POSIX
Sockets

TCP Concurrent Echo Server

In questo esempio, creeremo un semplice server echo che ascolterà sulla porta specificata e sarà in grado di gestire nuove connessioni:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <time.h>

/**
  Connection handler - this will be executed in
  the new process, after forking, and it will read
  all the data from the socket, while available and
  to echo it on the local terminal.

  Params:
    sd = socket to the client
*/
#define BUF_SIZE (1024)

int echo_client(int sd) 
{
    int result = 0; 

    char buf[BUF_SIZE + 1] = {0};

    ssize_t n_read;
    while (0 < (n_read = read(sd, buf, BUF_SIZE))) 
    {
        buf[n_read] = '\0';
        printf("%s\n", buf);
    }

    if (0 > n_read)
    {
      perror("read() failed");
      result = -1;
    }
    else
    {
      fprintf(stderr, "The other side orderly shut down the connection.\n");
    }

    close(sd);

    return result;
}


int main(void)
{
    // Create a listening socket
    int listening_socket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (listening_socket == -1)
    {
        perror("socket() failed");
        return EXIT_FAILURE;
    }

    // Bind it to port 15000.
    unsigned short listening_port = 15000;

    struct sockaddr_in addr = {0};
    addr.sin_family = AF_INET;
    addr.sin_port = htons(listening_port);

    socklen_t sock_len = sizeof(addr);

    if (0 > (bind(listening_socket, (const struct sockaddr*) &addr, sock_len)))
    {
        perror("bind() failed");
        return EXIT_FAILURE;
    }

    // Start listening
    if (0 > listen(listening_socket, 0))
    {
        perror("listen() failed");
        return EXIT_FAILURE;
    }

    // Accept new connections, fork the new process for handling
    // and handle the connection in the new process, while the parent
    // is waiting for another connection to arrive.
    int accepted_socket = 0;
    while (0 < (accepted_socket =
                  accept(listening_socket, (struct sockaddr*) &addr, &sock_len)))
    {
        pid_t pid_child = fork();

        if (0 > pid_child)
        {
            perror("fork() failed");
            return EXIT_FAILURE;
        }
        else if (0 == pid_child)
        {
            // inside the forked child here
            close(listening_socket); // The child does not need this any more. 

            echo_client(accepted_socket);

            return EXIT_SUCCESS;
        }

        // Inside parent process, since file descriptors are reference
        // counted, we need to close the client socket
        close(accepted_socket);
    }

    if (0 > accepted_socket)
    {
        perror("accept() failed");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

Abilitare TCP keepalive sul lato server

Questo è un esempio client-server. Il processo esegue il fork e esegue il client nel processo padre e nel server nel processo figlio:

• il client si connette al server e attende fino all'uscita dal server;
• il server accetta la connessione dal client, abilita keepalive e attende qualsiasi segnale.

Keepalive è configurato utilizzando le seguenti opzioni descritte nelle pagine man socket(7) e tcp(7) :

• SO_KEEPALIVE : consente l'invio di messaggi keep-alive
• TCP_KEEPIDLE - il tempo (in secondi) per cui la connessione deve rimanere inattiva prima che TCP inizi ad inviare sonde keepalive
• TCP_KEEPINTVL - il tempo (in secondi) tra le singole sonde keepalive
• TCP_KEEPCNT : il numero massimo di sonde keepalive che TCP deve inviare prima di rilasciare la connessione

Codice sorgente:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/signal.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/tcp.h>

#define check(expr) if (!(expr)) { perror(#expr); kill(0, SIGTERM); }

void enable_keepalive(int sock) {
    int yes = 1;
    check(setsockopt(
        sock, SOL_SOCKET, SO_KEEPALIVE, &yes, sizeof(int)) != -1);

    int idle = 1;
    check(setsockopt(
        sock, IPPROTO_TCP, TCP_KEEPIDLE, &idle, sizeof(int)) != -1);

    int interval = 1;
    check(setsockopt(
        sock, IPPROTO_TCP, TCP_KEEPINTVL, &interval, sizeof(int)) != -1);

    int maxpkt = 10;
    check(setsockopt(
        sock, IPPROTO_TCP, TCP_KEEPCNT, &maxpkt, sizeof(int)) != -1);
}

int main(int argc, char** argv) {
    check(argc == 2);

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(12345);
    check(inet_pton(AF_INET, argv[1], &addr.sin_addr) != -1);

    int server = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    check(server != -1);

    int yes = 1;
    check(setsockopt(server, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) != -1);

    check(bind(server, (struct sockaddr*)&addr, sizeof(addr)) != -1);
    check(listen(server, 1) != -1);

    if (fork() == 0) {
        int client = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
        check(client != -1);
        check(connect(client, (struct sockaddr*)&addr, sizeof(addr)) != -1);
        printf("connected\n");
        pause();
    }
    else {
        int client = accept(server, NULL, NULL);
        check(client != -1);
        enable_keepalive(client);
        printf("accepted\n");
        wait(NULL);
    }

    return 0;
}

I pacchetti keepalive possono essere monitorati usando tcpdump .

