and (sob!) retired questions.
The notation is based on that of the standard `diff' program: `<'
indicates old, changed, or deleted text; `>' indicates new text.
(For my fellow packrats out there: don't worry, the deleted
questions, and most of the other deleted text, will live on
in the web version...)
First, the new questions. (Some of these are migrated from the
book-length version, but most of them are brand new.)
==========
====================
Next, here are the significant changes.
==========
[Q1.1 How should I decide which integer type to use?]
If for some reason you need to declare something with an
*exact* size... be sure to encapsulate the choice behind
< an appropriate typedef.
---
==========
[Q1.4 What should the 64-bit type be on a machine that can support it?]
< A: The forthcoming revision to the C Standard (C9X) specifies type
< long long as effectively being at least 64 bits, and this type
< has been implemented by a number of compilers for some time.
< (Others have implemented extensions such as __longlong.)
< On the other hand, there's no theoretical reason why a compiler
< couldn't implement type short int as 16, int as 32, and long int
< as 64 bits, and some compilers do indeed choose this
< arrangement.
---
==========
[Q1.7 What's the best way to declare and define global variables...]
A: First, though there can be many "declarations" (and in many
< translation units) of a single "global" (strictly speaking,
< "external") variable or function, there must be exactly one
< "definition". (The definition is the declaration that actually
< allocates space, and provides an initialization value, if any.)
---
==========
(Unix compilers and linkers typically use a "common model" which
allows multiple definitions, as long as at most one is
initialized; this behavior is mentioned as a "common extension"
< by the ANSI Standard, no pun intended. A few very odd systems
< may require an explicit initializer to distinguish a definition
< from an external declaration.)
---
==========
[Q1.25 My compiler is complaining about an invalid redeclaration...]
< A: Functions which are called without a declaration in scope
< (perhaps because the first call precedes the function's
< definition) are assumed to be declared as returning int (and
without any argument type information), leading to discrepancies
< if the function is later declared or defined otherwise. Non-int
< functions must be declared before they are called.
---
without any argument type information), leading to discrepancies
==========
[Q1.30 What am I allowed to assume about the initial values...]
==========
[Q1.31 This code, straight out of a book, isn't compiling]
< A: Perhaps you have a pre-ANSI compiler, which doesn't allow
< initialization of "automatic aggregates" (i.e. non-static
< local arrays, structures, and unions). (As a workaround, and
< depending on how the variable a is used, you may be able to make
< it global or static, or replace it with a pointer, or initialize
< it by hand with strcpy() when f() is called.)
---
==========
[Q1.34 I finally figured out the syntax for... pointers to functions...]
< An explicit declaration for the function is normally needed,
< since implicit external function declaration does not happen in
< this case (because the function name in the initialization is
< not part of a function call).
---
==========
[Q2.4 How can I implement opaque (abstract) data types in C?]
A: One good way is for clients to use structure pointers (perhaps
additionally hidden behind typedefs) which point to structure
==========
[Q2.6 I came across some code that declared a structure like this...]
these "chummy" structures must be used with care, since the
programmer knows more about their size than the compiler does.
< (In particular, they can generally only be manipulated via
< pointers.)
< C9X will introduce the concept of a "flexible array member",
< which will allow the size of an array to be omitted if it is
< the last member in a structure, thus providing a well-defined
< solution.
---
==========
[Q2.10 How can I pass constant... structure arguments?]
< A: As of this writing, C has no way of generating anonymous
< structure values. You will have to use a temporary structure
< variable or a little structure-building function.
---
< The C9X Standard will introduce "compound literals"; one form of
< compound literal will allow structure constants. For example,
< to pass a constant coordinate pair to a plotpoint() function
< which expects a struct point, you will be able to call
---
< Combined with "designated initializers" (another C9X feature),
< it will also be possible to specify member values by name:
---
==========
2.12: My compiler is leaving holes in structures, which is wasting
< space and preventing "binary" I/O to external data files. Can I
< turn off the padding, or otherwise control the alignment of
---
< A: Your compiler may provide an extension to give you this control
< (perhaps a #pragma; see question 11.20), but there is no
< standard method.
---
==========
[Q2.14 How can I determine the byte offset of a field within a structure?]
< A: ANSI C defines the offsetof() macro, which should be used if
< available; see <stddef.h>. If you don't have it, one possible
< implementation is
---
==========
[2.15 How can I access structure fields by name at run time?]
< A: Build a table of names and offsets, using the offsetof() macro.
< The offset of field b in struct a is
<
< offsetb = offsetof(struct a, b)
<
< If structp is a pointer to an instance of this structure, and
< field b is an int (with offset as computed above), b's value can
< be set indirectly with
---
==========
[Q2.20 Can I initialize unions?]
< A: The current C Standard allows an initializer for the first-named
< member of a union. C9X will introduce "designated initializers"
< which can be used to initialize any member.
---
==========
[Q3.2 Under my compiler... Regardless of the order of evaluation...]
example, the compiler chose to multiply the previous value by
< itself and to perform both increments afterwards.
---
[Q3.8 How can I understand these complex expressions?]
The second sentence can be difficult to understand. It says
that if an object is written to within a full expression, any
< and all accesses to it within the same expression must be for
< the purposes of computing the value to be written. This rule
effectively constrains legal expressions to those in which the
accesses demonstrably precede the modification.
---
This rule effectively constrains legal expressions to those in
==========
[3.9 So... we don't know which..., but i does get incremented..., right?]
< A: *No*. Once an expression or program becomes undefined, *all*
---
undefined, *all* aspects of it become undefined.
