Learn Blocking and Nonblocking IO | Building Network Applications

Definition

Nonblocking mode offers an alternative. In this mode, socket operations return immediately, even if they would normally block. You can attempt to read or write, and if the operation cannot be completed right away, the function will return with an error (such as EWOULDBLOCK). This allows your program to continue doing other work, but you need a way to know when sockets are ready for reading or writing.

Understanding how your network program interacts with sockets is crucial for building efficient applications. By default, most socket operations in C are blocking. This means when you call functions like recv() or accept(), your program will pause and wait until the operation completes, such as when data arrives or a client connects. This approach is simple, but it can cause your application to become unresponsive if a socket takes a long time to respond.

blocking_nonblocking_select.c


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#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/select.h>

#define PORT 9090

int main() {
    int listen_fd, client_fd, max_fd, activity;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addrlen = sizeof(client_addr);
    char buffer[1024];
    fd_set readfds;

    // Create socket
    listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (listen_fd < 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // Set to nonblocking
    int flags = fcntl(listen_fd, F_GETFL, 0);
    fcntl(listen_fd, F_SETFL, flags | O_NONBLOCK);

    // Bind
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(PORT);

    if (bind(listen_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind failed");
        close(listen_fd);
        exit(EXIT_FAILURE);
    }

    if (listen(listen_fd, 5) < 0) {
        perror("listen failed");
        close(listen_fd);
        exit(EXIT_FAILURE);
    }

    printf("Listening on port %d...\n", PORT);

    while (1) {
        FD_ZERO(&readfds);
        FD_SET(listen_fd, &readfds);
        max_fd = listen_fd;

        // Wait for activity on the listening socket (no connected clients in this simple example)
        activity = select(max_fd + 1, &readfds, NULL, NULL, NULL);

        if (activity < 0 && errno != EINTR) {
            perror("select error");
        }

        if (FD_ISSET(listen_fd, &readfds)) {
            client_fd = accept(listen_fd, (struct sockaddr *)&client_addr, &addrlen);
            if (client_fd < 0) {
                if (errno != EWOULDBLOCK && errno != EAGAIN) {
                    perror("accept failed");
                }
            } else {
                printf("New connection accepted.\n");
                // Set client socket to nonblocking
                int cflags = fcntl(client_fd, F_GETFL, 0);
                fcntl(client_fd, F_SETFL, cflags | O_NONBLOCK);

                // Try to read data (will not block)
                ssize_t bytes = recv(client_fd, buffer, sizeof(buffer) - 1, 0);
                if (bytes > 0) {
                    buffer[bytes] = '\0';
                    printf("Received: %s\n", buffer);
                } else if (bytes == 0) {
                    printf("Client disconnected.\n");
                } else {
                    if (errno != EWOULDBLOCK && errno != EAGAIN) {
                        perror("recv failed");
                    }
                }
                close(client_fd);
            }
        }
    }

    close(listen_fd);
    return 0;
}

This is where the select() function comes in. select() enables you to monitor multiple sockets at once, waiting until at least one is ready for some operation (like reading or writing). With select(), you can efficiently handle many connections within a single-threaded program, responding only when sockets are ready, and avoiding unnecessary blocking.

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Section 2. Chapter 4

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Definition

blocking_nonblocking_select.c


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#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/select.h>

#define PORT 9090

int main() {
    int listen_fd, client_fd, max_fd, activity;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addrlen = sizeof(client_addr);
    char buffer[1024];
    fd_set readfds;

    // Create socket
    listen_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (listen_fd < 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // Set to nonblocking
    int flags = fcntl(listen_fd, F_GETFL, 0);
    fcntl(listen_fd, F_SETFL, flags | O_NONBLOCK);

    // Bind
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(PORT);

    if (bind(listen_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind failed");
        close(listen_fd);
        exit(EXIT_FAILURE);
    }

    if (listen(listen_fd, 5) < 0) {
        perror("listen failed");
        close(listen_fd);
        exit(EXIT_FAILURE);
    }

    printf("Listening on port %d...\n", PORT);

    while (1) {
        FD_ZERO(&readfds);
        FD_SET(listen_fd, &readfds);
        max_fd = listen_fd;

        // Wait for activity on the listening socket (no connected clients in this simple example)
        activity = select(max_fd + 1, &readfds, NULL, NULL, NULL);

        if (activity < 0 && errno != EINTR) {
            perror("select error");
        }

        if (FD_ISSET(listen_fd, &readfds)) {
            client_fd = accept(listen_fd, (struct sockaddr *)&client_addr, &addrlen);
            if (client_fd < 0) {
                if (errno != EWOULDBLOCK && errno != EAGAIN) {
                    perror("accept failed");
                }
            } else {
                printf("New connection accepted.\n");
                // Set client socket to nonblocking
                int cflags = fcntl(client_fd, F_GETFL, 0);
                fcntl(client_fd, F_SETFL, cflags | O_NONBLOCK);

                // Try to read data (will not block)
                ssize_t bytes = recv(client_fd, buffer, sizeof(buffer) - 1, 0);
                if (bytes > 0) {
                    buffer[bytes] = '\0';
                    printf("Received: %s\n", buffer);
                } else if (bytes == 0) {
                    printf("Client disconnected.\n");
                } else {
                    if (errno != EWOULDBLOCK && errno != EAGAIN) {
                        perror("recv failed");
                    }
                }
                close(client_fd);
            }
        }
    }

    close(listen_fd);
    return 0;
}

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 4