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

NAME
       popen, pclose - process I/O

SYNOPSIS
       #include <stdio.h>

       FILE *popen(const char *command, const char *type);

       int pclose(FILE *stream);

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 mode argument is a pointer to a null-terminated string which must
       be either `r' for reading or `w' for writing.

       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().  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.

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 returns an error, or some other
       error is detected.

ERRORS
       The popen function does not set errno if memory allocation fails.  If
       the underlying fork() or pipe() fails, errno is set appropriately.  If
       the mode 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.2

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.

HISTORY
       A popen() and a pclose() function appeared in Version 7 AT&T UNIX.

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

BSD MANPAGE                       1998-05-07                          POPEN(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | BUGS | HISTORY | SEE ALSO

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