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popen(3c)
Standard C Library Functions popen(3C)
NAME
popen, pclose - initiate a pipe to or from a process
SYNOPSIS
#include <stdio.h>
FILE *popen(const char *command, const char *mode);
int pclose(FILE *stream);
DESCRIPTION
The popen() function creates a pipe between the calling program and the
command to be executed. The arguments to popen() are pointers to null-
terminated strings. The command argument consists of a shell command
line. The mode argument is an I/O mode, either starting with 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 starts with 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
starts with 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. An additional e and/or f character can also be
included in the mode argument; it indicates that the underlying file
descriptor of the returned FILE has the FD_CLOEXEC and/or FD_CLOFORK
flag, respectively, set.
The environment of the executed command will be as if a child process
were created within the popen() call using fork(2). If the application
is standard-conforming (see standards(7)), the child is created as if
invoked with the call:
execl("/usr/xpg4/bin/sh", "sh", "-c", command, NULL);
otherwise, the child is created as if invoked with the call:
execl("/usr/bin/sh", "sh", "-c", command, NULL);
The pclose() function closes a stream opened by popen() by closing the
pipe. It waits for the associated process to terminate and returns the
termination status of the process running the command language inter‐
preter. This is the value returned by waitpid(3C). See wait.h(3HEAD)
for more information on termination status. If, however, a call to
waitpid() with a pid argument equal to the process ID of the command
line interpreter causes the termination status to be unavailable to
pclose(), then pclose() returns −1 with errno set to ECHILD to report
this condition.
RETURN VALUES
Upon successful completion, popen() returns a pointer to an open stream
that can be used to read or write to the pipe. Otherwise, it returns a
null pointer and may set errno to indicate the error.
Upon successful completion, pclose() returns the termination status of
the command language interpreter as returned by waitpid(). Otherwise,
it returns −1 and sets errno to indicate the error.
ERRORS
The pclose() function will fail if:
ECHILD The status of the child process could not be obtained, as
described in the DESCRIPTION.
The popen() function may fail if:
EMFILE There are currently FOPEN_MAX or STREAM_MAX streams open in
the calling process.
EINVAL The mode argument is invalid.
The popen() function may also set errno values as described by fork(2)
or pipe(2).
USAGE
If the original and popen() processes concurrently read or write a com‐
mon file, neither should use buffered I/O. Problems with an output fil‐
ter may be forestalled by careful buffer flushing, for example, with
fflush(3C).
The IFS and PATH environment variables should be set to values you con‐
trol before calling popen(), to avoid the users environment variables
causing the use of different programs than expected, especially if the
program is run with privileges the user does not normally have. PATH
should contain only absolute paths, with no relative paths or empty
entries. IFS should be set to space and tab: " \t".
The SIGCHLD signal does not apply to processes run from popen(). If a
process sets a signal handler for SIGCHLD, that signal handler will not
be called when the popen() process terminates.
A wait(3C) call from another thread in the same process will not inter‐
fere with the return value of pclose().
EXAMPLES
Example 1 popen() example
The following program will print on the standard output (see stdio(3C))
the names of files in the current directory with a .c suffix.
#include <stdio.h>
#include <stdlib.h>
main()
{
const 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);
(void) pclose(ptr);
}
return 0;
}
Example 2 system() replacement
The following function can be used in a multithreaded process in place
of the most common usage of the Unsafe system(3C) function:
int my_system(const char *cmd)
{
FILE *p;
if ((p = popen(cmd, "w")) == NULL)
return (-1);
return (pclose(p));
}
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelSafe _ Standard‐
See standards(7).
SEE ALSO
ksh(1), pipe(2), fclose(3C), fopen(3C), posix_spawn(3C), stdio(3C),
system(3C), wait(3C), waitpid(3C), wait.h(3HEAD), attributes(7), stan‐
dards(7)
Oracle Solaris 11.4 08 May 2020 popen(3C)