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From: ARaman@massey.ac.nz (A.V. Raman)
Newsgroups: comp.unix.questions
Subject: pipe() doesn't seem to work. (Pyramid OSx 4.4)
Message-ID: <611@massey.ac.nz>
Date: 22 Mar 90 03:18:06 GMT
Reply-To: ARaman@massey.ac.nz (A.V. Raman)
Organization: Massey University, Palmerston North, New Zealand
Lines: 61

When I tried to set up a pipe in one of my programs, I found that
it failed.  When I put in some debug printfs in the code, I
found that it worked whenever there was a printf done just before the
dup() in the parent.   I also found that write() does not work in
place of printf().

I'm posting a reduced version of the code that caused the problem.
I've also removed all error checks at dups() and closes().  However, I
must mention that no errors were encountered even in the code that
included the checking.

Any help (by email please) would be greatly appreciated.

---- Code begins ----

#include <stdio.h>
setup()
{
   int fd[2];

   if (pipe(fd) < 0) {  /* write to fd[1] and read fd[0] */
      perror("pipe");
      exit(1);
   }
   if (fork() == 0) {   /* set up child to read pipe as stdin */
      close(0);
      dup(fd[0]);
      close(fd[1]);
      execl("/bin/cat","cat","-n",NULL);  /* ucb cat -n to number lines */
      perror("exec");
   } else {            /* set up parent to write to pipe as stdout */
/**/  printf (" ");    /* program fails if this line is removed */
      close(1);
      dup(fd[1]);
      close(fd[0]);
   }
}

main(argc,argv)
char **argv;
{
   FILE *fp;
   char buf [BUFSIZ];
   
   if (argc == 1)
      exit(1);
   setup();			/* setup pipe with cat reading other end */
   if ((fp = fopen(argv[1],"r")) == NULL) {
      perror(argv[1]);
      exit(1);
   }
   while (fgets(buf,BUFSIZ,fp) != NULL)
      printf("%s", buf);        /* write into pipe */
   fclose(fp);
   close(1);
   return 0;
}
---- Code ends ----

Thanks.
&