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Full Discussion: Pipes in C
Top Forums Programming Pipes in C Post 302392587 by petrca on Thursday 4th of February 2010 11:53:16 PM
Old 02-05-2010
Pipes in C
Hello all, I am trying to learn more about programming Unix pipes in C.
I have created a pipe that does od -bc < myfile | head
Now, I am trying to create od -bc < myfile | head | wc
Here is my code, and I know I might be off, thats why I am here so I can get some clarification.
Code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/file.h>

int main(int argc, char *argv[]) {

   int i, filefd, pipefd[2], od_pid, head_pid, pid, wc_pid;

   for (i = 1; i < argc; i++) {
      filefd = open(argv[i], O_RDONLY);
      if (filefd < 0){
         printf("%s could not be opened\n", argv[i]);
         continue;
      }

      dup2(filefd, STDIN_FILENO);
      close(filefd);
      pipe(pipefd);

      if ((od_pid = fork()) == 0) {
         dup2(pipefd[1], STDOUT_FILENO);
         close(pipefd[0]);
         close(pipefd[1]);
         execl("/usr/bin/od", "od", "-bc", NULL);
         exit(EXIT_FAILURE);
      }

      if ((head_pid = fork()) == 0) {
         dup2(pipefd[1], STDOUT_FILENO);
         close(pipefd[0]);
         close(pipefd[1]);
         execl("/usr/bin/head","head", NULL);
         exit(EXIT_FAILURE);
      }

     if ((wc_pid = fork()) == 0) {
        dup2(pipefd[0], STDIN_FILENO);
        close(pipefd[0]);
        close(pipefd[1]);
        execl("/usr/bin/wc","wc", NULL);
        exit(EXIT_FAILURE);
     }

      close(pipefd[0]);
      close(pipefd[1]);

      do {
         pid = wait(NULL);
         if (pid == od_pid)
            od_pid = 0;
         if (pid == head_pid)
            head_pid = 0;
        if (pid == wc_pid)
            head_pid = 0;
      } while ((od_pid != 0 || head_pid != 0 || wc_pid !=0) && pid != -1);

     } /* end of for loop*/

   exit(EXIT_SUCCESS);
}

Any direction would be appreciated. Cheers.

---------- Post updated at 11:53 PM ---------- Previous update was at 11:01 PM ----------

oh ok...figured it out. I had to create another pipe to take the input from "head".

Cheers!
 

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LEARN ABOUT DEBIAN

replacekey

replacekey(1)						      General Commands Manual						     replacekey(1)

NAME

       replacekey - replace keywords in a FITS header

SYNOPSIS

       replacekey [options] <in>

DESCRIPTION

       replacekey is aimed at replacing keywords in a FITS file's header by new keywords provided by the user on the command-line.

ALGORITHM

       The  user  can provide a new_keyword, an old_keyword, a value and a comment.  first searchs for old_keyword if provided. If it finds it, it
       replaces the associated card by the new one (new_keyword=value/comment).  If old_keyword is either not found or	not  provided,	associated
       card by the new one (new_keyword=value/comment).  new_keyword has to be provided.

       Examples: You want to change the value of VALUE from 32 to 16 in myfile.fits: % replacekey -k "VALUE" -v "16" myfile.fits

       If  you	want  to  replace  the	card with the keyword OLD by "NEW = 3.14 / This is a usefull key" in myfile.fits: % replacekey -k "NEW" -v
       "3.14" -c "This is a usefull key" -K "OLD" myfile.fits

       If your do not care about the HISTORY cards you have in the header, and want to replace the first one by VALUE=32, you would do:
       % replacekey -k "VALUE" -v "32" -K "HISTORY" myfile.fits

OPTIONS

       -k or --key key
	      To provide the new keyword.

       -K or --old key
	      To provide the old keyword.

       -v or --val val
	      To provide the new value.

       -c or --com com
	      To provide the new comment.

SEE ALSO

       hierarch28 to convert HIERARCH ESO keywords to regular 8-char keywords.

FILES

       Files shall all comply with FITS format

BUGS

       Modifications are so far only possible in the main header, not in the extensions.

								    28 Oct 1999 						     replacekey(1)
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