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diff --git a/man-pages-posix-2013/man3p/system.3p b/man-pages-posix-2013/man3p/system.3p new file mode 100644 index 0000000..9275c56 --- /dev/null +++ b/man-pages-posix-2013/man3p/system.3p @@ -0,0 +1,459 @@ +'\" et +.TH SYSTEM "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual" +.SH 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. + +.SH NAME +system +\(em issue a command +.SH SYNOPSIS +.LP +.nf +#include <stdlib.h> +.P +int system(const char *\fIcommand\fP); +.fi +.SH DESCRIPTION +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\(hy2008 defers to the ISO\ C standard. +.P +If +.IR command +is a null pointer, the +\fIsystem\fR() +function shall determine whether the host environment has a command +processor. If +.IR command +is not a null pointer, the +\fIsystem\fR() +function shall pass the string pointed to by +.IR command +to that command processor to be executed in an implementation-defined +manner; this might then cause the program calling +\fIsystem\fR() +to behave in a non-conforming manner or to terminate. +.P +The +\fIsystem\fR() +function shall behave as if a child process were created using +\fIfork\fR(), +and the child process invoked the +.IR sh +utility using +\fIexecl\fR() +as follows: +.sp +.RS 4 +.nf +\fB +execl(<\fIshell path\fP>, "sh", "-c", \fIcommand\fP, (char *)0); +.fi \fR +.P +.RE +.P +where <\fIshell path\fP> is an unspecified pathname for the +.IR sh +utility. It is unspecified whether the handlers registered with +\fIpthread_atfork\fR() +are called as part of the creation of the child process. +.P +The +\fIsystem\fR() +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 +\fIsystem\fR() +and take whatever action is appropriate to the application if the +command terminated due to receipt of a signal. +.P +The +\fIsystem\fR() +function shall not affect the termination status of any child of the +calling processes other than the process or processes it itself +creates. +.P +The +\fIsystem\fR() +function shall not return until the child process has terminated. +.P +The +\fIsystem\fR() +function need not be thread-safe. +.SH "RETURN VALUE" +If +.IR command +is a null pointer, +\fIsystem\fR() +shall return non-zero to indicate that a command processor is +available, or zero if none is available. +The +\fIsystem\fR() +function shall always return non-zero when +.IR command +is NULL. +.P +If +.IR command +is not a null pointer, +\fIsystem\fR() +shall return the termination status of the command language interpreter +in the format specified by +\fIwaitpid\fR(). +The termination status shall be as defined for the +.IR 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 +\fIsystem\fR() +shall be as if the command language interpreter had terminated using +.IR exit (127) +or +.IR _exit (127). +If a child process cannot be created, or if the termination status for +the command language interpreter cannot be obtained, +\fIsystem\fR() +shall return \(mi1 and set +.IR errno +to indicate the error. +.SH ERRORS +The +\fIsystem\fR() +function may set +.IR errno +values as described by +.IR "\fIfork\fR\^(\|)". +.br +.P +In addition, +\fIsystem\fR() +may fail if: +.TP +.BR ECHILD +The status of the child process created by +\fIsystem\fR() +is no longer available. +.LP +.IR "The following sections are informative." +.SH EXAMPLES +None. +.SH "APPLICATION USAGE" +If the return value of +\fIsystem\fR() +is not \(mi1, its value can be decoded through the use of the macros +described in +.IR <sys/wait.h> . +For convenience, these macros are also provided in +.IR <stdlib.h> . +.P +Note that, while +\fIsystem\fR() +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 +\fIfork\fR() +and +.IR exec . +For example, if SIGINT is being caught or is set to SIG_DFL when +\fIsystem\fR() +is called, then the child is started with SIGINT handling set to +SIG_DFL. +.P +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 +.BR '!' +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 +\fIsystem\fR() +to perform some task invisible to the user. If the user typed the +interrupt character (\c +.BR \(dq^C\(dq , +for example) while +\fIsystem\fR() +is being used in this way, one would expect the application to be +killed, but only the executed command is killed. Applications that use +\fIsystem\fR() +in this way should carefully check the return status from +\fIsystem\fR() +to see if the executed command was successful, and should take +appropriate action when the command fails. +.P +Blocking SIGCHLD while waiting for the child to terminate prevents the +application from catching the signal and obtaining status from +\fIsystem\fR()'s +child process before +\fIsystem\fR() +can