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 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 writ-
       ing.  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 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  appropri-
       ately.  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
       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 4.15 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 https://www.kernel.org/doc/man-pages/.

GNU                                                                 2017-09-15                                                            POPEN(3)