popen(3) - NetBSD Manual Pages

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POPEN(3)                  NetBSD 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 result-
     ing 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 argu-
     ment 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 re-
     spects 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 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 stream is not associated with a
     ``popened'' command, if stream already ``pclosed'', or if wait4 returns
     an error.


ERRORS

     The popen() function does not reliably set errno.


SEE ALSO

     fork(2),  sh(1),  pipe(2),  wait4(2),  fflush(3),  fclose(3),  fopen(3),
     stdio(3),  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 expect-
     ed.  Similarly, the output from a command opened for writing may become
     intermingled with that of the original process.  The latter can be avoid-
     ed 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,  never calls csh.


HISTORY

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

4.4BSD                           June 4, 1993                                2