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

NAME

     popen, pclose - pipe stream to or from a process

SYNOPSIS

     #include <stdio.h>

     FILE *popen(const char *command, const char *type);

     int pclose(FILE *stream);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     popen(), pclose():
         _POSIX_C_SOURCE >= 2
             || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION

     The  popen()  function opens a process by creating a pipe, forking, and
     invoking the shell.  Since a pipe is by definition unidirectional,  the
     type  argument  may  specify  only  reading  or  writing, not both; the
     resulting stream is correspondingly read-only or write-only.

     The command argument is a pointer to a null-terminated string  contain-
     ing  a shell command line.  This command is passed to /bin/sh using the
     -c flag; interpretation, if any, is performed by the shell.

     The type argument is a pointer to a null-terminated string  which  must
     contain  either  the letter 'r' for reading or the letter 'w' for writ-
     ing.  Since glibc 2.9, this argument can additionally include the  let-
     ter  'e', which causes the close-on-exec flag (FD_CLOEXEC) to be set on
     the underlying file descriptor; see the description  of  the  O_CLOEXEC
     flag in open(2) for reasons why this may be useful.

     The  return  value  from popen() is a normal standard I/O stream in all
     respects save  that  it  must  be  closed  with  pclose()  rather  than
     fclose(3).   Writing  to  such a stream writes to the standard input of
     the command; the command's standard output is the same as that  of  the
     process  that  called  popen(),  unless  this is altered by the command
     itself.  Conversely, reading from the stream reads the command's  stan-
     dard  output,  and  the command's standard input is the same as that of
     the process that called popen().

     Note that output popen() streams are block buffered by default.

     The pclose() function waits for the associated process to terminate and
     returns the exit status of the command as returned by wait4(2).

RETURN VALUE

     popen():  on  success,  returns a pointer to an open stream that can be
     used to read or write to the pipe; if  the  fork(2)  or  pipe(2)  calls
     fail, or if the function cannot allocate memory, NULL is returned.

     pclose():  on  success,  returns  the  exit  status  of the command; if
     wait4(2) returns an error, or some  other  error  is  detected,  -1  is
     returned.

     Both  functions  set  errno  to  an appropriate value in the case of an
     error.

ERRORS

     The popen() function does not set errno if memory allocation fails.  If
     the  underlying  fork(2)  or pipe(2) fails, errno is set appropriately.
     If the type argument is invalid, and this condition is detected,  errno
     is set to EINVAL.

     If pclose() cannot obtain the child status, errno is set to ECHILD.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     +------------------+---------------+---------+
     |Interface         | Attribute     | Value   |
     +------------------+---------------+---------+
     |popen(), pclose() | Thread safety | MT-Safe |
     +------------------+---------------+---------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

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

NOTES

     Note: carefully read Caveats in system(3).

BUGS

     Since  the  standard  input  of a command opened for reading shares its
     seek offset with the process  that  called  popen(),  if  the  original
     process  has done a buffered read, the command's input position may not
     be as expected.  Similarly, the output from a command opened for  writ-
     ing  may  become  intermingled  with that of the original process.  The
     latter can be avoided by calling fflush(3) before popen().

     Failure to execute the shell  is  indistinguishable  from  the  shell's
     failure  to  execute command, or an immediate exit of the command.  The
     only hint is an exit status of 127.