POPEN(3) BSD Programmer's Manual POPEN(3)
popen, pclose - process I/O
#include <stdio.h>
FILE *
popen(const char *command, const char *type);
int
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
error.
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