About -lm switch used for linking math.h

[Avinash Sonawane]

Sir,
I am using gcc 4.6.1.
I have few questions regarding -lm switch. Please kindly guide me for the same.

1) Why I need to use -lm flag for math.h even if I had declared the
#include <math.h> in the code?
2) If it's all about linking the library then why I don't have to do
the same thing with stdio.h too?
3) Or we have to add the -l'library' switch for every library excluding stdio.h?
4) In case if the -l'library' switch is used for some specific
libraries only then please direct me to the complete list of such
libraries.

Thanks.

 

[Keith Thompson]

I am using gcc 4.6.1.
I have few questions regarding -lm switch. Please kindly guide me for the same.

1) Why I need to use -lm flag for math.h even if I had declared the
#include <math.h> in the code?

"include <math.h>" tells the compiler to, in effect, include the
contents of "math.h" (declarations) in your source file. "-lm" tells
the linker to use the math library, which contains the *implementations*
of the functions declared in math.h.

the same thing with stdio.h too?

No, because the functions declared in stdio.h are defined in a library
that's linked by default.

3) Or we have to add the -l'library' switch for every library excluding stdio.h?

You need "-l" for *some* libraries. For gcc, most of the functions
defined in standard header are in the standard library, which is linked
by default (<math.h> and "-lm" are probably the one exception).
Headers that aren't defined by the C standard may or may not require an
explicit "-l" option.

4) In case if the -l'library' switch is used for some specific
libraries only then please direct me to the complete list of such
libraries.

There is no such complete list. The libraries that are installed will
vary tremendously from one system to another. You'll just have to read
the documentation for whatever function you want to use. For example,
if you're on a Unix-like system, "man sqrt" will show you the
declaration of the sqrt() function and tell you that you need
"#include <math.h>" and "-lm".

The need to use "-lm" is widely criticized, and there are
implementations that don't require it (that put the math library into
the standard library).

 

[James Kuyper]

Sir,
I am using gcc 4.6.1.
I have few questions regarding -lm switch. Please kindly guide me for the same.

1) Why I need to use -lm flag for math.h even if I had declared the
#include <math.h> in the code?

#include <math.h> tells the compiler which library header to insert into
your program when compiling. -lm tells the linker which library to
search when linking your compiled program. A compiler could connect the
two, but most of the ones I'm familiar with don't.

2) If it's all about linking the library then why I don't have to do
the same thing with stdio.h too?

By convention dating back to the early days of C, most of the C standard
library is often stored in one library file, except for the part
described by <math.h>, which is stored in a different one. This approach
became conventional because, especially at that time, most programs made
no use of the math library, and adding all of those functions to the
linker's search would significantly slowed down linkage. This is far
less of an issue now than it used to be, but some habits, once started,
are hard to change.

3) Or we have to add the -l'library' switch for every library excluding stdio.h?
No.

4) In case if the -l'library' switch is used for some specific
libraries only then please direct me to the complete list of such
libraries.

Any such list could, in principle, be different for different compilers,
and there's no way it could be complete - there's millions of libraries
out there. The best you can hope for is a list of C standard library
parts. However, for most systems I've familiar with, <math.h> is the
only part of the C standard library that has it's own separate library file.

 

[jacob navia]

Le 26/01/12 22:59, Avinash Sonawane a écrit :

Sir,
I am using gcc 4.6.1.
I have few questions regarding -lm switch. Please kindly guide me for the same.

1) Why I need to use -lm flag for math.h even if I had declared the
#include<math.h> in the code?

Because you do not have a Macintosh. If you would have one, you
wouldn't need that.

2) If it's all about linking the library then why I don't have to do
the same thing with stdio.h too?

Because of hysterical reasons.

3) Or we have to add the -l'library' switch for every library excluding stdio.h?

Yes. For each library you should add the corresponding name to the link
command.

4) In case if the -l'library' switch is used for some specific
libraries only then please direct me to the complete list of such
libraries.

The -l'library' switch is needed for all the libraries not included
by default. Most linkers include some libraries by default, at least
the C library (that is why you do not need to include the stdio
library).

In more advanced systems, there is no distinction between the stdio
library and the mathematical library: they are all part of the C
library and included by default. Those systems do NOT need any -lm
switch. There are other systems that provide many more libraries
included by default.

There are some linkers/IDE combinations that have a small database
of which functions are included in which library so that it isn't
needed to remember the name of the library, the IDE just consults
its database and figures out the correct switch for the linker.

 

[Eric Sosman]

#include<math.h> tells the compiler which library header to insert into
your program when compiling. -lm tells the linker which library to
search when linking your compiled program. A compiler could connect the
two, but most of the ones I'm familiar with don't.


