popen(3) - NetBSD Manual Pages

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POPEN(3)                NetBSD Library Functions Manual               POPEN(3)


NAME

     popen, pclose -- process I/O


LIBRARY

     Standard C Library (libc, -lc)


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 an IPC connection,
     forking, and invoking the shell.  Historically, popen was implemented
     with a unidirectional pipe; hence many implementations of popen only
     allow the type argument to specify reading or writing, not both.  Since
     popen is now implemented using sockets, the type may request a bidirec-
     tional data flow.  The type argument is a pointer to a null-terminated
     string which must be `r' for reading, `w' for writing, or `r+' for read-
     ing and writing.

     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 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, read-
     ing 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 VALUES

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

     The pclose() function returns -1 if stream is not associated with a
     ``popened'' command, if stream has already been ``pclosed'', or if
     wait4(2) returns an error.


ERRORS

     The popen() function does not reliably set errno.


SEE ALSO

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


STANDARDS

     The popen() and pclose() functions conform to IEEE Std 1003.2-1992
     (``POSIX.2'').


HISTORY

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


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 fail-
     ure to execute command, or an immediate exit of the command.  The only
     hint is an exit status of 127.

     The popen() argument always calls sh(1), never calls csh(1).

NetBSD 3.0                       July 14, 2004                      NetBSD 3.0