Implementation of system function in Linux

#include <errno.h> #include <signal.h> #include <unistd.h> int system (const char  
    *cmdstring)/* with appropriate signal handling * * pid_t pid;  
    int status;  
    struct Sigaction Ignore, saveintr, Savequit;  
      
    sigset_t Chldmask, 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  
    TR, NULL) < 0) return (-1);  
    if (Sigaction (Sigquit, &savequit, NULL) < 0) return (-1);  if (Sigprocmask (Sig_setmask, &savemask, NULL) < 0)
        Return (-1);  
return (status); }

System (execute shell command)

Related functions

Fork,execve,waitpid,popen

Table header File

#i nclude<stdlib.h>

Defining functions

int system (const char * string);

Function description

System () invokes the fork () to produce the child process, which invokes the/BIN/SH-C string to execute the command represented by the parameter string string, which then returns the process of the original call. The SIGCHLD signal is temporarily shelved during the call to System () and the SIGINT and sigquit signals are ignored.

return value

=-1: Error occurred

= 0: The call succeeded but no child processes appear

>0: ID of the child process that successfully exited

If system () fails while calling/bin/sh, 127 is returned, and the other failure reason returns-1. If the argument string is a null pointer (null), a value of not 0 > is returned.

If the system () call succeeds, the return value after executing the shell command is finally returned, but this return value is also likely to return 127 for system () call/bin/sh failure, so it is best to check errno again to confirm execution success.