unix 下的C开发手册,还用详细的例程。

																				system																The Single UNIX ® Specification, Version 2				Copyright © 1997 The Open Group												 NAME				system - issue a command				 SYNOPSIS												#include <stdlib.h>								int system(const char *command);												 DESCRIPTION				The				system()				function passes the string pointed to by				command				to the host environment to be executed by a command processor in an				implementation-dependent manner.  If the implementation supports				the XCU specification commands, the environment of the executed				command will be as if a child process were created using				fork(),				and the child process invoked the				sh				utility (see				sh				in the XCU specification) 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 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, 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 will not affect the termination status of				any child of the calling processes other than the process or				processes it itself creates.								The				system()				function will not return until the child process has terminated.				 RETURN VALUE				If				command				is a null pointer,				system()				returns non-zero only if a command processor is available.								If				command				is not a null pointer,				system()				returns the termination status of the				command language interpreter in the format specified by				waitpid().				The termination status of the command language interpreter is as				specified for the				sh				utility, except that if some error prevents the command language interpreter				from executing after the child process is created, the return value from				system()				will 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()				returns -1 and sets				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.												 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 convenience, these macros are also provided in				<stdlib.h>.								To determine whether or not the XCU specification's environment is present, use:												sysconf(_SC_2_VERSION)																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 will be 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 will be 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 will be closed, and the process ID and parent process ID				will be different.				Also,				if the executed utility changes its environment				variables or its current working				directory, that change will not be 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				that is guaranteed to find the				sh				utility.				 FUTURE DIRECTIONS				None.				 SEE ALSO				exec,				pipe(),				waitpid(),				<limits.h>,				<signal.h>,				<stdlib.h>,				the XCU specification.				DERIVATION				Derived from Issue 1 of the SVID.												UNIX ® is a registered Trademark of The Open Group.				Copyright © 1997 The Open Group				 [ Main Index | XSH | XCU | XBD | XCURSES | XNS ]