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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SYSTEM(3P) POSIX Programmer's Manual SYSTEM(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.
system — issue a command
#include <stdlib.h>
int system(const char *command);
The functionality described on this reference page is aligned
with the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
If command is a null pointer, the system() function shall
determine whether the host environment has a command processor.
If command is not a null pointer, the system() function shall
pass the string pointed to by command to that command processor
to be executed in an implementation-defined manner; this might
then cause the program calling system() to behave in a non-
conforming manner or to terminate.
The system() function shall behave as if a child process were
created using fork(), and the child process invoked the sh
utility using execl() as follows:
execl(<shell path>, "sh", "-c", command, (char *)0);
where <shell path> is an unspecified pathname for the sh utility.
It is unspecified whether the handlers registered with
pthread_atfork() are called as part of the creation of the child
process.
The system() function shall ignore the SIGINT and SIGQUIT
signals, and shall block 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, then 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.
The system() function shall not affect the termination status of
any child of the calling processes other than the process or
processes it itself creates.
The system() function shall not return until the child process
has terminated.
The system() function need not be thread-safe.
If command is a null pointer, system() shall return non-zero to
indicate that a command processor is available, or zero if none
is available. The system() function shall always return non-zero
when command is NULL.
If command is not a null pointer, system() shall return the
termination status of the command language interpreter in the
format specified by waitpid(). The termination status shall be
as defined for the sh utility; otherwise, the termination status
is unspecified. If some error prevents the command language
interpreter from executing after the child process is created,
the return value from system() shall be as if the command
language interpreter had terminated using exit(127) or
_exit(127). If a child process cannot be created, or if the
termination status for the command language interpreter cannot be
obtained, system() shall return -1 and set errno to indicate the
error.
The system() function may set errno values as described by
fork(3p).
In addition, system() may fail if:
ECHILD The status of the child process created by system() is no
longer available.
The following sections are informative.
None.
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() and exec. For example, if SIGINT is being caught or is
set to SIG_DFL when system() is called, then the child is started
with SIGINT handling set to SIG_DFL.
Ignoring SIGINT and SIGQUIT in the parent process prevents
coordination problems (two processes reading from the same
terminal, for example) when the executed command ignores or
catches one of the signals. It is also usually the correct action
when the user has given a command to the application to be
executed synchronously (as in the '!' command in many
interactive applications). In either case, the signal should be
delivered only to the child process, not to the application
itself. There is one situation where ignoring the signals might
have less than the desired effect. This is when the application
uses system() to perform some task invisible to the user. If the
user typed the interrupt character ("^C", for example) while
system() is being used in this way, one would expect the
application to be killed, but only the executed command is
killed. Applications that use system() in this way should
carefully check the return status from system() to see if the
executed command was successful, and should take appropriate
action when the command fails.
Blocking SIGCHLD while waiting for the child to terminate
prevents the application from catching the signal and obtaining
status from system()'s child process before system() can get the
status itself.
The context in which the utility is ultimately executed may
differ from that in which system() was called. For example, file
descriptors that have the FD_CLOEXEC flag set are closed, and the
process ID and parent process ID are different. Also, if the
executed utility changes its environment variables or its current
working directory, that change is not reflected in the caller's
context.
There is no defined way for an application to find the specific
path for the shell. However, confstr() can provide a value for
PATH that is guaranteed to find the sh utility.
Using the system() function in more than one thread in a process
or when the SIGCHLD signal is being manipulated by more than one
thread in a process may produce unexpected results.
The system() 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.
There are three levels of specification for the system()
function. The ISO C standard gives the most basic. It requires
that the function exists, and defines a way for an application to
query whether a command language interpreter exists. It says
nothing about the command language or the environment in which
the command is interpreted.
POSIX.1‐2008 places additional restrictions on system(). It
requires that if there is a command language interpreter, the
environment must be as specified by fork() and exec. This
ensures, for example, that close-on-exec works, that file locks
are not inherited, and that the process ID is different. It also
specifies the return value from system() when the command line
can be run, thus giving the application some information about
the command's completion status.
Finally, POSIX.1‐2008 requires the command to be interpreted as
in the shell command language defined in the Shell and Utilities
volume of POSIX.1‐2017.
Note that, system(NULL) is required to return non-zero,
indicating that there is a command language interpreter. At first
glance, this would seem to conflict with the ISO C standard which
allows system(NULL) to return zero. There is no conflict,
however. A system must have a command language interpreter, and
is non-conforming if none is present. It is therefore
permissible for the system() function on such a system to
implement the behavior specified by the ISO C standard as long as
it is understood that the implementation does not conform to
POSIX.1‐2008 if system(NULL) returns zero.
