FILE
*
popen(
const char *command
, const char *type
)
int
pclose(
FILE *stream
)
)
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(
).
)
function returns
NULL
if the
fork(2),
pipe(2),
or
socketpair(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
has already been
``pclosed'',
or if
wait4(2)
returns an error.
)
function does not reliably set
errno.
)
and
pclose(
)
functions conform to
IEEE Std 1003.2-1992 (``POSIX.2'') .
)
and a
pclose(
)
function appeared in
Version 7 AT&T UNIX
.
),
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),
never calls
csh(1).