CODEWITHSUNDEEP
csocketsmultiprocessing
user2373043
user2373043

Reputation: 21

multi-process in C : global variable's value

Here is the code from a website. It used multi-processing to create a server. My question is: will the parent process close(newsockfd) executed before the child process doprocessing(newsockfd) ?

#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);
   }

}

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

code is from this website: http://www.tutorialspoint.com/unix_sockets/socket_server_example.htm

Upvotes: 0

Views: 1176

Answers (1)

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137517

When the process is forked, the gets a copy of the open set of file desriptors, and the reference count on the open filess is incremented accordingly. The close only happens in the parent process, so the child still maintains a reference to the open file. The order of execution doesn't really matter. On a multi-cpu system, it may happen truly simultaneously.

Global variables are not shared by forked child processes. A forked process runs in its own virtual memory space.

Upvotes: 1

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