The popen function "opens" a process by creating a bidirectional pipe forking, and invoking the shell. Any streams opened by previous popen calls in the parent process are closed in the new child process. 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 a bidirectional pipe, the type argument may request a bidirectional 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 reading 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 commands 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 commands standard output, and the commands 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(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)
fflush(3) sh(1), csh(1).