I have a loop handling an accept(2) call. I want to be able to perform some cleanup when a SIGINT is sent to the program. My first thought was to use the signal function.

void signal_handler(int signal) {
    printf("Caught signal\n");
}

int main() {
    signal(SIGINT, &signal_handler);
    // ...
    int accept_fd = accept(sock, NULL, NULL);
    if (accept_fd == -1) {
        close(sock);
        perror("accept");
        return 1;
    }
    // ...
}

However, this simply prints "Caught signal" and then the program continues on.

If I modify main to use sigaction, the program works as expected.

int main() {
    struct sigaction sa;
    sa.sa_handler = &signal_handler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGINT, &sa, NULL);
    // ...
    int accept_fd = accept(sock, NULL, NULL);
    if (accept_fd == -1) {
        close(sock);
        perror("accept");
        return 1;
    }
    // ...
}

Upon sending SIGINT, I get Caught Signal, followed by accept: Interrupted system call. From the man page for accept(2)

ERRORS

...

EINTR The system call was interrupted by a signal that was caught before a valid connection arrived; see signal(7).

I understand that sigaction is more modern and that I should be using it over signal, but I'm quite curious why it provides this difference in functionality.

Below I've included a full usable example program for each case.

Example with signal(2)

#include <netdb.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

#define BUFFER_SIZE 32

void signal_handler(int signal) {
    printf("Caught signal\n");
}

int main() {
    signal(SIGINT, &signal_handler);
    struct addrinfo hints;
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    struct addrinfo *addr_info;
    int info_result = getaddrinfo("localhost", "8080", &hints, &addr_info);
    if (info_result != 0) {
        printf("Getting address failed\n");
        return 1;
    }
    int sock = socket(addr_info->ai_family, addr_info->ai_socktype, addr_info->ai_protocol);
    if (sock == -1) {
        printf("Socket Failed\n");
        return 1;
    }
    int bind_result = bind(sock, addr_info->ai_addr, addr_info->ai_addrlen);
    if (bind_result == -1) {
        close(sock);
        printf("Bind Failed\n");
        return 1;
    }
    int listen_result = listen(sock, 8);
    if (listen_result == -1) {
        close(sock);
        printf("Listen Failed\n");
        return 1;
    }
    printf("Waiting...\n");
    int accept_fd = accept(sock, NULL, NULL);
    if (accept_fd == -1) {
        close(sock);
        perror("accept");
        return 1;
    }
    printf("Got fd %d\n", accept_fd);
    char *buffer = malloc(BUFFER_SIZE * sizeof(char));
    int n;
    while ((n = read(accept_fd, buffer, BUFFER_SIZE)) > 0) {
        printf("%.*s\n", n, buffer);
    }
    close(sock);
}

Example with sigaction(2)

#include <netdb.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

#define BUFFER_SIZE 32

void signal_handler(int signal) {
    printf("Caught signal\n");
}

int main() {
    struct sigaction sa;
    sa.sa_handler = &signal_handler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    sigaction(SIGINT, &sa, NULL);
    struct addrinfo hints;
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    struct addrinfo *addr_info;
    int info_result = getaddrinfo("localhost", "8080", &hints, &addr_info);
    if (info_result != 0) {
        printf("Getting address failed\n");
        return 1;
    }
    int sock = socket(addr_info->ai_family, addr_info->ai_socktype, addr_info->ai_protocol);
    if (sock == -1) {
        printf("Socket Failed\n");
        return 1;
    }
    int bind_result = bind(sock, addr_info->ai_addr, addr_info->ai_addrlen);
    if (bind_result == -1) {
        close(sock);
        printf("Bind Failed\n");
        return 1;
    }
    int listen_result = listen(sock, 8);
    if (listen_result == -1) {
        close(sock);
        printf("Listen Failed\n");
        return 1;
    }
    printf("Waiting...\n");
    int accept_fd = accept(sock, NULL, NULL);
    if (accept_fd == -1) {
        close(sock);
        perror("accept");
        return 1;
    }
    printf("Got fd %d\n", accept_fd);
    char *buffer = malloc(BUFFER_SIZE * sizeof(char));
    int n;
    while ((n = read(accept_fd, buffer, BUFFER_SIZE)) > 0) {
        printf("%.*s\n", n, buffer);
    }
    close(sock);
}