POPEN(3) Linux Programmer's Manual POPEN(3)
popen, pclose - process I/O
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
resulting stream is correspondingly read-only or write-only.
The command argument is a pointer to a null-terminated string contain-
ing 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 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
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 pro-
cess that called popen(), unless this is altered by the command itself.
Conversely, reading from a ''popened'' stream reads the command's stan-
dard 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().
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 wait4() returns an error, or some
other error is detected.
The popen() function does not set errno if memory allocation fails. If
the underlying fork() or pipe() fails, errno is set appropriately. If
the type argument is invalid, and this condition is detected, errno is
set to EINVAL.
If pclose() cannot obtain the child status, errno is set to ECHILD.
Since the standard input of a command opened for reading shares its
seek offset with the process that called popen(), if the original pro-
cess has done a buffered read, the command's input position may not be
as expected. 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.
A popen() and a pclose() function appeared in Version 7 AT&T UNIX.
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3),
BSD MANPAGE 1998-05-07 POPEN(3)