【Linux+C语言】你真的了解system接口的调用吗？

​本期给大家介绍了一下Linux下system()函数的正确用法！

废话不多说，相信你肯定也在system接口上填过坑！先上一把它的正确操作：

#include 
#include 

#define Debuging(fmt, arg...)    printf(fmt, ##arg)

int new_system(char *cmd, char *resultout, int buflen, int *lenout)
{
    int status = system(cmd);
    Debuging("cmd=%s\n", cmd);
    if (-1 == status)  
    {  
        Debuging("system error!\n");  
    }  
    else  
    {  
        Debuging("exit status value = [0x%x]\n", status);  
  
        if (WIFEXITED(status))  
        {  
            if (0 == WEXITSTATUS(status))  
            {  
                Debuging("run shell script successfully.\n");  
                return 0;
            }  
            else  
            {  
                Debuging("run shell script fail, script exit code: %d\n", WEXITSTATUS(status));  
            }  
        }  
        else  
        {  
            Debuging("exit status = [%d]\n", WEXITSTATUS(status));  
        }  
    }  
  
    return -1;  
}

示例代码中很好地展示了，如何判断system接口的返回值！

以下是linux man中关于system接口的介绍：

SYSTEM(3)                                               Linux Programmer's Manual                                               SYSTEM(3)
NAME
      system - execute a shell command
SYNOPSIS
      #include
      int system(const char *command);
DESCRIPTION
      The system() library function uses fork(2) to create a child process that executes the shell command specified in command using
      execl(3) as follows:
          execl("/bin/sh", "sh", "-c", command, (char *) 0);
      system() returns after the command has been completed.
      During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be ignored, in the process that calls system()
      (these signals will be handled according to their defaults inside the child process that executes command).
      If command is NULL, then system() returns a status indicating whether a shell is available on the system
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.
ATTRIBUTES
      For an explanation of the terms used in this section, see attributes(7).
      ┌──────────┬───────────────┬─────────┐
      │Interface │ Attribute     │ Value   │
      ├──────────┼───────────────┼─────────┤
      │system() │ Thread safety │ MT-Safe │
      └──────────┴───────────────┴─────────┘
CONFORMING TO
      POSIX.1-2001, POSIX.1-2008, C89, C99.
NOTES
      system() provides simplicity and convenience: it handles all of the details of calling fork(2), execl(3), and waitpid(2), as well as
      the necessary manipulations of signals; in addition, the shell performs the usual substitutions and I/O redirections for command.
      The main cost of system() is inefficiency: additional system calls are required to create the process that runs the shell and to
      execute the shell.
      If the _XOPEN_SOURCE feature test macro is defined (before including any header files), then the macros described in waitpid(2)
      (WEXITSTATUS(), etc.) are made available when including .
      As mentioned, system() ignores SIGINT and SIGQUIT. This may make programs that call it from a loop uninterruptible, unless they
      take care themselves to check the exit status of the child. For example:
          while (something) {
              int ret = system("foo");
              if (WIFSIGNALED(ret) &&
                  (WTERMSIG(ret) == SIGINT || WTERMSIG(ret) == SIGQUIT))
                      break;
          }
      Do not use system() from a program with set-user-ID or set-group-ID privileges, because strange values for some environment vari‐
      ables might be used to subvert system integrity. Use the exec(3) family of functions instead, but not execlp(3) or execvp(3). sys‐
      tem() will not, in fact, work properly from programs with set-user-ID or set-group-ID privileges on systems on which /bin/sh is bash
      version 2, since bash 2 drops privileges on startup. (Debian uses a modified bash which does not do this when invoked as sh.)
      In versions of glibc before 2.1.3, the check for the availability of /bin/sh was not actually performed if command was NULL; instead
      it was always assumed to be available, and system() always returned 1 in this case. Since glibc 2.1.3, this check is performed
      because, even though POSIX.1-2001 requires a conforming implementation to provide a shell, that shell may not be available or exe‐
      cutable if the calling program has previously called chroot(2) (which is not specified by POSIX.1-2001).
      It is possible for the shell command to terminate with a status of 127, which yields a system() return value that is indistinguish‐
      able from the case where a shell could not be executed in the child process.
SEE ALSO
      sh(1), sigaction(2), sigprocmask(2), fork(2), wait(2), exec(3), signal(7)
COLOPHON
      This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs,
      and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/.
​

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总结：

1.Linux系统接口使用前务必了解其接口说明；

2.类似system接口，返回值为0的时候，并不代表命令一定执行成功了，这个需要特别注意；

3.读懂man说明，也是一项基本的技能。

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好了，本期的介绍就到这里了，你掌握了吗？