system()函数-苹果udid定制软件-程序博客网

函数说明：system()会调用fork()产生子进程, 由子进程来调用/bin/sh-c string 来执行参数string 字符串所代表的命令, 此命令执行完后随即返回原调用的进程. 在调用system()期间SIGCHLD 信号会被暂时搁置,SIGINT 和SIGQUIT 信号则会被忽略.

　　返回值：
　　1、如果 system()在调用/bin/sh 时失败则返回127, 其他失败原因返回-1.。
　　2、若参数string 为空指针(NULL), 则返回非零值.
　　3、如果system()调用成功则最后会返回执行shell 命令后的返回值, 但是此返回值也有可能为system()调用/bin/sh 失败所返回的127, 因此最好能再检查errno 来确认执行成功.
　　附加说明：在编写具有 SUID/SGID 权限的程序时请勿使用system(), system()会继承环境变量, 通过环境变量可能会造成系统安全的问题.

linux版system函数的源码：

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

int system(const char * cmdstring)
{
pid_t pid;
int status;

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

    if((pid = fork())<0){
            status = -1;
    }

    else if(pid = 0){
        execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
        -exit(127); //子进程正常执行则不会执行此语句
        }

    else{
           while(waitpid(pid, &status, 0) < 0){
                if(errno != EINTER){
                    status = -1;
                    break;
                }
            }
        }

        return status;
}

man system

SYSTEM(3) Linux Programmer’s Manual SYSTEM(3)

NAME
system - execute a shell command

SYNOPSIS
#include <stdlib.h>

int system(const char *command);

DESCRIPTION
       system() executes a command specified in command by calling /bin/sh -c command, and returns after the command
       has been completed. During execution of the command, SIGCHLD will be blocked, and SIGINT and SIGQUIT will be
       ignored.

RETURN VALUE
       The value returned is -1 on error (e.g. fork() failed), and the return status of the command otherwise. This
       latter return status is in the format specified in wait(2). Thus, the exit code of the command will be WEXIT-
       STATUS(status).   In case /bin/sh could not be executed, the exit status will be that of a command that does
       exit(127).

If the value of command is NULL, system() returns non-zero if the shell is available, and zero if not.

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

CONFORMING TO
C89, C99, POSIX..1-2001.

NOTES
If the _XOPEN_SOURCE feature test macro is defined, then the macros described in wait(2) (WEXITSTATUS(), etc.)
are made available when including <stdlib.h>.

As mentioned, system() ignores SIGINT and SIGQUIT. This may make programs that call it from a loop uninter-
ruptible, unless they take care themselves to check the exit status of the child. E.g.

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 variables might be used to subvert system integrity. Use the exec(3) family of functions instead,
       but not execlp(3) or execvp(3). system() 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 com-
       mand 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 implementa-
       tion to provide a shell, that shell may not be available or executable 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 return 127, so that code is not a sure indication that the execve()
call failed.

SEE ALSO
sh(1), signal(2), wait(2), exec(3)

2004-12-20 SYSTEM(3)