==========
[Q4.2 I'm trying to declare a pointer and allocate some space for it...]
< A: The pointer you declared is p, not *p. To make a pointer point
< somewhere, you just use the name of the pointer:
---
==========
[4.3 Does *p++ increment p, or what it points to?]
< A: Postfix ++ essentially has higher precedence than the prefix
---
precedence than the prefix unary operators.
==========
[Q4.10 ...How can I pass a constant by reference?]
< A: You can't do this directly. You will have to declare
---
a temporary variable, and then pass its address to the function:
==========
[Q5.6 If NULL were defined as follows...]
==========
5.20: What does a run-time "null pointer assignment" error mean?
< How can I track it down?
< A debugger may let you set a data watchpoint on location 0.
< Alternatively, you could write a bit of code to stash away a
< copy of 20 or so bytes from location 0, and periodically check
< that the memory at location 0 hasn't changed. See also question
< 16.8.
==========
[Q6.15 How can I declare local arrays of a size matching a passed-in array?]
< A: Until recently, you couldn't. Array dimensions in C
< traditionally had to be compile-time constants. C9X will
< introduce variable-length arrays (VLA's) which will solve this
---
problem; local arrays may have sizes set by variables or other
expressions, perhaps involving function parameters.
==========
[Q6.16 How can I dynamically allocate a multidimensional array?]
==========
[Q6.19 How do I write functions which accept two-dimensional arrays...]
< C9X will allow variable-length arrays, and once compilers which
< accept C9X's extensions become widespread, this will probably
---
become the preferred solution.
==========
[Q7.27 So can I query the malloc package to find out how big...]
==========
[Q7.32 What is alloca() and why is its use discouraged?]
==========
[Q8.6 How can I get the numeric (character set) value corresponding to...]
==========
[Q10.7 Is it acceptable for one header file to #include another?]
the prestigious Indian Hill Style Guide (see question 17.9)
disparages them; they can make it harder to find relevant
definitions; they can lead to multiple-definition errors if a file
< is #included twice;
---
and they make manual Makefile maintenance very difficult.
==========
< 10.11: I seem to be missing the system header file <sgtty.h>.
< Can someone send me a copy?
---
Standard headers exist in part so that definitions appropriate
to your compiler, operating system, and processor can be
supplied. You cannot just pick up a copy of someone else's
header file and expect it to work, unless that person is using
< exactly the same environment. Ask your compiler vendor why the
< file was not provided (or to send a replacement copy).
---
==========
[Q10.26 How can I write a macro which takes a variable number of arguments?]
< C9X will introduce formal support for function-like macros with
< variable-length argument lists. The notation ... will appear at
---
< definition will be replaced by the variable arguments during
---
==========
[Q11.1 What is the "ANSI C Standard?"]
< More recently, the Standard has been adopted as an international
< standard, ISO/IEC 9899:1990, and this ISO Standard replaces the
< earlier X3.159 even within the United States (where it is known
---
< As of this writing, a complete revision of the Standard is in
< its final stages. The new Standard is nicknamed "C9X" on the
< assumption that it will be finished by the end of 1999. (Many
< of this article's answers have been updated to reflect new C9X
< features.)
---
< The original ANSI Standard included a "Rationale," explaining
< many of its decisions, and discussing a number of subtle points,
< including several of those covered here. (The Rationale was
< "not part of ANSI Standard X3.159-1989, but... included for
< information only," and is not included with the ISO Standard.
< A new one is being prepared for C9X.)
---
==========
[Q11.2 How can I get a copy of the Standard?]
< Note that ANSI derives revenues to support its operations
< from the sale of printed standards, so electronic copies
< are *not* available.
---
< The last time I checked, the cost was $130.00 from ANSI or
< $400.50 from Global. Copies of the original X3.159 (including
< the Rationale) may still be available at $205.00 from ANSI or
< $162.50 from Global.
< In the U.S., it may be possible to get a copy of the original
< ANSI X3.159 (including the Rationale) as "FIPS PUB 160" from
<
< National Technical Information Service (NTIS)
< U.S. Department of Commerce
< Springfield, VA 22161
< 703 487 4650
==========
[Q11.10 Why can't I pass a char ** to a function which expects...]
< You must use explicit casts (e.g. (const char **) in this case)
< when assigning (or passing) pointers which have qualifier
< mismatches at other than the first level of indirection.
---
==========
< 11.27: Why does the ANSI Standard not guarantee more than six case-
< insensitive characters of external identifier significance?
---
The limitation is only that identifiers be *significant*
< in the first six characters, not that they be restricted to
< six characters in length. This limitation is marked in the
< Standard as "obsolescent", and will be removed in C9X.
---
==========
[Q11.33 ...implementation-defined, unspecified, and undefined behavior.]
==========
[Q12.2 Why does the code... copy the last line twice?]
Usually, you should just check the return value of
< the input routine (in this case, fgets() will return NULL on end-
< of-file); often, you don't need to use feof() at all.
---
==========
[Q12.21 How can I tell how much... buffer space... for a... sprintf call?]
< The "obvious" solution to the overflow problem is a length-
< limited version of sprintf(), namely snprintf(). It would be
< used like this:
---
< It will be standardized in C9X.
---
< When the C9X snprintf() arrives, it will also be possible to use
< it to predict the size required for an arbitrary sprintf() call.
< C9X snprintf() will return the number of characters it would
< have placed in the buffer, not just how many it did place.
< Furthermore, it may be called with a buffer size of 0 and a
< null pointer as the destination buffer. Therefore, the call
---
< will compute the number of characters required for the fully-
---
==========
[Q12.23 Why does everyone say not to use gets()?]
