Allow Unix sockets to be treated like normal files.

某天忽然想要用echo/cat的方式直接對unix socket做存取，結果得到error，只好有請google大神，覓得此良方Allow Unix sockets to be treated like normal files，try了一下沒問題。

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <stdlib.h>

#define handle_error(msg) \
        do { perror(msg); exit(EXIT_FAILURE); } while (0)

int main(int argc, char *argv[])
{
    int srv_fd, cli_fd;
    socklen_t cli_len;
    struct sockaddr_un srv_addr, cli_addr;
    char buf[128] = "Brook: ";
    ssize_t len;

    unlink("server_socket");
    if ((srv_fd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
        handle_error("socket");
    }

    srv_addr.sun_family = AF_UNIX;
    strcpy(srv_addr.sun_path, "/tmp/unix_sock");
    if (bind(srv_fd, (struct sockaddr *)&srv_addr, sizeof(srv_addr)) < 0) {
        handle_error("bind");
    }

    if (listen(srv_fd, 1) < 0) {
         handle_error("listen");
    }

    while (1) {
        cli_fd = accept(srv_fd, (struct sockaddr *)&cli_addr, &cli_len);
        len = read(cli_fd, buf + 6, sizeof(buf) - 6);
        buf[6 + len] = 0;
        write(cli_fd, buf, strlen(buf));
        close(cli_fd);
    }
    return 0;
}

參考資料：
• http://lwn.net/Articles/415651/ , net/unix: Allow Unix sockets to be treated like normal files.

github：
https://github.com/brook-kuo/Linux_Module/tree/master/socket/unix_as_normal_file

socket programming in bash

關於Bash的Socket部份您可以在man page中看到這段描述：

/dev/tcp/host/port
        If host is a valid hostname or Internet address, and port is an integer
        port number or service name, bash attempts to open a TCP connection to
        the corresponding socket.

/dev/udp/host/port
        If host is a valid hostname or Internet address, and port is an integer
        port number or service name, bash attempts to open a UDP connection to 
        the corresponding socket.

用一個簡單的例子就會懂了

simple TCP

這是最簡單的tcp的架構，server只有接受(accept)一個client，然後收送資料，接著就結束。
Server

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h> // inet_xx()
#include <unistd.h> // close()
#include <netinet/in.h>

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)


int main(int argc, char *argv[])
{
    int sd, af, asd; // socket descriptor, address family, accepted sd
    struct sockaddr_in sa, peer; // socket address
    uint16_t port = 0;
    char *ip = NULL, buf[128];
    socklen_t addrlen;
    ssize_t len;

    if (argc > 1) {
        ip = argv[1];
    }
    af = AF_INET;
    if ((sd = socket(af, SOCK_STREAM, 0)) < 0) {
        handle_error("socket");
    }

    memset(&sa, 0, sizeof(sa));
    sa.sin_family = af;
    sa.sin_port = htons(port);
    sa.sin_addr.s_addr = INADDR_ANY;
    if (ip) {
        if (inet_pton(af, ip, &sa.sin_addr) != 1) {
            handle_error("inet_pton");
        }
    }
    if (bind(sd, (struct sockaddr*) &sa, sizeof(sa)) < 0) {
        handle_error("bind");
    }
    addrlen = sizeof(sa);
    if (getsockname(sd, (struct sockaddr*)&sa, &addrlen) < 0) {
        handle_error("getsockname");
    }

    printf("bind on %s/%d\n",
            inet_ntop(sa.sin_family, &sa.sin_addr, buf, sizeof(buf)),
            ntohs(sa.sin_port));

    if (listen(sd, 1) < -1) { // waiting to be accepted
        handle_error("listen");
    }

    addrlen = sizeof(peer);
    if ((asd = accept(sd, (struct sockaddr*) &peer, &addrlen)) < 0) {
        handle_error("accept");
    }

    if ((len = recv(asd, buf, sizeof(buf), 0)) < 0) {
        handle_error("recv");
    }
    buf[len] = 0;
    printf("Server recv: %s\n", buf);

    strcpy(buf, "bye bye, peer!");
    if (send(asd, buf, strlen(buf), 0) < 0) {
        handle_error("send");
    }

    close(asd);
    close(sd);
    return 0;
}

Client

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h> // inet_xx()
#include <unistd.h> // close()
#include <netinet/in.h>

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)


int main(int argc, char *argv[])
{
    int sd, af = AF_INET; // socket descriptor, address family
    struct sockaddr_in serv; // socket address
    char *serv_ip = NULL, buf[128];
    socklen_t addrlen;
    ssize_t len;
    uint16_t port;

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <serv-ip> <port>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    serv_ip = argv[1];
    port = atoi(argv[2]);

    if ((sd = socket(af, SOCK_STREAM, 0)) < 0) {
        handle_error("socket");
    }

    memset(&serv, 0, sizeof(serv));
    serv.sin_family = af;
    serv.sin_port = htons(port);
    if (inet_pton(af, serv_ip, &serv.sin_addr) != 1) {
        handle_error("inet_pton");
    }

    if (connect(sd, (struct sockaddr*) &serv, sizeof(serv)) < 0) {
        handle_error("connect");
    }

    strcpy(buf, "hello server!");
    if (send(sd, buf, strlen(buf), 0) < 0) {
        handle_error("send");
    }

    if ((len = recv(sd, buf, sizeof(buf), 0)) < 0) {
        handle_error("recv");
    }
    buf[len] = 0;
    printf("Client recv: %s", buf);

    close(sd);
    return 0;
}