Stopping a while loop with user input ?

Discussion in 'C Programming' started by ern, Oct 12, 2005.

  1. ern

    ern Guest

    I have a program that runs scripts. If the user types "script
    myScript.dat" the program will grab commands from the text file, verify
    correctness, and begin executing the script UNTIL...

    I need a way to stop the execution with user input. I was thinking
    something like this:

    while(user hasn't pressed 'any key'){

    How can I do the "user hasn't pressed 'any key' " of this logic?
    Running Linux Red Hat V3.

    ern, Oct 12, 2005
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  2. ern

    Michael Mair Guest

    This cannot done in standard C; try comp.unix.programmer

    Michael Mair, Oct 12, 2005
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  3. ern

    Default User Guest

    ern wrote:

    Default User, Oct 12, 2005
  4. If you're allowed to choose which key the user uses to interrupt the
    program, there's a good chance you can do this without going beyond what
    the standard defines.
    (This may or may not be something you'd want to do instead of using
    something system-specific instead.)

    Most implementations have a way to cause SIGINT to be raised in a running
    program, so you can just install a handler that sets a flag indicating
    it's time to stop, and check that in your main loop.
    (Note that the way you arrange for SIGINT to be raised asynchronously
    is entirely dependent on your system and there may not be a way at all.
    (All of the systems that used nontrivially use ctrl-C by default, but
    check your documentation.) So you're not actually gaining portability
    to systems that don't support user-caused SIGINT, you're just gaining
    not having to change the code to move between systems that do.)

    #include <signal.h>

    volatile sig_atomic_t time_to_stop=0;

    void sigint_handler(int ignored)
    /*Revert to the default signal handler (usually immediate
    termination) for if the user gets impatient and interrupts

    void do_main_loop(some_args args)
    void (*old_sighandler)(int);

    /*Assume it was ignored for a good reason, re-ignore*/


    /*Depending on what comes next, we may want to restore the original

    Note that the things you can do in a handler for an asynchronous signal
    are seriously limited. Storing a value in an object of type volatile
    sig_atomic_t is guaranteed to do what you'd expect (store the value
    completely and correctly and become available the next time it's checked
    outside the signal handler), and using signal() to install a new handler
    for the signal you're handling is also well-defined. Calling most library
    functions isn't safe, examining and/or modifying most objects isn't safe,
    and there's really not a whole lot that you can do without trying to do
    one or the other of those.

    Dave Vandervies, Oct 20, 2005
  5. ern

    Mabden Guest

    getch() might work.
    Mabden, Oct 26, 2005
  6. ern

    Jordan Abel Guest

    however, you need curses for that. The only "portable" way to do
    this would be to use SIGINT and limit the user's ability to stop
    the "script" to causing SIGINT (in an implementation-defined way:
    on any unix system this is controlled by the tty driver and is
    caused by default by hitting ctrl-c - it's not unreasonable to
    use ctrl-c and/or ctrl-break on windows/dos, etc.)

    your signal handler could set a global variable that is then
    checked in the while loop.

    it's also a lot easier than using curses, and a lot more in
    keeping with how other unix-ish applications do things.
    Jordan Abel, Oct 26, 2005
  7. ern

    Jordan Abel Guest

    there is a getch() that exists on DOS and one that exists on unix.
    they do not have the same semantics. K&R uses the name, but it is
    for neither of the two commonly-existing functions of that name, but
    rather of its own example of how one might write a getc-ish function
    that could play nice with an ungetc-ish one. Moreover, the getch in
    K&R, the one in unix/curses under some circumstances, and the dos
    one, will ALL block until the user actually presses a key - none of
    them [except the curses one under SOME sets of options] will return
    immediately if there is no key pressed by the user.
    he _can_ do it, using the curses getch()
    the problem is that getch() will not return if there is _not_ a
    character. [well, you can do it with curses, but it's tricky]
    Jordan Abel, Oct 26, 2005
  8. ern

    Mabden Guest

    Curses? Unixish? what's that? My 'puter speaks DOS. My compiler knows
    getch() and so does my bible, the K&R.