A: Unlike fgets(), gets() cannot be told the size of the buffer
it's to read into, so it cannot be prevented from overflowing
< that buffer. As a general rule, always use fgets().
---
==========
< 12.26: How can I flush pending input so that a user's typeahead isn't
---
There is no standard way to discard unread characters from a
stdio input stream, nor would such a way necessarily be
sufficient, since unread characters can also accumulate in
< other, OS-level input buffers. You may be able to read and
< discard characters until \n, or use the curses flushinp()
< function, or use some system-specific technique. See also
< questions 19.1 and 19.2.
---
==========
[Q12.30 I'm trying to update a file in place...]
writing in the read/write "+" modes. Also, remember that you
can only overwrite characters with the same number of
replacement characters, and that overwriting in text mode may
< truncate the file at that point. See also question 19.14.
---
==========
[Q12.34 Once I've used freopen(), how can I get the original stdout... back?]
< It is barely possible to save away information about a stream
< before calling freopen(), such that the original stream can
< later be restored, but the methods involve system-specific calls
< such as dup(), or copying or inspecting the contents of a FILE
< structure, which is exceedingly nonportable and unreliable.
---
==========
12.36b: How can I arrange to have output go two places at once,
e.g. to the screen and to a file?
==========
[Q13.2 Why does strncpy() not always place a '\0' terminator...]
You can get around the problem by using strncat() instead
of strncpy(): if the destination string starts out empty
does what you probably wanted strncpy() to do.
==========
[Q13.12 How can I get the current date or time of day in a C program?]
< printf("It's %.24s.\n", ctime(&now));
---
==========
[Q13.14 How can I add N days to a date?]
A: The ANSI/ISO Standard C mktime() and difftime() functions
< provide some support for both problems.
---
< These solutions are only guaranteed to work correctly
---
for dates in the range which can be represented as time_t's.
Another approach to both problems,
is to use "Julian day numbers".
==========
[Q13.15 I need a random number generator.]
If you do find yourself needing to implement your own random
number generator, there is plenty of literature out there; see
< the References. There are also any number of packages on the
< net: look for r250, RANLIB, and FSULTRA (see question 18.16).
---
==========
[Q13.17 Each time I run my program, I get the same sequence of numbers...]
A: You can call srand() to seed the pseudo-random number generator
< with a truly random initial value. Popular seed values are the
< time of day, or the elapsed time before the user presses a key
< (although keypress times are hard to determine portably;
---
==========
[Q14.1 When I set a float... 3.1, why is printf printing it as 3.0999999?]
A: Most computers use base 2 for floating-point numbers as well as
< for integers. In base 2, one divided by ten is an infinitely-
< repeating fraction (0.0001100110011...), so fractions such as
< 3.1 (which look like they can be exactly represented in decimal)
---
cannot be represented exactly in binary.
[Q14.3 ...I keep getting "undefined: sin" compilation errors.]
A: Make sure you're actually linking with the math library. For
< instance, under Unix, you usually need to use the -lm option, at
---
the command line, when compiling/linking.
==========
[Q14.5 What's a good way to check for "close enough" floating-point equality?]
use something like
#include <math.h>
if(fabs(a - b) <= epsilon * fabs(a))
< for some suitably-chosen degree of closeness epsilon (as long as
< a is nonzero!).
---
==========
[Q14.8 The predefined constant M_PI seems to be missing...]
machine's copy of <math.h>.
If you need pi, you'll have to define it yourself, or compute it
< with 4*atan(1.0).
---
==========
[Q14.9 How do I test for IEEE NaN and other special values?]
< C9X will provide isnan(), fpclassify(), and several other
---
classification routines.
==========
[Q14.11 What's a good way to implement complex numbers in C?]
< C9X will support complex as a standard type.
---
==========
[Q15.6 How can I write a function analogous to scanf()...]
< A: C9X will support vscanf(), vfscanf(), and vsscanf().
< (Until then, you may be on your own.)
---
==========
[Q16.1b I'm getting baffling syntax errors which make no sense at all...]
< A: Check for unclosed comments or mismatched #if/#ifdef/#ifndef/
< #else/#endif directives; remember to check header files, too.
---
to check header files, too.
[Section 18. Tools and Resources]
==========
[Q18.1 I need...]
tools to compute code ccount, Metre, lcount, or csize,
< metrics or see URL http://www.qucis.queensu.ca/
< Software-Engineering/Cmetrics.html ;
---
metrics there is also a package sold by
McCabe and Associates
==========
[Q18.2 How can I track down these pesky malloc problems?]
< MEMDEBUG from ftp.crpht.lu in pub/sources/memdebug .
---
< Bounds-Checker for DOS, from Nu-Mega Technologies,
< P.O. Box 7780, Nashua, NH 03060-7780, USA, 603-889-2386.
CodeCenter (formerly Saber-C) from Centerline Software,
< 10 Fawcett Street, Cambridge, MA 02138, USA, 617-498-3000.
---
< Insight, from ParaSoft Corporation, 2500 E. Foothill
< Blvd., Pasadena, CA 91107, USA, 818-792-9941,
< (e-mail address removed) .
---
< Purify, from Pure Software, 1309 S. Mary Ave., Sunnyvale,
< CA 94087, USA, 800-224-7873, http://www.pure.com ,
< (e-mail address removed) .
< (I believe Pure was recently acquired by Rational.)
---
< Final Exam Memory Advisor, from PLATINUM Technology
< (formerly Sentinel from AIB Software), 1815 South Meyers
< Rd., Oakbrook Terrace, IL 60181, USA, 630-620-5000,
< 800-442-6861, (e-mail address removed), www.platinum.com .
