每个应用程序(程序)都是通过进程来执行的，进程是程序的一个运行实例。进程是通过不同的系统调用创建的，最常用的是fork()和exec()

fork()

pid_t pid = fork();

fork()通过复制调用进程来创建一个新进程，这个新进程被称为子进程，是调用进程的精确副本，被称为父进程，除了以下情况:

1. 子进程有自己唯一的进程ID，并且这个PID与任何现有进程组的ID不匹配。
2. 子进程的父进程ID与父进程ID相同。
3. 子进程不会继承父进程的内存锁和信号量调整。
4. 子进程不会从父进程继承未完成的异步I/O操作，也不会从父进程继承任何异步I/O上下文。

fork()如果成功，子进程的PID在父进程中返回，子进程返回0。失败时，父进程中返回-1，不创建子进程，并适当地设置errno。关于fork系统调用的详细文章

exec()

exec()系列函数用一个新的进程映像替换当前进程映像。它将程序加载到当前进程空间中，并从入口点运行它。exec()家族由以下函数组成，我在下面的C程序中实现了execv()，您可以尝试休息作为练习

int execl(const char *path, const char *arg, ...);
int execlp(const char *file, const char *arg, ...);
int execle(const char *path, const char *arg, ..., 
                               char * const envp[]);
int execv(const char *path, char *const argv[]);
int execvp(const char *file, char *const argv[]);
int execvpe(const char *file, char *const argv[], 
                              char *const envp[]);

fork与exec

• Fork启动一个新进程，它是调用它的进程的副本，而exec用另一个(不同的)进程映像替换当前进程映像。
• 在fork()的情况下，父进程和子进程会同时执行，而Control永远不会返回到原始程序，除非出现exec()错误。

// C program to illustrate  use of fork() &
// exec() system call for process creation
 
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h> 
#include <sys/wait.h>
 
int main(){
   pid_t  pid;
   int ret = 1;
   int status;
   pid = fork();
 
   if (pid == -1){
 
      // pid == -1 means error occurred
      printf("can't fork, error occurred\n");
      exit(EXIT_FAILURE);
   }
   else if (pid == 0){
 
      // pid == 0 means child process created
      // getpid() returns process id of calling process
      // Here It will return process id of child process
      printf("child process, pid = %u\n",getpid());
      // Here It will return Parent of child Process means Parent process it self
      printf("parent of child process, pid = %u\n",getppid());
 
      // the argv list first argument should point to 
      // filename associated with file being executed
      // the array pointer must be terminated by NULL
      // pointer
      char * argv_list[] = {"ls","-lart","/home",NULL};
 
      // the execv() only return if error occurred.
      // The return value is -1
      execv("ls",argv_list);
      exit(0);
   }
   else{
      // a positive number is returned for the pid of
      // parent process
      // getppid() returns process id of parent of
      // calling process
// Here It will return parent of parent process's ID
      printf("Parent Of parent process, pid = %u\n",getppid());
      printf("parent process, pid = %u\n",getpid());
 
        // the parent process calls waitpid() on the child
        // waitpid() system call suspends execution of
        // calling process until a child specified by pid
        // argument has changed state
        // see wait() man page for all the flags or options
        // used here
        if (waitpid(pid, &status, 0) > 0) {
             
            if (WIFEXITED(status) && !WEXITSTATUS(status))
              printf("program execution successful\n");
             
            else if (WIFEXITED(status) && WEXITSTATUS(status)) {
                if (WEXITSTATUS(status) == 127) {
 
                    // execv failed
                    printf("execv failed\n");
                }
                else
                    printf("program terminated normally,"
                       " but returned a non-zero status\n");               
            }
            else
               printf("program didn't terminate normally\n");           
        }
        else {
           // waitpid() failed
           printf("waitpid() failed\n");
        }
      exit(0);
   }
   return 0;
}

输出:

parent process, pid = 11523
child process, pid = 14188
Program execution successful

让我们用表格的形式来看看它们的区别: