Linux中的system函數的實現和解釋

#include<sys/wait.h>#include<errno.h>#include<signal.h>#include<unistd.h>intsystem(const char *cmdstring)/* with appropriate signal handling */{pid_tpid;intstatus;struct sigactionignore, saveintr, savequit;sigset_tchldmask, savemask;if (cmdstring == NULL)return(1);/* always a command processor with UNIX */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 actions & 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);}