Linux C fork函数使用

基本使用

#include
#include
#include
 
int main(){
        printf("main\n");
        pid_t myId = getpid();
        printf("myId:%d\n", myId);
        pid_t pid = fork();
        if(pid == 0){
                printf("i'am child process\n");
        }else if(pid > 0){
                printf("i'am parent process, new child process:%d\n", pid);
        }else{
                printf("fork failed");
        }
        printf("%d, exit\n", getpid());
        return 0;
}

输出

main
myId:8628
i'am parent process, new child process:8629
8628, exit
i'am child process
8629, exit

注意，fork()调用之后的代码两个进程都会执行，通过判断fork()返回值是否为0确认是否是子进程。

变量的拷贝

#include
#include
#include
 
int main(){
        pid_t myId = getpid();
        printf("[%d] main\n", myId);
        int i = 0;
        while(i < 2){
                printf("[%d] [@%ld]i=%d\n",  getpid(), &i,  i);
                pid_t pid = fork();
                if(pid == 0){
                        printf("[%d] i'am child process:%d\n",getpid(), getpid());
                }else if(pid > 0){
                        printf("[%d] i'am parent process:%d, new child process:%d\n", getpid(), getpid(), pid);
                }else{
                        printf("[%d] fork failed", getpid());
                }
                i++;
                printf("[%d] set i = %d\n", getpid(), i);
        }
        printf("[%d] end\n", getpid());
        sleep(60);
        return 0;
}

运行结果

yeqiang@yeqiang-PC:~/tmp$ ./a.out 
[11145] main
[11145] [@140732325423988]i=0
[11145] i'am parent process:11145, new child process:11146
[11145] set i = 1
[11145] [@140732325423988]i=1
[11146] i'am child process:11146
[11146] set i = 1
[11145] i'am parent process:11145, new child process:11147
[11146] [@140732325423988]i=1
[11145] set i = 2
[11145] end
[11147] i'am child process:11147
[11147] set i = 2
[11147] end
[11146] i'am parent process:11146, new child process:11148
[11146] set i = 2
[11146] end
[11148] i'am child process:11148
[11148] set i = 2
[11148] end

pstree

yeqiang@yeqiang-PC:~/tmp$ pstree -p 11145
a.out(11145)─┬─a.out(11146)───a.out(11148)
             └─a.out(11147)

说明，每一次fork，都会拷贝变量到新的进程，其进程看到的虚拟地址是一样的。