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 || _XOPEN_SOURCE || _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 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 a "popened" 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 fully 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

       The popen() function returns NULL if the fork(2) or pipe(2) calls fail, or if it cannot allocate memory.

       The  pclose()  function returns -1 if wait4(2) returns an error, or some other error is detected.  In the
       event of an error, these functions set errno to indicate the cause of the 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.

CONFORMING TO

       POSIX.1-2001.

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

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 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.

SEE ALSO

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.