SYSTEM(3P) POSIX Programmer's Manual SYSTEM(3P)
PROLOG
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.
NAME
system - issue a command
SYNOPSIS
#include <stdlib.h>
int system(const char *command);
DESCRIPTION
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 implementa-
tion-defined manner; this might then cause the program calling system()
to behave in a non-conforming manner or to terminate.
The environment of the executed command shall be 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.
The system() function shall ignore the SIGINT and SIGQUIT signals, and
shall block the SIGCHLD signal, while waiting for the command to termi-
nate. 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 applica-
tion 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 ter-
minated.
RETURN VALUE
If command is a null pointer, system() shall return non-zero to indi-
cate that a command processor is available, or zero if none is avail-
able. 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 util-
ity; 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, sys-
tem() shall return -1 and set errno to indicate the error.
ERRORS
The system() function may set errno values as described by fork().
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.
EXAMPLES
None.
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 conve-
nience, 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 sig-
nals 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 per-
form 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 exe-
cuted 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 sys-
tem()'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.
RATIONALE
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.
IEEE Std 1003.1-2001 places additional restrictions on system(). It
requires that if there is a command language interpreter, the environ-
ment must be as specified by fork() and exec. This ensures, for exam-
ple, 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 appli-
cation some information about the command's completion status.
Finally, IEEE Std 1003.1-2001 requires the command to be interpreted as
in the shell command language defined in the Shell and Utilities volume
of IEEE Std 1003.1-2001.
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 com-
mand 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
IEEE Std 1003.1-2001 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 inter-
preter actually exists with the correct mode, that there are enough
processes to execute it, and so on. The call system(NULL) could, theo-
retically, check for such problems as too many existing child pro-
cesses, 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-con-
forming.
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 undesir-
able 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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001.
#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 por-
table, and is not intended to be.
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 spec-
ify a different command interpreter. Such an implementation is discour-
aged. If the alternate command interpreter did not follow the command
line syntax specified in the Shell and Utilities volume of
IEEE Std 1003.1-2001, 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
IEEE Std 1003.1-2001 does not mention that SHELL affects system(), the
application would not know that it needed to unset SHELL.
FUTURE DIRECTIONS
None.
SEE ALSO
exec(), pipe(), waitpid(), the Base Definitions volume of
IEEE Std 1003.1-2001, <limits.h>, <signal.h>, <stdlib.h>, <sys/wait.h>,
the Shell and Utilities volume of IEEE Std 1003.1-2001, sh
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 SYSTEM(3P)
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