< ZeroFault, from The Kernel Group, 1250 Capital of Texas
< Highway South, Building Three, Suite 601, Austin,
< TX 78746, 512-433-3333, http://www.tkg.com, (e-mail address removed) .
---
==========
[Q18.3 What's a free or cheap C compiler I can use?]
< compiler, or gcc. It is available by anonymous ftp from
< prep.ai.mit.edu in directory pub/gnu, or at several other FSF
< archive sites.
---
< There are currently no viable shareware compilers for the
< Macintosh.
---
< There is a shareware compiler called PCC, available as
< PCC12C.ZIP .
< Another recently-developed compiler is lcc, available for
< anonymous ftp from ftp.cs.princeton.edu in pub/lcc/.
---
< Archives associated with comp.compilers contain a great deal of
< information about available compilers, interpreters, grammars,
< etc. (for many languages). The comp.compilers archives
< (including an FAQ list), maintained by the moderator, John R.
< Levine, are at iecc.com . A list of available compilers and
< related resources, maintained by Mark Hopkins, Steven Robenalt,
< and David Muir Sharnoff, is at ftp.idiom.com in pub/compilers-
< list/. (See also the comp.compilers directory in the
< news.answers archives at rtfm.mit.edu and ftp.uu.net; see
< question 20.40.)
---
==========
[Q18.7 Where can I get an ANSI-compatible lint?]
< A: Products called PC-Lint and FlexeLint (in "shrouded source
< form," for compilation on 'most any system) are available from
<
< Gimpel Software
< 3207 Hogarth Lane
< Collegeville, PA 19426 USA
< (+1) 610 584 4261
< (e-mail address removed)
---
Another ANSI-compatible lint (which can also perform higher-
< level formal verification) is LCLint, available via anonymous
< ftp from larch.lcs.mit.edu in pub/Larch/lclint/.
---
==========
[Q18.8 Don't ANSI function prototypes render lint obsolete?]
< A: No.
---
==========
[Q18.9 Are there any C tutorials or other resources on the net?]
< Tom Torfs has a nice tutorial at
< http://members.xoom.com/tomtorfs/cintro.html .
---
"Notes for C programmers," by Christopher Sawtell, are
< available from svr-ftp.eng.cam.ac.uk in misc/sawtell_C.shar and
< garbo.uwasa.fi in /pc/c-lang/c-lesson.zip .
---
The Coronado Enterprises C tutorials are available on Simtel
< mirrors in pub/msdos/c or on the web at http://www.swcp.com/~dodrill .
---
< Rick Rowe has a tutorial which is available from ftp.netcom.com
< as pub/rowe/tutorde.zip or ftp.wustl.edu as
< pub/MSDOS_UPLOADS/programming/c_language/ctutorde.zip .
< There is evidently a web-based course at
< http://www.strath.ac.uk/CC/Courses/CCourse/CCourse.html .
---
==========
[Q18.10 What's a good book for learning C?]
Several sets of annotations and errata are available on the net, see
e.g. http://www.csd.uwo.ca/~jamie/.Refs/.Footnotes/C-annotes.html ,
http://www.eskimo.com/~scs/cclass/cclass.html , and
< http://www.lysator.liu.se/c/c-errata.html#main .
---
< Mitch Wright maintains an annotated bibliography of C and Unix
< books; it is available for anonymous ftp from ftp.rahul.net in
< directory pub/mitch/YABL/.
< Scott McMahon has a nice set of reviews at
< http://www.skwc.com/essent/cyberreviews.html .
The Association of C and C++ Users (ACCU) maintains a
< comprehensive set of bibliographic reviews of C/C++ titles, at
< http://bach.cis.temple.edu/accu/bookcase or
< http://www.accu.org/accu .
<
---
< This FAQ list's editor has a large collection of assorted
< old recommendations which various people have posted; it
< is available upon request.
==========
[Q18.13 Where can I find the sources of the standard C libraries?]
< A: One source (though not public domain) is _The Standard C
< Library_, by P.J. Plauger (see the Bibliography).
< Implementations of all or part of the C library have been
< written and are readily available as part of the NetBSD and GNU
< (also Linux) projects.
---
==========
[Q18.15 Where can I get a BNF or YACC grammar for C?]
A: The definitive grammar is of course the one in the ANSI
< standard; see question 11.2. Another grammar (along with
< one for C++) by Jim Roskind is in pub/c++grammar1.1.tar.Z
< at ics.uci.edu (or perhaps ftp.ics.uci.edu, or perhaps
< OLD/pub/c++grammar1.1.tar.Z), or at ftp.eskimo.com in
< u/s/scs/roskind_grammar.Z .
---
< A fleshed-out, working instance of the ANSI grammar
---
==========
[Q18.15d I need code for performing multiple precision arithmetic.]
A: Some popular packages are the "quad" functions within the BSD
Unix libc sources (ftp.uu.net, /systems/unix/bsd-sources/.../
< /src/lib/libc/quad/*), the GNU MP library, the MIRACL package
< (see http://indigo.ie/~mscott/ ), and the old Unix libmp.a.
---
==========
[Q19.1 How can I read a single character from the keyboard...]
< Note that the answers are often not unique even across different
< variants of a system; bear in mind when answering system-
---
that the answer that applies to your system may not apply to
everyone else's.
==========
[Q19.3 How can I display a percentage-done indication...]
current line. The character '\b' is a backspace, and will
< usually move the cursor one position to the left.
---
==========
[Q19.8 How can I direct output to the printer?]
