How do I use setsockopt(SO_REUSEADDR)?

Question

I am running my own http server on a raspberry pi. The problem is when I stop the program and restart it, the port is no longer available. Sometimes I get the same issue when receiving lots of requests.
I want to use SO_REUSEADDR so that I can keep using the port even when the error occurs but have had no luck getting it set up. Below is my code.
The error I get is "ERROR on binding:Address already in use".

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/types.h>  #include <sys/socket.h> #include <netinet/in.h>  void error(const char *msg) {     perror(msg);     exit(1); }  int main(int argc, char *argv[]) {     printf("Starting Listener\n");      int sockfd, newsockfd, portno;      socklen_t clilen;      char buffer[256];      struct sockaddr_in serv_addr, cli_addr;      int n;      if (argc < 2) {          fprintf(stderr,"ERROR, no port provided\n");          exit(1);      }      sockfd = socket(AF_INET, SOCK_STREAM, 0);      if (sockfd < 0)          error("ERROR opening socket");      bzero((char *) &serv_addr, sizeof(serv_addr));      portno = atoi(argv[1]);      serv_addr.sin_family = AF_INET;      serv_addr.sin_addr.s_addr = INADDR_ANY;      serv_addr.sin_port = htons(portno);      if (bind(sockfd, (struct sockaddr *) &serv_addr,               sizeof(serv_addr)) < 0)                error("ERROR on binding");       printf("about to listen\n");      listen(sockfd,5);      printf("finished listening\n");      clilen = sizeof(cli_addr);      printf("About to accept\n");       int i;      for(i=0; i<100; i++){          newsockfd = accept(sockfd,                   (struct sockaddr *) &cli_addr,                   &clilen);           if (newsockfd < 0)               error("ERROR on accept");          bzero(buffer,256);          n = read(newsockfd,buffer,255);          if (n < 0) error("ERROR reading from socket");          printf("Here is the message: %s\n",buffer);          n = write(newsockfd,"I got your message",18);          if (n < 0) error("ERROR writing to socket");          close(newsockfd);      }      close(sockfd);      return 0;  } 

Answer 1

After :

sockfd = socket(AF_INET, SOCK_STREAM, 0); if (sockfd < 0)      error("ERROR opening socket"); 

You can add (with standard C99 compound literal support) :

if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &(int){1}, sizeof(int)) < 0)     error("setsockopt(SO_REUSEADDR) failed"); 

Or :

int enable = 1; if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(int)) < 0)     error("setsockopt(SO_REUSEADDR) failed");