1.
#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/wait.h> int x; int main(int argc, char *argv[]) { printf("This program pid is (%d), x is %d\n", (int)getpid(), x); int rc = fork(); if (rc < 0) { fprintf(stderr, "fork failed\n"); exit(1); } else if (rc == 0) { wait(NULL); printf("I am child (pid:%d), my x is %d\n", (int)getpid(), x); x = 3; printf("x in child now changed to %d\n", x); } else { x = 100; printf("I am parent of %d (pid:%d), my x is %d\n", rc, (int)getpid(), x); } return 0; }结果如下图所示： 可以看出，当主进程改变变量x的值时，子进程x变量的值不会改变，当子进程和父进程都改变x的值时，x在父子进程会分别变成不一样的值。
2.
#include <stdio.h> #include <stdlib.h> #include <fcntl.h> #include <unistd.h> int main(int argc, char *argv[]) { char buffer[13]; int in, out; in = open("./52.txt", O_RDONLY); out = open("./52.txt", O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR); int cread, pread; int rc = fork(); if (rc < 0) { fprintf(stderr, "fork failed\n"); exit(1); } else if (rc == 0) { cread = read(in, buffer, 13); if (cread == -1) { printf("child cannot visit file\n"); } else { printf("child can visit file\n"); } write(out, "I am child\n", 10); } else { pread = read(in, buffer, 13); if (pread == -1) { printf("parent cannot visit file\n"); } else { printf("parent can visit file\n"); } write(out, "I am parent\n", 12); } return 0; }结果如下所示： 因此，子进程和父进程都可以访问open()返回的文件描述符，当它们并发写入文件时，父子进程都能将内容写入文件。
3.
#include <stdio.h> #include <stdlib.h> #include <unistd.h> int main(int argc, char *argv[]) { printf("This program pid is (%d)\n", (int)getpid()); int rc = vfork(); if (rc < 0) { fprintf(stderr, "fork failed\n"); exit(1); } else if (rc == 0) { printf("I am child (pid:%d), hello\n", (int)getpid()); exit(0); } else { printf("I am parent of %d (pid:%d), goodbye\n", rc, (int)getpid()); } return 0; }使用vfork创建一个子进程能保证子进程先运行，子进程调用exec或者exit之前父进程处于阻塞等待状态。