==========
[Q19.10 How can I do graphics?]
==========
[Q19.12 How can I find out the size of a file, prior to reading it in?]
You can fseek() to the end and then use
ftell(), or maybe try fstat(), but these tend to have the same
sorts of problems: fstat() is not portable, and generally tells
you the same thing stat() tells you; ftell() is not guaranteed
< to return a byte count except for binary files. Some systems
---
filesize() or filelength(), but these are obviously not
portable, either.
==========
[Q19.20 How can I read a directory in a C program?]
(MS-DOS also has FINDFIRST and FINDNEXT routines which
< do essentially the same thing.)
---
==========
[Q19.23 How can I allocate arrays or structures bigger than 64K?]
to allocate huge amounts of it contiguously. (The C Standard
does not guarantee that single objects can be 32K or larger,
< or 64K for C9X.) Often it's a good idea to use data
---
==========
[Q19.25 How can I access memory (a memory-mapped device...]
< Then, *magicloc refers to the location you want.
---
==========
[Q19.27 How can I invoke another program...]
==========
[Q19.37 How can I implement a delay... with sub-second resolution?]
< A: Unfortunately, there is no portable way. V7 Unix, and derived
< systems, provided a fairly useful ftime() function with
< resolution up to a millisecond, but it has disappeared from
< System V and POSIX. Other routines you might look for on your
---
==========
[Q19.40 How do I... Use sockets? Do networking?]
and W. R. Stevens's _UNIX Network Programming_. There is also
plenty of information out on the net itself, including the
< "Unix Socket FAQ" at http://kipper.york.ac.uk/~vic/sock-faq/
---
==========
[Q20.14 Are pointers really faster than arrays?]
< It is "usually" faster to march through large arrays with
---
==========
[Q20.20 Why don't C comments nest? ...]
< Note also that // comments, as in C++, are not yet legal in C,
< so it's not a good idea to use them in C programs (even if your
< compiler supports them as an extension).
---
==========
< 20.20b: Is C a great language, or what? Where else could you write
---
something like a+++++b ?
==========
[Q20.27 ... Can I use a C++ compiler to compile C code?]
< compilation modes. See also questions 8.9 and 20.20.
---
==========
< 20.32: Will 2000 be a leap year? Is (year % 4 == 0) an accurate test
< for leap years?
---
< A: Yes and no, respectively. The full expression for the present
< Gregorian calendar is
---
==========
[Q20.34 ...how do you write a program which produces its own source code...?]
< (This program, like many of the genre, neglects to #include
quote character " has the value 34, as it does in ASCII.)and assumes that the double-quote character " has the said:
==========
[Q20.35 What is "Duff's Device"?]
< A: It's a devastatingly deviously unrolled byte-copying loop,
< devised by Tom Duff while he was at Lucasfilm. In its "classic"
< form, it looks like:
---
==========
[Q20.38 Where does the name "C" come from, anyway?]
A: C was derived from Ken Thompson's experimental language B, which
was inspired by Martin Richards's BCPL (Basic Combined
Programming Language), which was a simplification of CPL
< (Cambridge Programming Language).
---
==========
[Q20.40 Where can I get extra copies of this list?]
< What about back issues?
< This list is an evolving document containing questions which
< have been Frequent since before the Great Renaming; it is not
< just a collection of this month's interesting questions. Older
< copies are obsolete and don't contain much, except the
< occasional typo, that the current list doesn't.
==========
[Bibliography]
Samuel P. Harbison and Guy L. Steele, Jr., _C: A Reference Manual_,
< Fourth Edition, Prentice-Hall, 1995, ISBN 0-13-326224-3. [H&S]
---
Donald E. Knuth, _The Art of Computer Programming_. Volume 1:
< _Fundamental Algorithms_, Second Edition, Addison-Wesley, 1973, ISBN
< 0-201-03809-9. Volume 2: _Seminumerical Algorithms_, Second Edition,
< Addison-Wesley, 1981, ISBN 0-201-03822-6. Volume 3: _Sorting and
< Searching_, Addison-Wesley, 1973, ISBN 0-201-03803-X. (New editions
< are coming out!) [Knuth]
---
Robert Sedgewick, _Algorithms in C_, Addison-Wesley, 1990,
< ISBN 0-201-51425-7. (A new edition is being prepared;
< the first half is ISBN 0-201-31452-5.)