    Can't he read a line of script and then check if a key was hit? I mean,
    the one line of the script may take a little while to finish, but at
    least the whole script doesn't have to finish before the user gets

    SIGINT is fine, but seems overkill for processing a script, since you
    can just poll after every line. I use SIGINT for trapping Ctrl-C to stop
    something lengthy like parsing a directory tree, but a script file of
    presumably short-lived commands could be halted by just polling the
    keyboard buffer with getch(). Unless the script was something like:

    1 cure cancer
    2 cure Michael Jackson
    3 fix Madben
    Mabden, Oct 26, 2005
  9. ern

    Mabden Guest

    Oops.. so That's why I use SIGINT... I looked back at some DOS programs
    and there were more of them than I remembered. But they all seemed to be
    for Ctrl-C handling.

    Did the OP say they wanted to trap "The Any Key" or a specific key? Does
    SIGINT flag, say a space bar? I forget, since I used it to exit
    gracefully from a program when ctrl-c or ctrl-break was hit. What about
    the old ctrl-d ctrl-s combo for pausing and restarting a program, maybe
    that might work for the OP.

    I guess I need to read the first post. ;-)
    Mabden, Oct 26, 2005
  10. ern

    Joe Wright Guest

    Just when I thought you were learning.. getch() simply does not exist in
    Standard C, the topic of this newsgroup. Various implementations may
    provide it (an others) as extensions but it isn't C.
    Joe Wright, Oct 26, 2005
  11. ern

    Mabden Guest

    Awww.. how sweet, you thought...

    getch() is on page 78-79 of the only C standard that matters, K&R2.

    I've never compiled a program using getch() and had it fail. But as I
    said, I was surprized to find that I actually used SIGINT more than I
    thought, and getch() less than you think.

    So, while you are not wrong - you're wrong! ;-)
    Mabden, Oct 27, 2005
  12. ern

    Jordan Abel Guest

    You are incorrect. K&R2 does NOT claim getch() exists or a standard
    function. It uses the name in an example, which is in fact an
    implicit claim that it is NOT a standard function
    Jordan Abel, Oct 27, 2005
  13. ern

    Flash Gordon Guest

    Where it provides an implementation of getch which has different
    semantics to the implementation I have on my Linux box and different
    semantics to the implementation provided my MS in their C library. If
    you look at Appendix B which describes the standard library you will
    find absolutely NO reference to getch.

    Also, it is the ISO standard that defines C and therefore what is
    topical here, K&R2 is merely a very good, but not perfect, book about
    the language.
    Which proves nothing apart from your experience being limited.
    No, Joe is COMPLETELY correct and you are COMPLETELY wrong.

    Since you are obviously not improving I'll put you back in the kill file.
    Flash Gordon, Oct 27, 2005
  14. ern

    Mabden Guest

    Hey! I just got to stretch my legs! Haven't you ever been wrong!!
    Mabden, Oct 27, 2005
  15. ern

    pete Guest

    You are correct.
    You would think so, but there is a counterexample in K&R.

    119-121(§5.11): The qsort discussion needs recasting in several ways.
    First, qsort is a standard routine in ANSI/ISO C, so
    the rendition here should be given a different name, especially because
    the arguments to standard qsort are a bit different: the
    standard accepts a base pointer and a count, while this example uses a
    base pointer and two offsets.
    pete, Oct 28, 2005
  16. But using the name "qsort" in an example *is* an implicit claim that
    it's not a standard function. It happens to be an incorrect claim,
    which is why it's mentioned in the errata.
    Keith Thompson, Oct 28, 2005
  17. ern

    Mabden Guest

    So none of the programs named in K&R (2 in my version) are actually in
    The Standard? Is that canonical? Have you checked this yourself, or is
    it hearsay?
    Mabden, Oct 28, 2005
  18. ern

    Jordan Abel Guest

    No program which contains a definition of a standard function is
    permitted on a conforming hosted implementation. With the sole (i
    believe) exceptions of the qsort function and the unix-specific
    examples in chapter 8, all the examples in K&R conform to the
    Jordan Abel, Oct 28, 2005
  19. I believe thats incorrect. My understanding is that the names are only
    reserved if you include the header.

    7.1.3 Reserved Identifiers
    - Each identifier with file scope listed in any of the following
    subclauses (including the future library directions) is reserved for
    use as a macro name and as an identifier with file scope in the same
    name space if any of its associated headers is included.
    Mark McIntyre, Oct 28, 2005
  20. ern

    Flash Gordon Guest

    In n1124, section 7.13, it also says:
    — All identifiers with external linkage in any of the following
    subclauses (including the future library directions) are always
    reserved for use as identifiers with external linkage.157)

    So you can never use the names for functions unless you don't include
    the relevant header *and* you declare the function as static.
    Flash Gordon, Oct 28, 2005
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