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 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.
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
popen();
output streams are fully buffered by default.
In addition, fork handlers established using
pthread_atfork(3)
are not called when a multithreaded program calls
popen();.
The
pclose();
function waits for the associated process to terminate and returns the
exit status of the command as returned by
wait4(2).
RETURN VALUES
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.
ERRORS
The
popen();
function does not reliably set
errno.
A
popen();
and a
pclose();
function appeared in
Version 7 AT&T UNIX.
BUGS
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 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.
The
popen();
argument always calls
sh(1).