pclose

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
       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).

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
       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 5.03 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)