client.c

#define _XOPEN_SOURCE 700

#include "assert.h"
#include "stdbool.h"
#include "stdio.h"
#include "stdlib.h"
#include "string.h"

#include <sys/socket.h>
#include <sys/un.h> /* sockaddr_un */
#include "unistd.h"

int main(int argc, char** argv) {
    /* Name of the socket file server and client must agree on it. */
    char name[] = "socket.tmp";

    /* Sockets are accessible via file descriptors. */
    int sockfd;
    int len;

    //this is the struct used by UNix addresses
    struct sockaddr_un address;
    char ch_init = 'a';
    char ch = ch_init;

    /*
    #socket

        Create the socket, and get a file descrpitor to it.

        This must be done by both clients and servers.

            int socket(int domain, int type, int protocol);

        - protocol:

            For a given domain, select which protocol id to use.

            `0` uses a default protocol for the domain.

            Many domains have a single protocol.

            Other do not, for example `AF_INET` has both `tcp` adn `udp`.

            To get a protocol id, use `struct protoent *protoent = getprotobyname('tcp')`,
            and then extract `protoent->p_proto`.
    */

        sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
        if (sockfd == -1) {
            perror("socket");
            exit(EXIT_FAILURE);
        }

    /*
    # connect

        Request connection to the socket on the given address.

        If the socket file does not exist fails.
    */

        /* type of socket */
        address.sun_family = AF_UNIX;

        /* give a name to the socket */
        strcpy(address.sun_path, name);

        len = sizeof(address);

    if (connect(sockfd, (struct sockaddr*)&address, len) == -1) {
        perror("connect");
        exit(EXIT_FAILURE);
    }

    if (write(sockfd, &ch, 1) == -1) {
        perror("write");
        exit(EXIT_FAILURE);
    }
    if (read(sockfd, &ch, 1) == -1) {
        perror("read");
        exit(EXIT_FAILURE);
    }

    /* You should close the connection on both client and server. */
    close(sockfd);

    /* Assert that the server did its job of increasing the char we gave it. */
    assert(ch == ch_init + 1);

    exit(EXIT_SUCCESS);
}