popen(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       popen, pclose - pipe stream to or from a process

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

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

       ┌──────────────────┬───────────────┬─────────┐
       │Interface         Attribute     Value   │
       ├──────────────────┼───────────────┼─────────┤
       │popen(), pclose() │ Thread safety │ MT-Safe │
       └──────────────────┴───────────────┴─────────┘

CONFORMING TO         top

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

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

NOTES         top

       Note: carefully read Caveats in system(3).

BUGS         top

       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         top

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

COLOPHON         top

       This page is part of release 5.10 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)

Pages that refer to this page: gawk(1)pipe(2)getexeccon(3)__pmprocessexec(3)__pmprocesspipe(3)