---
Michael Bresnahan, Walter Briscoe, Vincent Broman, Robert T. Brown, Stan
Brown, John R. Buchan, Joe Buehler, Kimberley Burchett, Gordon Burditt,
J.M. Cain, Christopher Calabrese, Ian Cargill, Vinit Carpenter, Paul
Carter, Mike Chambers, Billy Chambless, C. Ron Charlton, Franklin Chen,
Jonathan Chen, Raymond Chen, Richard Cheung, Avinash Chopde, Steve
Clamage, Ken Corbin, Dann Corbit, Ian Cottam, Russ Cox, Jonathan
Ehrlich, Arno Eigenwillig, Yoav Eilat, Dave Eisen, Joe English, Bjorn
Alexander Forst, Steve Fosdick, Jeff Francis, Ken Fuchs, Tom Gambill,
Wilhelm Harder, Elliotte Rusty Harold, Joe Harrington, Guy Harris, John
Lawrence Kirby, Kin-ichi Kitano, Peter Klausler, John Kleinjans, Andrew
Stuart MacMartin, John R. MacMillan, Robert S. Maier, Andrew Main,
Bob Makowski, Evan Manning, Barry Margolin, George Marsaglia, George
Matas, Brad Mears, Wayne Mery, De Mickey, Rich Miller, Roger Miller,
Bill Mitchell, Mark Moraes, Darren Morby, Bernhard Muenzer, David Murphy,
Walter Murray, Ralf Muschall, Ken Nakata, Todd Nathan, Taed Nelson,
Nulsen, David O'Brien, Richard A. O'Keefe, Adam Kolawa, Keith Edward
Francois Pinard, Nick Pitfield, Wayne Pollock, (e-mail address removed), Dan Pop,
Matthew Saltzman, Rich Salz, Chip Salzenberg, Matthew Sams, Paul Sand,
DaviD W. Sanderson, Frank Sandy, Christopher Sawtell, Jonas Schlein,
Paul Schlyter, Doug Schmidt, Rene Schmit, Russell Schulz, Dean Schulze,
James Stern, Zalman Stern, Michael Sternberg, Geoff Stevens, Alan
Nikos Triantafillis, Ilya Tsindlekht, Andrew Tucker, Goran Uddeborg,
Verhoeff, Ed Vielmetti, Larry Virden, Chris Volpe, Mark Warren, Alan
Watson, Kurt Watzka, Larry Weiss, Martin Weitzel, Howard West, Tom
Ozan S. Yigit, and Zhuo Zang, who have contributed, directly or
indirectly, to this article.
====================
Next, here are all sorts of minor wording changes and cosmetic
differences:
==========
< [Last modified February 7, 1999 by scs.]
---
==========
Certain topics come up again and again on this newsgroup. They are good
questions, and the answers may not be immediately obvious, but each time
they recur, much net bandwidth and reader time is wasted on repetitive
< responses, and on tedious corrections to the incorrect answers which are
< inevitably posted.
---
==========
If you have a question about C which is not answered in this article,
< first try to answer it by checking a few of the referenced books, or by
< asking knowledgeable colleagues, before posing your question to the net
---
your question to the net at large.
==========
(However, this is a large and heavy document, so don't
< assume that everyone on the newsgroup has managed to read all of it in
---
and please don't roll it up and thwack people over the head with it just
because they missed their answer in it.)
==========
< be able to obtain the most up-to-date copy on the web at
---
http://www.eskimo.com/~scs/C-faq/top.html or http://www.faqs.org/faqs/ ,
==========
< or from one of the ftp sites mentioned in question 20.40.
---
==========
Since this list is modified from time to time, its question numbers
may not match those in older or newer copies which are in circulation;
< be careful when referring to FAQ list entries by number alone.
---
==========
< Other versions of this document are also available. Posted along
---
with it are an abridged version and (when there are changes) a list of
==========
< is available on the web at the aforementioned URL. Finally, for those
< who might prefer a bound, hardcopy version (and even longer answers to
< even more questions!), a book-length version has been published by
< Addison-Wesley (ISBN 0-201-84519-9).
---
==========
< This article is always being improved. Your input is welcomed. Send
---
your comments to (e-mail address removed) .
==========
< 1.1: How do you decide which integer type to use?
---
==========
< float and double. None of the above rules apply if the address
< of a variable is taken and must have a particular type.
---
==========
< 1.4: What should the 64-bit type on a machine that can support it?
---
==========
The .c file containing the definition should also #include the
< same header file, so that the compiler can check that the definition
---
matches the declarations.
==========
clear. The problem with the NODEPTR example is that the typedef
< has not been defined at the point where the "next" field is
---
declared.
==========
< 1.21: How do I declare an array of N pointers to functions returning
< pointers to functions returning pointers to characters?
---
==========
< A: The first part of this question can be answered in at least
< three ways:
---
==========
cdecl can also explain complicated declarations, help with
< casts, and indicate which set of parentheses the arguments
---
go in
==========
< Any good book on C should explain how to read these complicated C
---
declarations "inside out" to understand them ("declaration
mimics use").
==========
1.30: What am I allowed to assume about the initial values
< of variables which are not explicitly initialized?
---
==========
A: Uninitialized variables with "static" duration (that is, those
declared outside of functions, and those declared with the
< storage class static), are guaranteed to start out as zero,
---
as if the programmer had typed "= 0".
==========
A: Is the declaration of a static or non-local variable? Function
< calls are allowed only in initializers for automatic variables
---
==========
A: A string literal can be used in two slightly different ways. As
< an array initializer (as in the declaration of char a[]), it
---
that array.
==========
Anywhere else, it turns into an unnamed, static array of
characters, which may be stored in read-only memory,
< which is why you can't safely modify it.
---
==========
(For compiling old code, some compilers have a switch
< controlling whether strings are writable or not.)
---
==========
< When the name of a function appears in an expression like this,
---
it "decays" into a pointer (that is, it has its address
implicitly taken), much as an array name does.
==========
< 2.4: What's the best way of implementing opaque (abstract) data types
< in C?
---
==========
< A: No. There is no single, good way for a compiler to implement
< implicit structure comparison (i.e. to support the == operator
---
for structures) which is consistent with C's low-level flavor.
==========
A simple byte-by-byte comparison could founder on random bits
< present in unused "holes" in the structure (such padding is used
< to keep the alignment of later fields correct; see question 2.12).
---
==========
< Note also that if the structure contains any pointers, only
---
the pointer values will be written, and they are most unlikely
to be valid when read back in.
==========
Finally, note that for widespread portability you must use the
< "b" flag when fopening the files; see question 12.38.
---
==========
< A: Structures may have this padding (as well as internal padding),
< if necessary, to ensure that alignment properties will be
< preserved when an array of contiguous structures is allocated.
