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

6       popen, pclose - pipe stream to or from a process
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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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15   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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17       popen(), pclose(): _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE || _BSD_SOURCE
18       || _SVID_SOURCE
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DESCRIPTION

21       The popen() function opens a process by creating a pipe,  forking,  and
22       invoking  the shell.  Since a pipe is by definition unidirectional, the
23       type argument may specify  only  reading  or  writing,  not  both;  the
24       resulting stream is correspondingly read-only or write-only.
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26       The  command argument is a pointer to a null-terminated string contain‐
27       ing a shell command line.  This command is passed to /bin/sh using  the
28       -c  flag;  interpretation, if any, is performed by the shell.  The type
29       argument is a pointer to a null-terminated string  which  must  contain
30       either the letter 'r' for reading or the letter 'w' for writing.  Since
31       glibc 2.9, this argument can additionally include the letter 'e', which
32       causes  the close-on-exec flag (FD_CLOEXEC) to be set on the underlying
33       file descriptor; see the description of the O_CLOEXEC flag  in  open(2)
34       for reasons why this may be useful.
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36       The  return  value  from popen() is a normal standard I/O stream in all
37       respects save  that  it  must  be  closed  with  pclose()  rather  than
38       fclose(3).   Writing  to  such a stream writes to the standard input of
39       the command; the command's standard output is the same as that  of  the
40       process  that  called  popen(),  unless  this is altered by the command
41       itself.  Conversely, reading from a "popened"  stream  reads  the  com‐
42       mand's standard output, and the command's standard input is the same as
43       that of the process that called popen().
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45       Note that output popen() streams are fully buffered by default.
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47       The pclose() function waits for the associated process to terminate and
48       returns the exit status of the command as returned by wait4(2).
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RETURN VALUE

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

58       The popen() function does not set errno if memory allocation fails.  If
59       the underlying fork(2) or pipe(2) fails, errno  is  set  appropriately.
60       If  the type argument is invalid, and this condition is detected, errno
61       is set to EINVAL.
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63       If pclose() cannot obtain the child status, errno is set to ECHILD.
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CONFORMING TO

66       POSIX.1-2001.
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68       The 'e' value for type is a Linux extension.
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BUGS

71       Since the standard input of a command opened  for  reading  shares  its
72       seek  offset  with  the  process  that  called popen(), if the original
73       process has done a buffered read, the command's input position may  not
74       be  as expected.  Similarly, the output from a command opened for writ‐
75       ing may become intermingled with that of  the  original  process.   The
76       latter can be avoided by calling fflush(3) before popen().
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78       Failure  to  execute  the  shell  is indistinguishable from the shell's
79       failure to execute command, or an immediate exit of the  command.   The
80       only hint is an exit status of 127.
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SEE ALSO

83       sh(1),  fork(2),  pipe(2),  wait4(2),  fclose(3),  fflush(3), fopen(3),
84       stdio(3), system(3)
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COLOPHON

87       This page is part of release 3.25 of the Linux  man-pages  project.   A
88       description  of  the project, and information about reporting bugs, can
89       be found at http://www.kernel.org/doc/man-pages/.
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93GNU                               2010-02-03                          POPEN(3)
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