NAME
system() — issue a shell command
SYNOPSIS
#include <stdlib.h>
int system(const char *command);
DESCRIPTION
system()
executes the command specified by the string pointed to by
command.
The environment of the executed command
is as if a child process were created using
fork()
(see
fork(2)),
and the child process invoked the
sh-posix(1)
utility via a call to
execl()
(see
exec(2))
as follows:
execl("/usr/bin/sh", "sh", "-c", command, 0);
system()
ignores the
SIGINT
and
SIGQUIT
signals, and blocks the
SIGCHLD
signal, while waiting for the command to terminate.
If this might cause the application to miss a signal
that would have killed it,
the application should examine the return value from
system()
and take whatever action is appropriate to the application
if the command terminated due to receipt of a signal.
system()
does not affect the termination status of any child
of the calling processes
other than the process or processes it itself creates.
system()
does not return until the child process has terminated.
APPLICATION USAGE
If the return value of
system()
is not -1,
its value can be decoded through the use of the macros described in
<sys/wait.h>.
For convenience, these macros are also provided in
<stdlib.h>.
Note that, while
system()
must ignore
SIGINT
and
SIGQUIT
and block
SIGCHLD
while waiting for the child to terminate,
the handling of signals in the executed command is as specified by
fork(2)
and
exec(2).
For example, if
SIGINT
is being caught or is set to
SIG_DFL
when
system()
is called, the child is started with
SIGINT
handling set to
SIG_DFL.
Ignoring
SIGINT
and
SIGQUIT
in the parent process prevents coordination problems
(such as two processes reading from the same terminal)
when the executed command ignores or catches one of the signals.
RETURN VALUE
If
command
is null,
system()
returns non-zero.
If
command
is not null,
system()
returns the termination status of the command language interpreter
in the format specified by
wait(2).
The termination status of the command language interpreter
is as specified for
sh-posix(1),
except that if some error prevents the command language interpreter
from executing after the child process is created,
the return value from
system()
is as if the command language interpreter had terminated using
_exit(127).
If a child process cannot be created, or if the termination status
for the command language interpreter cannot be obtained,
system()
returns -1 and sets
errno
to indicate the error.
DIAGNOSTICS
system()
forks to create a child process which, in turn,
exec()s
/usr/bin/sh
in order to execute
string.
If the fork fails,
system()
returns -1 and sets
errno.
If the exec fails,
system()
returns the status value returned by
waitpid()
(see
wait(2))
for a process that terminates with a call of
exit(127).
ERRORS
If errors are encountered,
system()
sets
errno
values as described by
fork(2).
STANDARDS CONFORMANCE
system(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, POSIX.2, ANSI C
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