By convention dating back to the early days of C, most of the C standard
library is often stored in one library file, except for the part
described by<math.h>, which is stored in a different one. This approach
became conventional because, especially at that time, most programs made
no use of the math library, and adding all of those functions to the
linker's search would significantly slowed down linkage. This is far
less of an issue now than it used to be, but some habits, once started,
are hard to change.

In at least one C implementation, the math functions have in
fact been combined with the rest of the library and all are linked
automatically with no need for -lm. This implementation also provides
a completely empty "libm" library, just to accommodate Makefiles and
so on that use -lm anyhow!

 

[Kaz Kylheku]

By convention dating back to the early days of C, most of the C standard
library is often stored in one library file, except for the part
described by <math.h>, which is stored in a different one. This approach
became conventional because, especially at that time, most programs made
no use of the math library, and adding all of those functions to the
linker's search would significantly slowed down linkage. This is far
less of an issue now than it used to be, but some habits, once started,
are hard to change.

I suspect there is one more reason and it is related to the reason why there
are separate /bin and /usr/bin directories as well as /lib and /usr/lib.

Disks used to be small, and so programs had to be separated into essential
(for booting the system) and nonessential.

If programs linked the math library by default, then the math library would
have to have been placed in the root file system (so that the utilities which
run early in the boot process could execute).

(The issue of what executable material to include in a minimal filesystem for
booting is still relevant.)

 

[Joe Pfeiffer]

Sir,
I am using gcc 4.6.1.
I have few questions regarding -lm switch. Please kindly guide me for the same.

1) Why I need to use -lm flag for math.h even if I had declared the
#include <math.h> in the code?

There are two distinct phases here: #include <math.h> tells the
compiler what the match functions look like, -lm actually links in the
code for them.

2) If it's all about linking the library then why I don't have to do
the same thing with stdio.h too?

Because that's *so* standard it's just assumed. More specifically,
there is a "standard C library" which is automatically linked with your
code, and all of the functions from <stdio.h> are in that library.

The better question would be why the C standard library function
prototypes aren't all automatically available, without a bunch of
#include's. To me it seems like they ought to be, but the people
writing the standards and the people writing the compilers don't agree
with me, so they aren't.

3) Or we have to add the -l'library' switch for every library
excluding stdio.h?

You've got a little bit of confusion going on here, which I've alluded
to above but I'll say more specifically here: a .h file is not a
library. A .h file contains function prototypes, which tell the
compiler how to generate code to make calls to the functions. That has
nothing at all to do with the code that's in the actual library --
that's what the -l flag pulls in.

OK, when I say it has "nothing to do with" that's overstating things a
bit: assuming no bugs, the .h file is designed to make sure calls to
the library are actually correct. But aside from that intention, they
are unrelated.

4) In case if the -l'library' switch is used for some specific
libraries only then please direct me to the complete list of such
libraries.

There is no complete list. Any particular system will have its own set
of libraries, depending on what's been installed on it. If you want,
you can create a library of your own that exists only on your own
system, and which won't be in any list.

If you're on a Unix-based system (including Linux and BSD), the man
page for a function will tell you both what .h file you need to #include
to compile it, and what library you need to use to link it. So, for
instance, the man page for the sin function says:

#include <math.h>

double sin(double x);
float sinf(float x);
long double sinl(long double x);

Link with -lm.

So, to be able to use sin(), sinf(), or sinl(), you need to
#include <math.h> and link with -lm

 

[jacob navia]

Le 27/01/12 07:09, Joe Pfeiffer a écrit :

The better question would be why the C standard library function
prototypes aren't all automatically available, without a bunch of
#include's. To me it seems like they ought to be, but the people
writing the standards and the people writing the compilers don't agree
with me, so they aren't.

The lcc-win compiler provides the "stdheaders.h" include file
that includes all the standard headers sparing you the need to figure
out what function is in what header.

I proposed that in comp.std.c years ago, but it was ignored in the
new standard.

 

[Kenny McCormack]

If you're on a Unix-based system (including Linux and BSD), the man
page for a function will tell you both what .h file you need to #include
to compile it, and what library you need to use to link it. So, for
instance, the man page for the sin function says:

This isn't necessarily true (and, in true CLC fashion, I'm going to go to
great pains to explain why your statement isn't true in absolute
generality). All versions of MS/PC DOS/Windows, since DOS 2.0, are
Unix-based, yet, when I type "man {anything}" on my Windows Server 2008 R2
Home Premium Super Deluxe (*), all I get is some dumb error message.

(*) With wheels and a sandwich...

--
Windows 95 n. (Win-doze): A 32 bit extension to a 16 bit user interface for
an 8 bit operating system based on a 4 bit architecture from a 2 bit company
that can't stand 1 bit of competition.

