POPEN(3) | Library Functions Manual | POPEN(3) |
popen
, popenve
,
pclose
—
#include <stdio.h>
FILE *
popen
(const
char *command, const char
*type);
FILE *
popenve
(const
char *path, char * const
*argv, char * const
*envp, const char
*type);
int
pclose
(FILE
*stream);
popen
() 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. In
addition if the character ‘e
’ is present
in the type string, the file descriptor used internally
is set to be closed on exec(3).
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 popenve
() function is similar to
popen
() but the first three arguments are passed to
execve(2) and there is no
shell involved in the command invocation.
The return value from popen
() and
popenve
() 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
().
popen
() function returns
NULL
if the
vfork(2),
pipe(2), or
socketpair(2) calls fail, or
if it cannot allocate memory, preserving the errno from those functions.
The pclose
() function returns -1 if
stream is not associated with a
“popened” command, if stream has already
been “pclosed”, setting errno to ESRCH
or if wait4(2) returns an
error, preserving the errno returned by
wait4(2).
popen
() and pclose
()
functions conform to IEEE Std 1003.2-1992
(“POSIX.2”).
popen
() and a pclose
()
function appeared in Version 7 AT&T UNIX.
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), never calls
csh(1).
The popenve
() function first appeared in
NetBSD 8.
January 19, 2015 | NetBSD 9.0 |