execve()系统调用

execve()系统调用学习

先来一个小程序, 它的功能是打印父子进程号, 传入的全部参数及两个环境变量(如果有):

process.c

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

int main(int argc,char *argv[],char *env[])
{
int i;
　　 char *p;

printf("\n=========I am a process image!============\n");
printf("\nMy pid = %d,  parentpid = %d\n", getpid(), getppid());

for(i=0; i< argc; i++)
printf("\nargv[%d]:  %s\n",i ,argv[i]);

p = getenv("env1");
if(p != NULL)
printf("\nenv1 = %s\n",p);

p = getenv("env2");
if(p != NULL)
printf("\nenv2 = %s\n\n",p);

printf("\n============= process =================\n\n");

sleep(atoi(argv[1]));

return 99;
}

另一段代码myexec.c如下:

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>

#define ERROR(flag)                 \
if(flag)                \
{                    \
printf("%d: ",__LINE__);    \
fflush(stdout);            \
perror("error");        \
exit(errno);            \
}

int main()
{
int ret = 0;

char *argv[] = {"11","22",NULL};
char *env[] = {"env1=1111111111111","env2=2222222222222",NULL};

printf("\nthis is myexec file, pid is %d,  ppid is %d\n",getpid(),getppid());

//    ret = execl("process","11","22",NULL);
//    ret = execlp("process","11","22",NULL);
//    ret = execle("process","11","22",NULL,NULL);
//    ret = execv("process",argv);
//    ret = execvp("process",argv);
ret = execve("process",argv,env);
ERROR(ret == -1);

return 0;
}

执行"make process myexec"后, 在当前目录下生成process和myexec两个可执行文件. 执行结果如下:



porcess.c代码将myexec.c代码执行时execve()系统调用传入的参数及环境变量息数打印了出来.

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将myexec.c改造成如下代码:

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

#define ERROR(flag)                 \
if(flag)                \
{                    \
printf("%d: ",__LINE__);    \
fflush(stdout);            \
perror("error");        \
exit(errno);            \
}

int main(int argc,char *argv[],char *env[])
{
pid_t pid;

pid = fork();
//pid = vfork();
ERROR(pid == -1);
if(pid == 0)
{
printf("\nChild process is running\n");
printf("\nMy pid = %d ,parent pid = %d\n",getpid(),getppid());

char *arglist[] = {"sleep","3",NULL};
execvp("./process",arglist);
printf("process never go to here!\n");

_exit(0);
}

int stat;
pid_t child_pid;

child_pid = wait(&stat);
printf("child process has exited, pid = %d\n\n",child_pid);

if(WIFEXITED(stat))
printf("child exited with code %d\n\n",WEXITSTATUS(stat));
else
printf("child exit abnormally\n\n");

return 0;
}

执行"make myexec process" 后, 执行结果如下图:



子进程通过execvp()执行process. 父进程通过wait()取得子进程号及其(正常)退出运行时(通过WEXITSTAYUS()取得)的返回值.

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一种通过c代码运行终端命令的方法:

mysystem,c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/types.h>
#include <errno.h>

int main(int argc, char *argv[])
{
system("ls --color=tty");
return 0;
}

编译链接后运行, 其执行效果与在终端运行"ls --color=tty"命令的效果相同.