Modern day upgrade --> Windows XP Professional x64: Windows is now a 64 bit
tweak of a 32 bit extension to a 16 bit user interface for an 8 bit
operating system based on a 4 bit architecture from a 2 bit company that
can't stand 1 bit of competition.

 

[Kleuske]

This isn't necessarily true (and, in true CLC fashion, I'm going to go
to great pains to explain why your statement isn't true in absolute
generality). All versions of MS/PC DOS/Windows, since DOS 2.0, are
Unix-based, yet, when I type "man {anything}" on my Windows Server 2008
R2 Home Premium Super Deluxe (*), all I get is some dumb error message.

(*) With wheels and a sandwich...

In what universe is MS-DOS "unix-based?".

 

[Wolfgang.Draxinger]

The better question would be why the C standard library function
prototypes aren't all automatically available, without a bunch of
#include's. To me it seems like they ought to be, but the people
writing the standards and the people writing the compilers don't agree
with me, so they aren't.

Because you may be able to compile and link without the C standard
library functions being available. Like when boostrapping the standard
library or implementing a operating system kernel, where the standard
library functions simply won't work. All compilers/linkers offer a
option, not to link against the standard library.


Wolfgang

 

[BartC]

I suppose it's simple enough to bundle all the standard headers into one big
header, and to just include that.

That's saves having the ridiculous situation of having to keep deleting or
adding include files, everything time you edit some code and the number of
references to something in a standard header, falls below 1, or rises above
zero. How do you keep track of something like that anyway? So you just keep
a whole bunch of headers in, whether they are essential or not.

The only problem is that an all-inclusive header file is not standardised
(or is it?) making it more difficult to share code.

Because you may be able to compile and link without the C standard
library functions being available. Like when boostrapping the standard
library or implementing a operating system kernel, where the standard
library functions simply won't work.

Those are special cases, ordinary programmers shouldn't be burdened with
maintaining lists of include files just so 1% of all programmers have the
option of leaving them out!

Actually it would be simplest for the compiler to just include everything
necessary (to be able to write C code), and for a compiler switch to say
headers are included manually.

All compilers/linkers offer a
option, not to link against the standard library.

So why not a switch to *not* include the maths library, rather than the
other way around? Is the maths library actually that big? And would it be
statically linked in?

 

[Ben Bacarisse]

I suppose it's simple enough to bundle all the standard headers into one big
header, and to just include that.

The only problem is that an all-inclusive header file is not standardised
(or is it?) making it more difficult to share code.

But since the list of standard headers is standardised, you could
distribute a .h file that includes them all along with the code you want
to share.

Those are special cases, ordinary programmers shouldn't be burdened
with maintaining lists of include files just so 1% of all programmers
have the option of leaving them out!

Actually it would be simplest for the compiler to just include everything
necessary (to be able to write C code), and for a compiler switch to say
headers are included manually.

Some environments let you make such a compiler in a few minutes. gcc
has a -include command-line option (and you can read options from a file
with @file) so an alias like "gcc -std=c99 -pedantic @std-includes" does
what you want.

<snip>

 

[Kenny McCormack]

In what universe is MS-DOS "unix-based?".

This one. Re-read what I wrote. It is well understood that as of
version 2 of DOS - i.e., DOS 2.0 - DOS was based on Unix - the hierarchical
file system, the general compilation model, etc. If necessary, re-read your
history books. And, of course, all versions of DOS/Windows since then share
the legacy.

Note: Obviously, I'm playing games with the word "based" - as in
"Unix-based". The previous poster was trying to be politically correct by
avoiding saying that Linux is Unix - weaseling out by saying that Linux is
merely "Unix-based". Well, the point is that that wording allows in
DOS/Windows, which is clearly not Unix, but it is equally clearly
"Unix-based".

And that's the way of CLC...

--
But the Bush apologists hope that you won't remember all that. And they
also have a theory, which I've been hearing more and more - namely,
that President Obama, though not yet in office or even elected, caused the
2008 slump. You see, people were worried in advance about his future
policies, and that's what caused the economy to tank. Seriously.

(Paul Krugman - Addicted to Bush)

 

[tom st denis]

This one.  Re-read what I wrote.  It is well understood that as of
version 2 of DOS - i.e., DOS 2.0 - DOS was based on Unix - the hierarchical
file system, the general compilation model, etc.  If necessary, re-readyour
history books.  And, of course, all versions of DOS/Windows since then share
the legacy.

Actually DOS is CP/M based ... which wasn't UNIX based.