It was explicitly decided that when command is NULL, system()
should not be required to check to make sure that the command
language interpreter actually exists with the correct mode, that
there are enough processes to execute it, and so on. The call
system(NULL) could, theoretically, check for such problems as too
many existing child processes, and return zero. However, it would
be inappropriate to return zero due to such a (presumably)
transient condition. If some condition exists that is not under
the control of this application and that would cause any system()
call to fail, that system has been rendered non-conforming.
Early drafts required, or allowed, system() to return with errno
set to [EINTR] if it was interrupted with a signal. This error
return was removed, and a requirement that system() not return
until the child has terminated was added. This means that if a
waitpid() call in system() exits with errno set to [EINTR],
system() must reissue the waitpid(). This change was made for
two reasons:
1. There is no way for an application to clean up if system()
returns [EINTR], short of calling wait(), and that could have
the undesirable effect of returning the status of children
other than the one started by system().
2. While it might require a change in some historical
implementations, those implementations already have to be
changed because they use wait() instead of waitpid().
Note that if the application is catching SIGCHLD signals, it will
receive such a signal before a successful system() call returns.
To conform to POSIX.1‐2008, system() must use waitpid(), or some
similar function, instead of wait().
The following code sample illustrates how system() might be
implemented on an implementation conforming to POSIX.1‐2008.
#include <signal.h>
int system(const char *cmd)
{
int stat;
pid_t pid;
struct sigaction sa, savintr, savequit;
sigset_t saveblock;
if (cmd == NULL)
return(1);
sa.sa_handler = SIG_IGN;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
sigemptyset(&savintr.sa_mask);
sigemptyset(&savequit.sa_mask);
sigaction(SIGINT, &sa, &savintr);
sigaction(SIGQUIT, &sa, &savequit);
sigaddset(&sa.sa_mask, SIGCHLD);
sigprocmask(SIG_BLOCK, &sa.sa_mask, &saveblock);
if ((pid = fork()) == 0) {
sigaction(SIGINT, &savintr, (struct sigaction *)0);
sigaction(SIGQUIT, &savequit, (struct sigaction *)0);
sigprocmask(SIG_SETMASK, &saveblock, (sigset_t *)0);
execl("/bin/sh", "sh", "-c", cmd, (char *)0);
_exit(127);
}
if (pid == -1) {
stat = -1; /* errno comes from fork() */
} else {
while (waitpid(pid, &stat, 0) == -1) {
if (errno != EINTR){
stat = -1;
break;
}
}
}
sigaction(SIGINT, &savintr, (struct sigaction *)0);
sigaction(SIGQUIT, &savequit, (struct sigaction *)0);
sigprocmask(SIG_SETMASK, &saveblock, (sigset_t *)0);
return(stat);
}
Note that, while a particular implementation of system() (such as
the one above) can assume a particular path for the shell, such a
path is not necessarily valid on another system. The above
example is not portable, and is not intended to be.
Note also that the above example implementation is not thread-
safe. Implementations can provide a thread-safe system()
function, but doing so involves complications such as how to
restore the signal dispositions for SIGINT and SIGQUIT correctly
if there are overlapping calls, and how to deal with
cancellation. The example above would not restore the signal
dispositions and would leak a process ID if cancelled. This does
not matter for a non-thread-safe implementation since canceling a
non-thread-safe function results in undefined behavior (see
Section 2.9.5.2, Cancellation Points). To avoid leaking a
process ID, a thread-safe implementation would need to terminate
the child process when acting on a cancellation.
One reviewer suggested that an implementation of system() might
want to use an environment variable such as SHELL to determine
which command interpreter to use. The supposed implementation
would use the default command interpreter if the one specified by
the environment variable was not available. This would allow a
user, when using an application that prompts for command lines to
be processed using system(), to specify a different command
interpreter. Such an implementation is discouraged. If the
alternate command interpreter did not follow the command line
syntax specified in the Shell and Utilities volume of
POSIX.1‐2017, then changing SHELL would render system() non-
conforming. This would affect applications that expected the
specified behavior from system(), and since the Shell and
Utilities volume of POSIX.1‐2017 does not mention that SHELL
affects system(), the application would not know that it needed
to unset SHELL.
None.
Section 2.9.5.2, Cancellation Points, exec(1p), pipe(3p),
pthread_atfork(3p), wait(3p)
The Base Definitions volume of POSIX.1‐2017, limits.h(0p),
signal.h(0p), stdlib.h(0p), sys_wait.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 SYSTEM(3P)
Pages that refer to this page: stdio.h(0p), stdlib.h(0p), getconf(1p), make(1p), sh(1p), exec(3p), popen(3p), wait(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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