NAME
popen, pclose
— process I/O
SYNOPSIS
#include
<stdio.h>
FILE *
popen(const
char *command, const char
*type);
int
pclose(FILE
*stream);
DESCRIPTION
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" or "re"
for reading or "w" or "we" for writing. If the letter
"e" is present in the string then the close-on-exec flag shall be
set on the file descriptor underlying the FILE that is
returned.
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.
SEE ALSO
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)
HISTORY
A popen() and a
pclose() function appeared in
Version 7 AT&T UNIX.
CAVEATS
Never supply the popen() function with a
command containing any part of an unsanitized user-supplied string. Shell
meta-characters present will be honored by the
sh(1) command
interpreter.
It is often simpler to bypass the shell entirely and use pipe(2), fork(2), dup2(2), execlp(3), and fdopen(3) directly instead of having to sanitize a string for shell consumption.
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.