c - C 中的多进程 : global variable's value

Question

这是来自网站的代码。它使用多处理来创建服务器。我的问题是:父进程 close(newsockfd) 会在子进程 doprocessing(newsockfd) 之前执行吗？

#include <stdio.h>
#include <stdlib.h>

#include <netdb.h>
#include <netinet/in.h>

#include <string.h>

void doprocessing (int sock);

int main( int argc, char *argv[] ) {
   int sockfd, newsockfd, portno, clilen;
   char buffer[256];
   struct sockaddr_in serv_addr, cli_addr;
   int n, pid;

   /* First call to socket() function */
   sockfd = socket(AF_INET, SOCK_STREAM, 0);

   if (sockfd < 0) {
      perror("ERROR opening socket");
      exit(1);
   }

   /* Initialize socket structure */
   bzero((char *) &serv_addr, sizeof(serv_addr));
   portno = 5001;

   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = INADDR_ANY;
   serv_addr.sin_port = htons(portno);

   /* Now bind the host address using bind() call.*/
   if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
      perror("ERROR on binding");
      exit(1);
   }

   /* Now start listening for the clients, here
      * process will go in sleep mode and will wait
      * for the incoming connection
   */

   listen(sockfd,5);
   clilen = sizeof(cli_addr);

   while (1) {
      newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);

      if (newsockfd < 0) {
         perror("ERROR on accept");
         exit(1);
      }

      /* Create child process */
      pid = fork();

      if (pid < 0) {
         perror("ERROR on fork");
         exit(1);
      }

      if (pid == 0) {
         /* This is the client process */
         close(sockfd);
         doprocessing(newsockfd);
         exit(0);
      }
      else {
         close(newsockfd);
      }

   } /* end of while */
}

===-=================
void doprocessing (int sock) {
   int n;
   char buffer[256];
   bzero(buffer,256);
   n = read(sock,buffer,255);

   if (n < 0) {
      perror("ERROR reading from socket");
      exit(1);
   }

   printf("Here is the message: %s\n",buffer);
   n = write(sock,"I got your message",18);

   if (n < 0) {
      perror("ERROR writing to socket");
      exit(1);
   }

}

==========-============================

代码来自这个网站: http://www.tutorialspoint.com/unix_sockets/socket_server_example.htm

Answer 1

当进程被 fork 时，将获取打开的文件描述符集的副本，并且打开文件的引用计数相应地增加。 close 仅发生在父进程中，因此子进程仍保留对打开文件的引用。执行顺序并不重要。在多 CPU 系统上，它可能真正同时发生。

全局变量不被 fork 的子进程共享。 fork 进程在其自己的虚拟内存空间中运行。