MirBSD manpage: pclose(3), popen(3)

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


     popen, pclose - process I/O


     #include <stdio.h>

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

     pclose(FILE *stream);


     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 NUL-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 NUL-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 except 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. Con-
     versely, reading from a "popened" stream reads the command's standard
     output, and the command's standard input is the same as that of the pro-
     cess that called popen().

     Note that popen() output 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(2).


     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(2) returns an


     The popen() function does not reliably set errno.


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


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


     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 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.

     The popen() argument always calls sh(1).

MirBSD #10-current               June 4, 1993                                1

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