07_system 库函数使用

System
   功能：system()函数调用“/bin/sh -c command”执行特定的命令，阻塞当前进程直到command命令执行完毕
   原型：
    int system(const char *command);
   返回值：
    如果无法启动shell运行命令，system将返回127；出现不能执行system调用的其他错误时返回-1。如果system能够顺利执行，返回那个命令的退出码。
   system函数执行时，会调用fork、execve、waitpid等函数。

#include <sys/types.h>
#include <unistd.h>

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <signal.h>
#include <errno.h>
#include <signal.h>
#include <errno.h>
#include <sys/wait.h>


/*
    system()函数的使用
    int system(const char *command);
RETURN VALUE
       The return value of system() is one of the following:

       *  If command is NULL, then a nonzero value if a shell is available, or 0 if  no
          shell is available.

       *  If  a  child  process  could  not  be  created,  or  its  status could not be
          retrieved, the return value is -1.

       *  If a shell could not be executed in the child process, then the return  value
          is  as  though the child shell terminated by calling _exit(2) with the status
          127.

       *  If all system calls succeed, then the return value is the termination  status
          of  the  child  shell  used to execute command.  (The termination status of a
          shell is the termination status of the last command it executes.)

       In the last two cases, the return value is a "wait status" that can be  examined
       using  the macros described in waitpid(2).  (i.e., WIFEXITED() WEXITSTATUS() and
       so on).

       system() does not affect the wait status of any other children.


*/
#define ERR_EXIT(m) 
    do 
    { 
        perror(m); 
        exit(-1); 
    }while(0)


    void pr_exit(int status)
{
    if( WIFEXITED(status)){
            printf("子进程正常终止，退出码 exit status= %dn",  WEXITSTATUS(status));
    }else if(   WIFSIGNALED(status)){
            printf("子进程异常终止，退出码 signal number= %d%sn",  WTERMSIG(status),
            #ifdef WCOREDUMP
                WCOREDUMP(status) ? "(core file generated)" : "");
            #else
                "");
            #endif

        }else if(   WIFSTOPPED(status)){
            printf("子进程 stopped，退出码 signal number= %dn",  WSTOPSIG(status));
        }
}

    //实现system的原理 
    //暂时先忽略信号处理 posix1.1要求system()忽略SIGINT和SIGQUIT,阻塞SIGCHLD
    int my_system(const char * command){
        pid_t pid;
        int status;

        if(command == NULL){
            return 1;
        }

        if((pid = fork()) < 0){
            status = -1;    
        }else if(pid == 0){
            execl("/bin/sh", "sh", "-c", command, NULL);//execle执行新的进程以后，pid不会发生变化
            printf("execl 执行失败,如同shell执行了exit(127)n");
            _exit(127); //不能用exit，是为了防止任一标准I/O缓冲
            //(这些缓冲会在fork中由父进程复制给子进程)在子进程中被冲洗
        }else{
            while(waitpid(pid, &status, 0) < 0){//waitpid记录了执行shell的终止状态
                if(errno == EINTR)
                    continue;
                    status = -1;
                    break;  
            }
        }
        return status;
    }

void test()
{
    int status;
    printf("-------------date-----------------------n");
    if((status = system("date")) < 0)
        ERR_EXIT("system() error");
    pr_exit(status);
    printf("---------------nosuchcommand---------------------n");
    if((status = system("nosuchcommand")) < 0)
        ERR_EXIT("system() error");
    pr_exit(status);
        printf("---------------who; exit 44---------------------n");
    if((status = system("who; exit 44")) < 0)
        ERR_EXIT("system() error"); 
    pr_exit(status);
}
int main(void )
{
    test();
    printf("------------my_system()------------------------n");    
    my_system("ls -l | wc -w");

    printf("hello...n");
    exit(0);
}

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