popen(3S) manual page
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popen, pclose - initiate pipe to/from a process
#include
<stdio.h>
FILE
*popen(const char *command, const char *type);
int pclose (FILE
*stream);
Unsafe
popen() creates a pipe between
the calling program and the command to be executed. The arguments to popen()
are pointers to null-terminated strings. command consists of a shell command
line. type is an I/O
mode, either r for reading or w for writing. The value
returned is a stream pointer such that one can write to the standard input
of the command, if the I/O
mode is w, by writing to the file stream (see
intro(3)
); and one can read from the standard output of the command, if
the I/O
mode is r, by reading from the file stream. Because open files are
shared, a type r command may be used as an input filter and a type w as
an output filter.
The environment of the executed command will be as if
a child process were created within the popen() call using fork(2)
, and
the child invoked the shell using the call:
- Solaris
- execl("/usr/bin/sh",
"sh", "-c", command, (char *)0);
- XPG4
- execl("/usr/bin/ksh", "ksh", "-c",
command, (char *)0);
A stream opened by popen() should be closed by pclose(),
which closes the pipe, and waits for the associated process to terminate
and returns the termination status of the process running the command
language interpreter. This is the value returned by waitpid(2)
. See wstat(5)
for more details on termination status.
popen() returns a null
pointer if files or processes cannot be created.
pclose() returns the termination
status of the command. pclose() returns -1 if stream is not associated
with a popen() command and sets errno to indicate the error.
The
following is an example of a typical call:
#include <stdio.h>
#include <stdlib.h>
main()
{
char *cmd = "/usr/bin/ls *.c";
char buf[BUFSIZ];
FILE *ptr;
if ((ptr = popen(cmd, "r")) != NULL)
while (fgets(buf, BUFSIZ, ptr) != NULL)
(void) printf("%s", buf);
return 0;
}
This program will print on the standard output (see stdio(3S)
) all the
file names in the current directory that have a .c suffix.
ksh(1)
,
pipe(2)
, wait(2)
, waitpid(2)
, fclose(3S)
, fopen(3S)
, stdio(3S)
, system(3S)
,
wstat(5)
, xpg4(5)
If the original and popen() processes concurrently
read or write a common file, neither should use buffered I/O
. Problems with
an output filter may be forestalled by careful buffer flushing, for example,
with fflush() (see fclose(3S)
). A security hole exists through the IFS
and PATH
environment variables. Full pathnames should be used (or PATH
reset) and IFS
should be set to space and tab (" \t").
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