[unix]system函数的一个实现

代码来源 unix环境高级编程 第三版 一书  这个system函数实现有缺陷  具体参详10.18节

#include 
#include 
#include 
#include 
int system(const char *cmdstring)
{
	pid_t pid;
	int status;
	struct sigaction ignore,saveintr,savequit;
	sigset_t chldmask, savemask;

	if(cmdstring == NULL)
		return(1);

	ignore.sa_handler = SIG_IGN; /* ignore SIGINT and SIGQUIT */
	sigemptyset(&ignore.sa_mask);
	ignore.sa_flags = 0;
	if(sigaction(SIGINT,&ignore,&saveintr) < 0)
		return(-1);

	if(sigaction(SIGQUIT,&ignore,&savequit) < 0)
		return(-1);

	sigemptyset(&chldmask);   /*now block SIGCHLD*/
	sigaddset(&chldmask,SIGCHLD);
	if(sigprocmask(SIG_BLOCK,&chldmask,&savemask) < 0)
		return(-1);
	if((pid = fork()) < 0)
		status = -1;  /* probably out of processes*/
	else if(pid == 0 ){  /* child */
		/* restore previous signal  action & reset signal mask*/
		sigaction(SIGINT,&saveintr,NULL);
		sigaction(SIGQUIT,&savequit,NULL);
		sigprocmask(SIG_SETMASK,&savemask,NULL);
		execl("/bin/sh","sh","-c",cmdstring,(char *)0);
		_exit(127);  /*exec error */
	}else{  /*parent*/
		while(waitpid(pid,&status,0) < 0){
			if(errno != EINTR){
				status = -1;  /* error other than EINTR from waitpid() */
				break;
			}
		}
	}
	/*restore previous signal actions & reset signal mask*/
	if(sigaction(SIGINT,&saveintr,NULL) < 0)
		return(-1);
	if(sigaction(SIGQUIT,&savequit,NULL) < 0)
		return(-1);
	if(sigprocmask(SIG_SETMASK,&savemask,NULL) < 0)
		return(-1);

	return(status);
}

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