exec家族函数

NAMEexecl, execlp, execle, execv, execvp, execvpe - execute a fileSYNOPSIS#include extern char **environ;int execl(const char *pathname, const char *arg, .../* (char  *) NULL */);int execlp(const char *file, const char *arg, .../* (char  *) NULL */);int execle(const char *pathname, const char *arg, .../*, (char *) NULL, char *const envp[] */);int execv(const char *pathname, char *const argv[]);int execvp(const char *file, char *const argv[]);int execvpe(const char *file, char *const argv[],char *const envp[]);

可以观察一些这一组函数的后缀名称，无非就是在exec函数后面加上 l，p，e，v这几个字母。

1. l : list => 函数参数列表指定进程参数。

2. p:path => 到PATH环境变量这个路径中查找可执行程序

3. e:environ => 父进程提供环境变量

4. v:数组的方式提供进程参数

看一下demo：

#include 
#include 
#include 
#include 
#include int main(int argc,char* argv[])
{char  pids[32] = {0};char* const ps_argv[] = {"pstree","-A","-p","-s",pids,NULL};char* const ps_envp[] = {"PATH=/bin:/usr/bin","TEST=Delphi",NULL};sprintf(pids,"%d",getpid());execl("/bin/pstree","pstree","-A","-p","-s",pids,NULL);execl("pstree","pstree","-A","-p","-s",pids,NULL);execle("/bin/pstree","pstree","-A","-p","-s",pids,NULL,ps_envp);execv("/bin/pstree",ps_argv);execvp("pstree",ps_argv);execve("/bin/pstree",ps_argv,ps_envp);return 0;
}

system函数

NAMEsystem - execute a shell commandSYNOPSIS#include int system(const char *command);

参数：程序名以及进程参数(如：pstree -A -p -s $$)

返回值：进程退出状态值

下面是system(...) 函数运行原理图，和我们自己创建的create_process(...)原理差不多。

来看一下demo1:

#include 
#include 
#include 
#include 
#include /*$$ : 表示当前进程的pid
*/int main(int argc,char* argv[])
{int result = 0;printf("current: %d\n",getpid());result = system("pstree -A -p -s $$");printf("result = %d\n",result);return 0;
}

编译输出：

wj@wj:~/WORK/Learning/DT/07/07-3$ gcc test.c -o test.out
wj@wj:~/WORK/Learning/DT/07/07-3$ ./test.out 
current: 72319
systemd(1)---systemd(1454)---gnome-terminal-(2307)---bash(57230)---test.out(72319)---sh(72320)---pstree(72321)
result = 0

看一下demo2:

//test.sh

echo "Hello world from shell..."
a=1
b=1
c=$(($a+$b))
echo "c = $c"

wj@wj:~/WORK/Learning/DT/07/07-3$ chmod 777 test.sh
wj@wj:~/WORK/Learning/DT/07/07-3$ ./test.sh
Hello world from shell...
c = 2

看一下demo3:

//test.c
#include 
#include 
#include 
#include 
#include /*wait: true  :代表父进程是否等待子进程结束wait: false :代表父进程不等待子进程结束
*/int create_process(char* path,char* const args[],char* const env[],int wait)
{int ret = fork();if(ret == 0){if(execve(path,args,env) == -1){exit(-1);}}if(wait && ret)  // 1.wait为true。2.并且父进程是否成功创建子进程？来共同解决父进程是否等待子进程结束？{waitpid(ret,&ret,0);}return ret;   //1.如果等待，ret为子进程的退出状态；2.如果不等待，返回为子进程的pid
}int main(int argc,char* argv[])
{char* target = argv[1];char* const ps_argv[] = {target,NULL};char* const ps_envp[] = {"PATH=/bin:/usr/bin","TEST=Delphi",NULL};int result =0;if(argc < 2) exit(-1);printf("current: %d\n",getpid());//result = create_process(target,ps_argv,ps_envp,1);result = system(target);printf("result = %d\n",result);return 0;
}

//helloworld.c
#include int main(int argc,char* argv[])
{printf("hello world\n");return 0;
}

编译运行输出：

wj@wj:~/WORK/Learning/DT/07$ gcc test.c -o test.out
wj@wj:~/WORK/Learning/DT/07$ ./test.out ./helloworld.out
current: 73126
hello world
result = 0

wj@wj:~/WORK/Learning/DT/07$ cp 07-3/test.sh ./
wj@wj:~/WORK/Learning/DT/07$ ls
07-3  helloworld.c  helloworld.out  test.c  test.out  test.shwj@wj:~/WORK/Learning/DT/07$ ./test.out ./test.sh
current: 73178
Hello world from shell...
c = 2
result = 0

system 和 create_process区别和联系

1. system创建的是shell进程，功能比较强大，可以直接调用shell进程函数，但是这个功能强大(相对于我们自己创建的create_process函数来说)是以牺牲性能为代价的。

2. 如果用 //result = create_process(target,ps_argv,ps_envp,1); 我们自己创建的create_process，则不能够调用shell进程函数。我们自己创建的create_process效率更高。