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readpipe.c

/* Open a pipe to read from a program without intermediary sh.
   Copyright (C) 1992, 1997 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/* Written by David MacKenzie.  */

#include <config.h>
#include <stdio.h>

#if __STDC__
#include <stdarg.h>
#else
#include <varargs.h>
#endif

#include <unistd.h>

/* Open a pipe to read from a program without intermediary sh.  Checks
   PATH.  Sample use:

   stream = readpipe ("progname", "arg1", "arg2", (char *) 0);

   Return 0 on error.  */

#if __STDC__
FILE *
readpipe (char *progname, ...)
#else
FILE *
readpipe (va_alist)
#endif
{
#if ! __STDC__
  char *progname;
#endif
  int fds[2];
  va_list ap;
  char *args[100];
  int argno = 0;

  /* Copy arguments into `args'. */
#if __STDC__
  va_start (ap, progname);
#else
  va_start (ap);
  progname = va_arg (ap, char *);
#endif
  args[argno++] = progname;
  while ((args[argno++] = va_arg (ap, char *)) != NULL)
    ;
  va_end (ap);

  if (pipe (fds) == -1)
    return 0;

  switch (fork ())
    {
    case 0:             /* Child.  Write to pipe. */
      close (fds[0]);         /* Not needed. */
      if (fds[1] != 1)        /* Redirect 1 (stdout) only if needed.  */
      {
        close (1);            /* We don't want the old stdout. */
        if (dup (fds[1]) == 0)/* Maybe stdin was closed. */
          {
            dup (fds[1]);     /* Guaranteed to dup to 1 (stdout). */
            close (0);
          }
        close (fds[1]); /* No longer needed. */
      }
      execvp (args[0], args);
      _exit (2);        /* 2 for `cmp'. */
    case -1:                  /* Error. */
      return 0;
    default:                  /* Parent.  Read from pipe. */
      close (fds[1]);         /* Not needed. */
      return fdopen (fds[0], "r");
    }
}

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