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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.1‐2008 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.1‐2008.
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.
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.1‐2008, 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.1‐2008 does not mention that SHELL affects
system(), the application would not know that it needed to unset
SHELL.
None.
exec(1p), pipe(3p), pthread_atfork(3p), wait(3p)
The Base Definitions volume of POSIX.1‐2008, limits.h(0p),
signal.h(0p), stdlib.h(0p), sys_wait.h(0p)
The Shell and Utilities volume of POSIX.1‐2008, sh(1p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 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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