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 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 command's 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 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().