server.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <signal.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <unistd.h>

#define PORT "3490"
#define BACKLOG 10 //how many pending connections in queue

void sigchld_handler(int s)
{
    int saved_errno = errno;
    while(waitpid(-1,NULL,WNOHANG) > 0);
    errno = saved_errno;
}

void *get_in_addr(struct sockaddr *sa)
{
    if(sa->sa_family == AF_INET){
        return &(((struct sockaddr_in*)sa)->sin_addr);
    }
    return &(((struct sockaddr_in6*)sa)->sin6_addr);
}

int main(int argc, char*argv[])
{
    int status;
    int yes = 1;
    int sockfd,new_fd; //listen on sock_fd,new connection on new_fd
    struct addrinfo hints,*servinfo,*p;
    struct sigaction sa;
    char s[INET6_ADDRSTRLEN];

    memset(&hints,0,sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE; // use my IP
    if((status = getaddrinfo(NULL,PORT,&hints,&servinfo))!=0){
        fprintf(stderr,"getaddrinfo: %s\n",gai_strerror(status));
        return 1;
    }

    for(p = servinfo;p!=NULL;p = p->ai_next){
        if((sockfd = socket(p->ai_family,p->ai_socktype,p->ai_protocol)) == -1){
            perror("server:socket");
            continue;
        }
        if(setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(int)) == -1){
            perror("setsockopt");
            exit(1);
        }
        if(bind(sockfd,p->ai_addr,p->ai_addrlen) == -1){
            close(sockfd);
            perror("server:bind");
            continue;
        }
        break;
    }
    freeaddrinfo(servinfo);

    if(p == NULL){
        fprintf(stderr,"server: failed to bind\n");
        exit(1);
    }
    if(listen(sockfd,BACKLOG) == -1){
        perror("listen");
        exit(1);
    }

    sa.sa_handler = sigchld_handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;
    if(sigaction(SIGCHLD,&sa,NULL) == -1){
        perror("sigaction");
        exit(1);
    }
    printf("server: waiting for connections...\n");

    while(1){
        socklen_t sin_size;
        struct sockaddr_storage their_addr;
        sin_size = sizeof their_addr;
        new_fd = accept(sockfd,(struct sockaddr *)&their_addr,&sin_size);
        if(new_fd == -1){
            perror("accept");
            continue;
        }
        inet_ntop(their_addr.ss_family,get_in_addr((struct sockaddr*)&their_addr),s,sizeof s);
        printf("server: got connection from %s\n",s);

        if(!fork()){
            close(sockfd);
            if(send(new_fd,"Hello, world",13,0) == -1){
                perror("send");
            }
            close(new_fd);
            exit(0);
        }
        close(new_fd);
    }
    return 0;
}