popen(3)

1POPEN(3)                   Linux Programmer's Manual                  POPEN(3)
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NAME

6       popen, pclose - process I/O
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SYNOPSIS

9       #include <stdio.h>
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11       FILE *popen(const char *command, const char *type);
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13       int pclose(FILE *stream);
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DESCRIPTION

16       The  popen()  function opens a process by creating a pipe, forking, and
17       invoking the shell.  Since a pipe is by definition unidirectional,  the
18       type  argument  may  specify  only  reading  or  writing, not both; the
19       resulting stream is correspondingly read-only or write-only.
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21       The command argument is a pointer to a null-terminated string  contain‐
22       ing  a shell command line.  This command is passed to /bin/sh using the
23       -c flag; interpretation, if any, is performed by the shell.   The  type
24       argument  is a pointer to a null-terminated string which must be either
25       `r' for reading or `w' for writing.
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27       The return value from popen() is a normal standard I/O  stream  in  all
28       respects  save  that  it  must  be  closed  with  pclose()  rather than
29       fclose().  Writing to such a stream writes to the standard input of the
30       command;  the  command's  standard  output  is  the same as that of the
31       process that called popen(), unless this  is  altered  by  the  command
32       itself.   Conversely,  reading from a ``popened'' stream reads the com‐
33       mand's standard output, and the command's standard input is the same as
34       that of the process that called popen().
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36       Note that output popen() streams are fully buffered by default.
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38       The pclose() function waits for the associated process to terminate and
39       returns the exit status of the command as returned by wait4().
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RETURN VALUE

42       The popen() function returns NULL if the fork(2) or pipe(2) calls fail,
43       or if it cannot allocate memory.
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45       The  pclose()  function returns -1 if wait4() returns an error, or some
46       other error is detected.
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ERRORS

49       The popen() function does not set errno if memory allocation fails.  If
50       the  underlying fork() or pipe() fails, errno is set appropriately.  If
51       the type argument is invalid, and this condition is detected, errno  is
52       set to EINVAL.
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54       If pclose() cannot obtain the child status, errno is set to ECHILD.
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CONFORMING TO

57       POSIX.1-2001.
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BUGS

60       Since  the  standard  input  of a command opened for reading shares its
61       seek offset with the process  that  called  popen(),  if  the  original
62       process  has done a buffered read, the command's input position may not
63       be as expected.  Similarly, the output from a command opened for  writ‐
64       ing  may  become  intermingled  with that of the original process.  The
65       latter can be avoided by calling fflush(3) before popen().
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67       Failure to execute the shell  is  indistinguishable  from  the  shell's
68       failure  to  execute command, or an immediate exit of the command.  The
69       only hint is an exit status of 127.
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HISTORY

72       A popen() and a pclose() function appeared in Version 7 AT&T UNIX.
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