How to implement a server on C sockets using the select () system call. You need to write an echo server to which several clients could connect without using fork () and without creating threads, that is, using select ().

Here I also found an example of a suitable code, it is clear what needs to be done in order for the server to send the same messages to the client that the client sent to him, but it is not clear how and where to use select ().

#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[]) { 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"); listen(sockfd,5); clilen = sizeof(cli_addr); 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; }

    1 answer 1

    Better to use poll (). Here is my example:

     #include <errno.h> #include <unistd.h> #include <stdint.h> #include <stdio.h> #include <string.h> #include <sys/socket.h> #include <arpa/inet.h> #include <poll.h> #define MAX_CLIENTS 8 #define SERVER_PORT 10000 int main() { int listen_sock_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (listen_sock_fd < 0) { printf("server: Cannot create TCP socket: %s\n", strerror(errno)); return -1; } int opt = 1; if (0 != setsockopt(listen_sock_fd, SOL_SOCKET, SO_REUSEADDR, (const void*) &opt, sizeof(opt))) { printf("server: Error: could not allow reuse address on socket: %s\n", strerror(errno)); close(listen_sock_fd); return 0; } struct sockaddr_in servaddr = {0}; servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htonl(INADDR_ANY); servaddr.sin_port = htons(SERVER_PORT); if (-1 == bind(listen_sock_fd, (struct sockaddr*)&servaddr, sizeof(servaddr))) { printf("server: Error on call 'bind': %s\n", strerror(errno)); return -1; } if (-1 == listen(listen_sock_fd, 1)) { printf("server: Error on call 'listen': %s\n", strerror(errno)); return -1; } struct pollfd pfd_array[1 /* listen */ + MAX_CLIENTS] = { { .fd = listen_sock_fd, .events = POLLIN } }; int i; for (i = 0; i < MAX_CLIENTS; i++) { pfd_array[1 + i].fd = -1; } struct pollfd *listen_sock = pfd_array; int clients_count = 0; while (1) { int ret = poll(pfd_array, 1 + MAX_CLIENTS, -1 /* infinity */); if (ret == -1) { printf("Error on call 'poll': %s", strerror(errno)); return -1; } else { // ΠΎΠ±Ρ€Π°Π±ΠΎΡ‚ΠΊΠ° ΠΏΠΎΠΏΡ‹Ρ‚ΠΊΠΈ ΠΏΠΎΠ΄ΠΊΠ»ΡŽΡ‡Π΅Π½ΠΈΡ ΠΊΠ»ΠΈΠ΅Π½Ρ‚Π° if ((listen_sock->revents & POLLIN) != 0) { listen_sock->revents &= ~POLLIN; struct sockaddr_in cliaddr; socklen_t clilen = sizeof(struct sockaddr_in); int sock_fd = accept(listen_sock_fd, (struct sockaddr*)&cliaddr, &clilen); if (sock_fd > 0) { // ΠΎΠ³Ρ€Π°Π½ΠΈΡ‡Π΅Π½ΠΈΠ΅ числа ΠΏΠΎΠ΄ΠΊΠ»ΡŽΡ‡Π΅Π½ΠΈΠΉ if (clients_count == MAX_CLIENTS) { close(sock_fd); printf("%d clients already connected, unexpected new connection have discarded\n", MAX_CLIENTS); } else { clients_count++; opt = 1; setsockopt(sock_fd, SOL_SOCKET, SO_REUSEADDR, (const void*)&opt, sizeof(opt)); for (i = 0; i < MAX_CLIENTS; i++) { if (pfd_array[1 + i].fd == -1) { pfd_array[1 + i].fd = sock_fd; pfd_array[1 + i].events = POLLIN; printf("Client %d has been connected\nClients count = %d\n", i, clients_count); break; } } } } } for (i = 0; i < MAX_CLIENTS; i++) { if (pfd_array[1 + i].fd != -1 && (pfd_array[1 + i].revents & POLLIN) != 0) { pfd_array[1 + i].revents &= ~POLLIN; uint8_t buf[64]; int ret = recv(pfd_array[1 + i].fd, buf, 64, 0); if (ret < 0) { printf("Error on call 'recv': %s\n", strerror(errno)); return -1; } else if (ret == 0) { close(pfd_array[1 + i].fd); pfd_array[1 + i].fd = -1; pfd_array[1 + i].events = 0; clients_count--; printf("Client %d has been disconnected\nClients count = %d\n\n", i, clients_count); } else { printf("%d bytes received from client %d\n", ret, i); printf("Message from client %d: %.*s\n", i, ret, (char *)buf); if(-1 == send(pfd_array[1 + i].fd, (void *)"Response", 8, 0)) { printf("Error on call 'send': %s\n", strerror(errno)); return -1; } } } } } } }
    • not that β€œbetter”, but rather a matter of taste and habit ... - Fat-Zer
    • well, not quite. I won't argue) - ASh