get the status itself. +.P +The context in which the utility is ultimately executed may differ from +that in which +\fIsystem\fR() +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. +.P +There is no defined way for an application to find the specific path +for the shell. However, +\fIconfstr\fR() +can provide a value for +.IR PATH +that is guaranteed to find the +.IR sh +utility. +.P +Using the +\fIsystem\fR() +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. +.SH RATIONALE +The +\fIsystem\fR() +function should not be used by programs that have set user (or group) +ID privileges. The +\fIfork\fR() +and +.IR exec +family of functions (except +\fIexeclp\fR() +and +\fIexecvp\fR()), +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. +.P +There are three levels of specification for the +\fIsystem\fR() +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. +.P +POSIX.1\(hy2008 places additional restrictions on +\fIsystem\fR(). +It requires that if there is a command language interpreter, the +environment must be as specified by +\fIfork\fR() +and +.IR exec . +This ensures, for example, that close-on-\c +.IR exec +works, that file locks are not inherited, and that the process ID is +different. It also specifies the return value from +\fIsystem\fR() +when the command line can be run, thus giving the application some +information about the command's completion status. +.P +Finally, POSIX.1\(hy2008 requires the command to be interpreted as in the shell +command language defined in the Shell and Utilities volume of POSIX.1\(hy2008. +.P +Note that, +.IR 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 +.IR 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 +\fIsystem\fR() +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\(hy2008 if +.IR system (NULL) +returns zero. +.P +It was explicitly decided that when +.IR command +is NULL, +\fIsystem\fR() +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 +.IR 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 +\fIsystem\fR() +call to fail, that system has been rendered non-conforming. +.P +Early drafts required, or allowed, +\fIsystem\fR() +to return with +.IR errno +set to +.BR [EINTR] +if it was interrupted with a signal. This error return was removed, and +a requirement that +\fIsystem\fR() +not return until the child has terminated was added. This means that if +a +\fIwaitpid\fR() +call in +\fIsystem\fR() +exits with +.IR errno +set to +.BR [EINTR] , +\fIsystem\fR() +must reissue the +\fIwaitpid\fR(). +This change was made for two reasons: +.IP " 1." 4 +There is no way for an application to clean up if +\fIsystem\fR() +returns +.BR [EINTR] , +short of calling +\fIwait\fR(), +and that could have the undesirable effect of returning the status of +children other than the one started by +\fIsystem\fR(). +.IP " 2." 4 +While it might require a change in some historical implementations, +those implementations already have to be changed because they use +\fIwait\fR() +instead of +\fIwaitpid\fR(). +.P +Note that if the application is catching SIGCHLD signals, it will +receive such a signal before a successful +\fIsystem\fR() +call returns. +.P +To conform to POSIX.1\(hy2008, +\fIsystem\fR() +must use +\fIwaitpid\fR(), +or some similar function, instead of +\fIwait\fR(). +.P +The following code sample illustrates how +\fIsystem\fR() +might be implemented on an implementation conforming to POSIX.1\(hy2008. +.sp +.RS 4 +.nf +\fB +#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); +} +.fi \fR +.P +.RE +.P +Note that, while a particular implementation of +\fIsystem\fR() +(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. +.P +One reviewer suggested that an implementation of +\fIsystem\fR() +might want to use an environment variable such as +.IR 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 +\fIsystem\fR(), +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\(hy2008, then changing +.IR SHELL +would render +\fIsystem\fR() +non-conforming. This would affect applications that expected the +specified behavior from +\fIsystem\fR(), +and since the Shell and Utilities volume of POSIX.1\(hy2008 does not mention that +.IR SHELL +affects +\fIsystem\fR(), +the application would not know that it needed to unset +.IR SHELL . +.SH "FUTURE DIRECTIONS" +None. +.SH "SEE ALSO" +.IR "\fIexec\fR\^", +.IR "\fIpipe\fR\^(\|)", +.IR "\fIpthread_atfork\fR\^(\|)", +.IR "\fIwait\fR\^(\|)" +.P +The Base Definitions volume of POSIX.1\(hy2008, +.IR "\fB<limits.h>\fP", +.IR "\fB<signal.h>\fP", +.IR "\fB<stdlib.h>\fP", +.IR "\fB<sys_wait.h>\fP" +.P +The Shell and Utilities volume of POSIX.1\(hy2008, +.IR "\fIsh\fR\^" +.SH COPYRIGHT +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 . |