Note: Obviously, I'm playing games with the word "based" - as in
"Unix-based".  The previous poster was trying to be politically correctby
avoiding saying that Linux is Unix - weaseling out by saying that Linux is
merely "Unix-based".  Well, the point is that that wording allows in
DOS/Windows, which is clearly not Unix, but it is equally clearly
"Unix-based".

Linux isn't UNIX based either, it's based off of MINIX which is a
variant OS that is similar to UNIX.

So not only are your posts tiresome and boring but they're wholesale
factually incorrect even when you are trying to be a smarty pants.

Tom

 

[Kenny McCormack]

So not only are your posts tiresome and boring but they're wholesale
factually incorrect even when you are trying to be a smarty pants.

It all depends on what the meaning of the word "based" is.

--

Some of the more common characteristics of Asperger syndrome include:

* Inability to think in abstract ways (eg: puns, jokes, sarcasm, etc)
* Difficulties in empathising with others
* Problems with understanding another person's point of view
* Hampered conversational ability
* Problems with controlling feelings such as anger, depression
and anxiety
* Adherence to routines and schedules, and stress if expected routine
is disrupted
* Inability to manage appropriate social conduct
* Delayed understanding of sexual codes of conduct
* A narrow field of interests. For example a person with Asperger
syndrome may focus on learning all there is to know about
baseball statistics, politics or television shows.
* Anger and aggression when things do not happen as they want
* Sensitivity to criticism
* Eccentricity
* Behaviour varies from mildly unusual to quite aggressive
and difficult

 

[Joe Pfeiffer]

This one. Re-read what I wrote. It is well understood that as of
version 2 of DOS - i.e., DOS 2.0 - DOS was based on Unix - the hierarchical
file system, the general compilation model, etc. If necessary, re-read your
history books. And, of course, all versions of DOS/Windows since then share
the legacy.

Note: Obviously, I'm playing games with the word "based" - as in
"Unix-based". The previous poster was trying to be politically correct by
avoiding saying that Linux is Unix - weaseling out by saying that Linux is
merely "Unix-based". Well, the point is that that wording allows in
DOS/Windows, which is clearly not Unix, but it is equally clearly
"Unix-based".

And that's the way of CLC...

You're using a definition of "Unix-based" that is completely foreign to
just about everybody.

 

[Rui Maciel]

All versions of MS/PC DOS/Windows, since DOS 2.0, are
Unix-based

Do you actually have any basis for that assertion?


Rui Maciel

 

[Stephen Sprunk]

1) Why I need to use -lm flag for math.h even if I had declared the
#include <math.h> in the code?

You are confusing libraries and headers. A library, eg. "libm",
contains the code for the function you want to call; A header file, eg.
"math.h", contains the declaration of the function you want to call.

You include a header file so the compiler knows about the function you
want to call. However, you must separately tell the linker to include
the library with the function itself; the compiler won't tell the linker
that for you.

(In programs with a single source file, you may not see the distinction
between compiler and linker, but it's there--and understanding this is
very important when you have multiple source files.)

2) If it's all about linking the library then why I don't have to do
the same thing with stdio.h too?

Most linkers are set up so that they automatically link your code
against one or more libraries by default, eg. "libc". However, you
still need to tell the compiler about the functions you want to call by
including the relevant header file, eg. "stdio.h".

3) Or we have to add the -l'library' switch for every library excluding
stdio.h?

For every library that your linker doesn't include by default. Check
your linker's documentation.

4) In case if the -l'library' switch is used for some specific
libraries only then please direct me to the complete list of such
libraries.

There is no complete list of libraries, nor could there be since new
ones are created every day. However, the documentation for the library
functions you use should specify what, if any, library you should link to.

S

 

[Kenny McCormack]

Do you actually have any basis for that assertion?


Rui Maciel

I do not mean "based" in the sense of "code based" - i.e., that they took
Unix code to build DOS. Only a moronic pedant would think that - which is
why we are seeing so much of it in this thread. See: "straw man".

I mean "based" in the sense of "ideas borrowed from". And I mean borrowed
rather heavily - not just a smidgen here and there. As I said, read your
history. DOS 1 was, as Tommy notes, basically a quick hack-up of CP/M, but
DOS 2 introduced a lot of Unixy stuff and made DOS quite Unix-like. And the
whole point is we are all better off because of it. It made DOS (and its
later progeny) much better systems.

Anyway, I've made my point. I'm done.

--
Windows 95 n. (Win-doze): A 32 bit extension to a 16 bit user interface for
an 8 bit operating system based on a 4 bit architecture from a 2 bit company
that can't stand 1 bit of competition.

Modern day upgrade --> Windows XP Professional x64: Windows is now a 64 bit
tweak of a 32 bit extension to a 16 bit user interface for an 8 bit
operating system based on a 4 bit architecture from a 2 bit company that
can't stand 1 bit of competition.