provide input to another program

This is a discussion on provide input to another program within the C Programming forums, part of the General Programming Boards category; hello- i have program that accepts input via getchar() and performs a task after receiving that input. i know i ...

  1. #1
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    provide input to another program

    hello-

    i have program that accepts input via getchar() and performs a task after receiving that input. i know i can redirect stdin like so: a.out < inputfile to give input, but i am looking for a different way.

    is there a way i can write a separate program to provide input for the first program? i know how to execute the first program, just not give it input, it doesn't take command line args.

    thanks for any help.

  2. #2
    Registered User hk_mp5kpdw's Avatar
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    Is this linux or windows? Use a pipe. The second program can open a pipe to write to the first program.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
    -Christopher Hitchens

  3. #3
    and the hat of int overfl Salem's Avatar
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    a.out < inputfile
    cat inputfile | a.out
    Both do much the same.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  4. #4
    pdc
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    Use popen(), defined in stdio.h. For example, for cross platform sockets :
    Code:
    #include <stdio.h>
    #define BUFSIZE 100
    
    int main(void) {
      char buf[BUFSIZE];
    
      FILE * f = popen("telnet example.com 80", "r+");
      if(f) {
        fprintf(f, "GET / HTTP/1.0\r\n\r\n");
        fread(buf, sizeof(char), BUFSIZE, f);
        buf[BUFSIZE-1] = '\0';
    
        printf("Result: '%s'\n", buf);
    
        pclose(f);
        return 0;
      }
      return -1;
    }
    The above will print the telnet header and the start of the response from example.com to a HTTP request. (Warning, it's untested, I'm behind a proxy at the moment.)
    Last edited by pdc; 07-29-2005 at 09:21 AM.

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    standard pipes are one-way.
    you cannot read and write to the pipe at the same time.
    If you have opened the pipe as "r", you cannot write to it.

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    thanks for the help guys, i got it to work with pipes.

    it worked for me to do write-only and read the output on the screen.

    just a quick question, if pipes are one-way, can you set up two pipes for two-way? and how would you make sure you are communicating with the same process?

  7. #7
    pdc
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    AFAIK: popen() can create bidirectional pipes on OS X and BSD, but only unidirectional on Linux and Solaris.
    Last edited by pdc; 07-29-2005 at 11:27 AM.

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    on unix, bidirectional streams could be created with
    socketpair() and fdopen().

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    I'm new here, but I wonder if this will work on windows?

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    Should do if it's in the standard library.

  11. #11
    pdc
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    popen() is standard, I don't think socketpair() is available on Windows as it is on UNIX.

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    Frequently Quite Prolix dwks's Avatar
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    popen() is probably part of the POSIX standard, which is what Linux uses, but Windows generally uses the ANSI standard.

    [edit]url[/edit]

    [edit2]
    This function is NOT included in 'C Programming Language' (ANSI) but can be found in 'The Standard C Library' book.
    Taken from here.
    [/edit2]
    Last edited by dwks; 07-30-2005 at 01:08 PM.
    dwk

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  13. #13
    pdc
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    Sorry, yeah, Windows calls it _popen(). It has much the same behaviour, except read-write mode is "rw" and not "r+".

  14. #14
    ATH0 quzah's Avatar
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    Quote Originally Posted by dwks
    popen() is probably part of the POSIX standard, which is what Linux uses, but Windows generally uses the ANSI standard.
    There is no ANSI C standard method for opening pipes. Just like there's no standard method for reading a keystroke.


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  15. #15
    Frequently Quite Prolix dwks's Avatar
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    That's what my edit said.
    dwk

    Seek and ye shall find. quaere et invenies.

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    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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