< Even when the structure is not part of an array, the end padding
---
remains, so that sizeof can always return a consistent size.
==========
< A: At the present time, there is little difference. The C Standard
---
says that enumerations may be freely intermixed with other
integral types, without errors.
==========
that they obey block scope. (A compiler may also generate
< nonfatal warnings when enumerations and integers are
< indiscriminately mixed, since doing so can still be considered
< bad style even though it is not strictly illegal.)
---
==========
(Loosely speaking, by "multiple, ambiguous side effects" we mean
< any combination of ++, --, =, +=, -=, etc. in a single expression
---
in a single expression which causes the same object either to be
modified twice or modified and then inspected.
==========
< Note that (long int)(a * b) would *not* have the desired effect.
---
==========
< or a delibrate but nonstandard extension if a particular
---
==========
< Whenever possible, you should choose appropriate pointer types
< in the first place, instead of trying to treat one type
---
as another.
==========
< void * acts as a generic pointer only because conversions are
---
are assigned to and from void *'s;
==========
< 4.12: I've seen different methods used for calling functions via
---
pointers. What's the story?
==========
pointers, and that "real" function names always decay implicitly
into pointers (in expressions, as they do in initializations;
< see question 1.34). This reasoning (which is in fact used in
< the ANSI standard) means that
---
==========
function pointer followed by an argument list except call the
< function pointed to.) An explicit * is still allowed.
---
==========
< 5.4: What is NULL and how is it #defined?
---
==========
< preprocessor macro NULL is #defined (by <stdio.h> and several
< other headers) with the value 0, possibly cast to (void *)
---
==========
< NULL should *only* be used for pointers; see question 5.9.
---
==========
< A: Not in general. The complication is that there are machines
---
are machines which use different internal representations for
pointers to different types of data.
==========
< pointer arguments of other types would still be problematical,
---
==========
This article uses the phrase "null pointer" (in lower case) for
< sense 1, the character "0" or the phrase "null pointer constant"
---
for sense 3, and the capitalized word "NULL" for sense 4.
==========
< A: C programmers traditionally like to know more than they might
< need to about the underlying machine implementation.
---
==========
integer zero instead of an error message, and if that uncast 0
< was supposed to be a null pointer constant, the code may not
< work.
---
==========
Some 64-bit Cray machines represent int * in the lower 48 bits
< of a word; char * additionally uses the upper 16 bits to
---
indicate a byte address within a word.
==========
A: This message, which typically occurs with MS-DOS compilers, means
< that you've written, via a null (perhaps because uninitialized)
< pointer, to an invalid location (probably offset 0 in the
< default data segment).
---
==========
< A: In one source file you defind an array of characters and in the
---
==========
< It is important to realize that a reference like x[3] generates
---
different code depending on whether x is an array or a pointer.
==========
< If you can't use C9X or gcc, you'll have to use malloc(), and
---
remember to call free() before the function returns.
==========
< Finally, in C9X you can use a variable-length array.
---
==========
A: The rule (see question 6.3) by which arrays decay into pointers
< is not applied recursively.
---
==========
not provide an automatically-managed string type. C compilers
< only allocate memory for objects explicitly mentioned in the
---
source code (in the case of strings, this includes character
==========
< A: You got lucky, I guess. The memory pointed to by the
< unitialized pointer p happened to be writable by you,
---
==========
A pointer value which has been freed is, strictly speaking,
< invalid, and *any* use of it, even if is not dereferenced,
---
can theoretically lead to trouble,
==========
A: In C, characters are represented by small integers corresponding
< to their values (in the machine's character set), so you
---
don't need a conversion function: if you have the character, you
have its value.
==========
< DEBUG("i = %d" _ i)
---
==========
controversial trigraph sequences). The ANSI C standard also
< formalizes the C run-time library support routines.
---
==========
< The text of the Rationale (not the full Standard) can be
< obtained by anonymous ftp from ftp.uu.net (see question 18.16)
< in directory doc/standards/ansi/X3.159-1989, and is also
< available on the web at http://www.lysator.liu.se/c/rat/title.html .
< The Rationale has also been printed by Silicon Press,
< ISBN 0-929306-07-4.
---
==========
< Public review drafts of C9X are available from ISO/IEC
---
==========
11.5: Why does the declaration
extern int f(struct x *p);
< give me an obscure warning message about "struct x introduced in
< prototype scope"?
---
==========
A: "const char *p" (which can also be written "char const *p")
declares a pointer to a constant character (you can't change
< the character); "char * const p" declares a constant pointer
---
pointer to a (variable) character (i.e. you can't change the
pointer).
==========
< A: The problem is older linkers which are under the control of
---
neither the ANSI/ISO Standard nor the C compiler developers
on the systems which have them.
==========
If you're interested in writing portable code, you can ignore
< the distinctions, as you'll want to avoid code that depends
---
depends on any of the three behaviors.
==========
A: The functions in <locale.h> begin to provide some support for
< these operations, but there is no standard routine for doing
---
either task.
==========
< If you're worried about using floating point, you could use
---
rand() / (RAND_MAX / N + 1)
==========
< A: In general, a header file contains only declarations.
---
==========
how carefully your compiler's binary/decimal conversion routines
(such as those used by printf) have been written, you may see
< discrepancies when numbers (especially low-precision floats) not
< exactly representable in base 2 are assigned or read in and then
---
are assigned or read in and then printed (i.e. converted from
base 10 to base 2 and back again).
==========
< Another possibility is to to format the value in question using
---
sprintf(): on many systems it generates strings like "NaN"
==========
< A: Some compilers for small machines, including Borland's
---
Ritchie's original PDP-11 compiler), leave out certain floating
==========
< char *vstrcat(char *first, ...)
