popen(3) - f38

1popen(3)                   Library Functions Manual                   popen(3)
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NAME

6       popen, pclose - pipe stream to or from a process
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <stdio.h>
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14       FILE *popen(const char *command, const char *type);
15       int pclose(FILE *stream);
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17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19       popen(), pclose():
20           _POSIX_C_SOURCE >= 2
21               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

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

55       popen():  on  success,  returns a pointer to an open stream that can be
56       used to read or write to the pipe; if  the  fork(2)  or  pipe(2)  calls
57       fail, or if the function cannot allocate memory, NULL is returned.
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59       pclose():  on  success,  returns  the  exit  status  of the command; if
60       wait4(2) returns an error, or some other error is detected, -1  is  re‐
61       turned.
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63       On failure, both functions set errno to indicate the error.
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ERRORS

66       The popen() function does not set errno if memory allocation fails.  If
67       the underlying fork(2) or pipe(2) fails, errno is set to  indicate  the
68       error.   If  the  type  argument  is invalid, and this condition is de‐
69       tected, errno is set to EINVAL.
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71       If pclose() cannot obtain the child status, errno is set to ECHILD.
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ATTRIBUTES

74       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
75       tributes(7).
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77       ┌────────────────────────────────────────────┬───────────────┬─────────┐
78       │Interface                                   │ Attribute     │ Value   │
79       ├────────────────────────────────────────────┼───────────────┼─────────┤
80       │popen(), pclose()                           │ Thread safety │ MT-Safe │
81       └────────────────────────────────────────────┴───────────────┴─────────┘
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VERSIONS

84       The 'e' value for type is a Linux extension.
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STANDARDS

87       POSIX.1-2008.
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HISTORY

90       POSIX.1-2001.
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CAVEATS

93       Carefully read Caveats in system(3).
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BUGS

96       Since  the  standard  input  of a command opened for reading shares its
97       seek offset with the process  that  called  popen(),  if  the  original
98       process  has done a buffered read, the command's input position may not
99       be as expected.  Similarly, the output from a command opened for  writ‐
100       ing  may  become  intermingled  with that of the original process.  The
101       latter can be avoided by calling fflush(3) before popen().
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103       Failure to execute the shell  is  indistinguishable  from  the  shell's
104       failure  to  execute command, or an immediate exit of the command.  The
105       only hint is an exit status of 127.
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