popen, pclose - pipe stream to or from a process


   #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


   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
   containing  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
   writing.  Since glibc 2.9, this argument can additionally  include  the
   letter  '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
   standard  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).


   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

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


   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.


   For   an   explanation   of   the  terms  used  in  this  section,  see

   │InterfaceAttributeValue   │
   │popen(), pclose() │ Thread safety │ MT-Safe │


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

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


   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
   writing 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.


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


   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at


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