---
==========
< void error(char *fmt, ...)
---
==========
< examples in questions 5.2 and 15.4). Finally, if their types
---
==========
< local arrays. Many systems have fixed-size stacks, and
---
those which perform dynamic stack allocation automatically
(e.g. Unix) can be confused when the stack tries to grow by a
huge chunk all at once.
==========
< 16.8: What do "Segmentation violation" and "Bus error" mean?
---
==========
< Finally, the author of this FAQ list teaches a C class
< and has placed its notes on the web; they are at
---
==========
< 18.10: What's a good book for learning C?
---
==========
< The GNU libplot package maintains the same spirit and supports
< many modern plot devices;
< see http://www.gnu.org/software/plotutils/plotutils.html .
---
==========
A: If the "size of a file" is the number of characters you'll be
< able to read from it in C, it is difficult or impossible to
---
determine this number exactly.
==========
< readdir() only returns file names; if you need more
---
information about the file, try calling stat().
==========
< (Also, remember to call pclose().)
---
==========
busy-wait, but this is only an option on a single-user, single-
< tasking machine as it is terribly antisocial to any other
---
processes. Under a multitasking operating system, be sure to
==========
It is possible, and desirable, for *most* of a program to be
ANSI-compatible, deferring the system-dependent functionality to
< a few routines in a few files which are rewritten for each
< system ported to.
---
==========
or have the function return a structure containing the
< desired values, or (in a pinch) consider global variables.
---
global variables.
==========
is not only clearer to the human reader, it is more likely to be
recognized by the compiler and turned into the most-efficient
< code (e.g. using a swap instruction, if available).
---
==========
< More information may be found in FORT.gz by Glenn Geers, available
< via anonymous ftp from suphys.physics.su.oz.au in the src
< directory.
<
< cfortran.h, a C header file, simplifies C/FORTRAN interfacing on
< many popular machines. It is available via anonymous ftp from
< zebra.desy.de or at http://www-zeus.desy.de/~burow .
---
==========
This FAQ list's maintainer also has available a list of a few
< other commercial translation products, and some for more obscure
< languages.
---
==========
< A: The contest is in a state of flux; see
---
====================
Finally, here are a few questions which don't really need to be in
the posted-to-Usenet list every month. (As mentioned, though, they'll
live on in the web-based version.)
==========
< 1.22: How can I declare a function that can return a pointer to a
< function of the same type? I'm building a state machine with
< one function for each state, each of which returns a pointer to
< the function for the next state. But I can't find a way to
< declare the functions.
<
< A: You can't quite do it directly. Either have the function return
< a generic function pointer, with some judicious casts to adjust
< the types as the pointers are passed around; or have it return a
< structure containing only a pointer to a function returning that
< structure.
==========
< 2.7: I heard that structures could be assigned to variables and
< passed to and from functions, but K&R1 says not.
<
< A: What K&R1 said (though this was quite some time ago by now) was
< that the restrictions on structure operations would be lifted
< in a forthcoming version of the compiler, and in fact structure
< assignment and passing were fully functional in Ritchie's
< compiler even as K&R1 was being published. A few ancient C
< compilers may have lacked these operations, but all modern
< compilers support them, and they are part of the ANSI C
< standard, so there should be no reluctance to use them.
<
< (Note that when a structure is assigned, passed, or returned,
< the copying is done monolithically; the data pointed to by any
< pointer fields is *not* copied.)
==========
< 13.14b: Does C have any Year 2000 problems?
<
< A: No, although poorly-written C programs do.
<
< The tm_year field of struct tm holds the value of the year minus
< 1900; this field will therefore contain the value 100 for the
< year 2000. Code that uses tm_year correctly (by adding or
< subtracting 1900 when converting to or from human-readable
< 4-digit year representations) will have no problems at the turn
< of the millennium. Any code that uses tm_year incorrectly,
< however, such as by using it directly as a human-readable
< 2-digit year, or setting it from a 4-digit year with code like
<
< tm.tm_year = yyyy % 100; /* WRONG */
<
< or printing it as an allegedly human-readable 4-digit year with
< code like
<
< printf("19%d", tm.tm_year); /* WRONG */
<
< will have grave y2k problems indeed. See also question 20.32.
==========
< 13.24: I'm trying to port this A: Those functions are variously
< old program. Why do I obsolete; you should
< get "undefined external" instead:
< errors for:
<
< index? use strchr.
< rindex? use strrchr.
< bcopy? use memmove, after
< interchanging the first and
< second arguments (see also
< question 11.25).
< bcmp? use memcmp.
< bzero? use memset, with a second
< argument of 0.
==========
< 15.7: I have a pre-ANSI compiler, without <stdarg.h>. What can I do?
<
< A: There's an older header, <varargs.h>, which offers about the
< same functionality.
==========
< 18.5: How can I shut off the "warning: possible pointer alignment
< problem" message which lint gives me for each call to malloc()?
<
< A: The problem is that traditional versions of lint do not know,
< and cannot be told, that malloc() "returns a pointer to space
< suitably aligned for storage of any type of object." It is
< possible to provide a pseudoimplementation of malloc(), using a
< #define inside of #ifdef lint, which effectively shuts this
< warning off, but a simpleminded definition will also suppress
< meaningful messages about truly incorrect invocations. It may
< be easier simply to ignore the message, perhaps in an automated
< way with grep -v. (But don't get in the habit of ignoring too
< many lint messages, otherwise one day you'll overlook a
< significant one.)
====================
Steve Summit
(e-mail address removed)