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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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POPEN(3P) POSIX Programmer's Manual POPEN(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
popen — initiate pipe streams to or from a process
#include <stdio.h>
FILE *popen(const char *command, const char *mode);
The popen() function shall execute the command specified by the
string command. It shall create a pipe between the calling
program and the executed command, and shall return a pointer to a
stream that can be used to either read from or write to the pipe.
The environment of the executed command shall be as if a child
process were created within the popen() call using the fork()
function, and the child invoked the sh utility using the call:
execl(shell path, "sh", "-c", command, (char *)0);
where shell path is an unspecified pathname for the sh utility.
The popen() function shall ensure that any streams from previous
popen() calls that remain open in the parent process are closed
in the new child process.
The mode argument to popen() is a string that specifies I/O mode:
1. If mode is r, when the child process is started, its file
descriptor STDOUT_FILENO shall be the writable end of the
pipe, and the file descriptor fileno(stream) in the calling
process, where stream is the stream pointer returned by
popen(), shall be the readable end of the pipe.
2. If mode is w, when the child process is started its file
descriptor STDIN_FILENO shall be the readable end of the
pipe, and the file descriptor fileno(stream) in the calling
process, where stream is the stream pointer returned by
popen(), shall be the writable end of the pipe.
3. If mode is any other value, the result is unspecified.
After popen(), both the parent and the child process shall be
capable of executing independently before either terminates.
Pipe streams are byte-oriented.
Upon successful completion, popen() shall return a pointer to an
open stream that can be used to read or write to the pipe.
Otherwise, it shall return a null pointer and may set errno to
indicate the error.
The popen() function shall fail if:
EMFILE {STREAM_MAX} streams are currently open in the calling
process.
The popen() function may fail if:
EMFILE {FOPEN_MAX} streams are currently open in the calling
process.
EINVAL The mode argument is invalid.
The popen() function may also set errno values as described by
fork(3p) or pipe(3p).
The following sections are informative.
Using popen() to Obtain a List of Files from the ls Utility
The following example demonstrates the use of popen() and
pclose() to execute the command ls* in order to obtain a list of
files in the current directory:
#include <stdio.h>
...
FILE *fp;
int status;
char path[PATH_MAX];
fp = popen("ls *", "r");
if (fp == NULL)
/* Handle error */;
while (fgets(path, PATH_MAX, fp) != NULL)
printf("%s", path);
status = pclose(fp);
if (status == -1) {
/* Error reported by pclose() */
...
} else {
/* Use macros described under wait() to inspect `status' in order
to determine success/failure of command executed by popen() */
...
}
Since open files are shared, a mode r command can be used as an
input filter and a mode w command as an output filter.
Buffered reading before opening an input filter may leave the
standard input of that filter mispositioned. Similar problems
with an output filter may be prevented by careful buffer
flushing; for example, with fflush(3p).
A stream opened by popen() should be closed by pclose().
The behavior of popen() is specified for values of mode of r and
w. Other modes such as rb and wb might be supported by specific
implementations, but these would not be portable features. Note
that historical implementations of popen() only check to see if
the first character of mode is r. Thus, a mode of robert the
robot would be treated as mode r, and a mode of anything else
would be treated as mode w.
If the application calls waitpid() or waitid() with a pid
argument greater than 0, and it still has a stream that was
called with popen() open, it must ensure that pid does not refer
to the process started by popen().
To determine whether or not the environment specified in the
Shell and Utilities volume of POSIX.1‐2017 is present, use the
function call:
sysconf(_SC_2_VERSION)
(See sysconf(3p)).
The popen() function should not be used by programs that have set
user (or group) ID privileges. The fork() and exec family of
functions (except execlp() and execvp()), should be used instead.
This prevents any unforeseen manipulation of the environment of
the user that could cause execution of commands not anticipated
by the calling program.
If the original and popen()ed processes both intend to read or
write or read and write a common file, and either will be using
FILE-type C functions (fread(), fwrite(), and so on), the rules
for sharing file handles must be observed (see Section 2.5.1,
Interaction of File Descriptors and Standard I/O Streams).
None.
Section 2.5, Standard I/O Streams, fork(3p), pclose(3p),
pipe(3p), sysconf(3p), system(3p), wait(3p), waitid(3p)
The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)
The Shell and Utilities volume of POSIX.1‐2017, sh(1p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any
discrepancy between this version and the original IEEE and The
Open Group Standard, the original IEEE and The Open Group
Standard is the referee document. The original Standard can be
obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 POPEN(3P)
Pages that refer to this page: stdio.h(0p), awk(1p), pclose(3p), stdin(3p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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