Esempio di utilizzo:

$ ./a.out 127.0.0.1 &
[1] 14010
connected
accepted

$ tcpdump -n -c4 -ilo port 12345
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 262144 bytes
18:00:35.173892 IP 127.0.0.1.12345 > 127.0.0.1.60998: Flags [.], ack 510307430, win 342, options [nop,nop,TS val 389745775 ecr 389745675], length 0
18:00:35.173903 IP 127.0.0.1.60998 > 127.0.0.1.12345: Flags [.], ack 1, win 342, options [nop,nop,TS val 389745775 ecr 389745075], length 0
18:00:36.173886 IP 127.0.0.1.12345 > 127.0.0.1.60998: Flags [.], ack 1, win 342, options [nop,nop,TS val 389745875 ecr 389745775], length 0
18:00:36.173898 IP 127.0.0.1.60998 > 127.0.0.1.12345: Flags [.], ack 1, win 342, options [nop,nop,TS val 389745875 ecr 389745075], length 0
4 packets captured
8 packets received by filter
0 packets dropped by kernel

TCP Daytime Iterative Server

Questo è un server iterativo di giorno TCP mantenuto il più semplice possibile.

#include <sys/types.h>   /* predefined types */ 
#include <unistd.h>      /* unix standard library */ 
#include <arpa/inet.h>   /* IP addresses conversion utilities */ 
#include <netinet/in.h>  /* sockaddr_in structure definition */
#include <sys/socket.h>  /* berkley socket library */ 
#include <stdio.h>       /* standard I/O library */ 
#include <string.h>      /* include to have memset */ 
#include <stdlib.h>      /* include to have exit */ 
#include <time.h>        /* time manipulation primitives */
 
#define MAXLINE 80 
#define BACKLOG 10 
 
int main(int argc, char *argv[]) 
{  
    int list_fd, conn_fd;  
    struct sockaddr_in serv_add; 
    char buffer[MAXLINE]; 
    time_t timeval; 
 
    /* socket creation third parameter should be IPPROTO_TCP but 0 is an
     * accepted value */  
    list_fd = socket(AF_INET, SOCK_STREAM, 0);
         
    /* address initialization */ 
    memset(&serv_add, 0, sizeof(serv_add));        /* init the server address */ 
    serv_add.sin_family = AF_INET;                 /* address type is IPV4 */ 
    serv_add.sin_port = htons(13);                 /* daytime port is 13 */ 
    serv_add.sin_addr.s_addr = htonl(INADDR_ANY);  /* connect from anywhere */ 
     
    /* bind socket */ 
    bind(list_fd, (struct sockaddr *)&serv_add, sizeof(serv_add));
        
    /* listen on socket */ 
    listen(list_fd, BACKLOG);

    while (1) 
    { 
        /* accept connection */ 
        conn_fd = accept(list_fd, (struct sockaddr *) NULL, NULL);
           
        timeval = time(NULL); 
        snprintf(buffer, sizeof(buffer), "%.24s\r\n", ctime(&timeval)); 
         
        write(conn_fd, buffer, strlen(buffer)); /* write daytime to client */ 
         
        close(conn_fd); 
    } 
     
    /* normal exit */ 
    close(list_fd); 
    exit(0); 
}

TCP Daytime Client

Questo è un client di giorno TCP mantenuto il più semplice possibile.

#include <unistd.h>      /* unix standard library */
#include <arpa/inet.h>   /* IP addresses manipulation utilities */
#include <netinet/in.h>  /* sockaddr_in structure definition */
#include <sys/socket.h>  /* berkley socket library */
#include <stdio.h>       /* standard I/O library */
#include <string.h>      /* include to have memset*/
#include <stdlib.h>      /* include to have exit*/

#define MAXLINE 1024

int main(int argc, char *argv[])
{
    int sock_fd;
    int nread;
    struct sockaddr_in serv_add;
    char buffer[MAXLINE];

    /* socket creation third parameter should be IPPROTO_TCP but 0 is an
     * accepted value*/
    sock_fd = socket(AF_INET, SOCK_STREAM, 0);

    /* address initialization */
    memset(&serv_add, 0, sizeof(serv_add));     /* init the server address */
    serv_add.sin_family = AF_INET;              /* address type is IPV4 */
    serv_add.sin_port = htons(13);              /* daytime post is 13 */

    /* using inet_pton to build address */
    inet_pton(AF_INET, argv[1], &serv_add.sin_addr);

    /* connect to the server */
    connect(sock_fd, (struct sockaddr *)&serv_add, sizeof(serv_add));

    /* read daytime from server */
    while ((nread = read(sock_fd, buffer, MAXLINE)) > 0)
    {
        buffer[nread] = 0;
        if (fputs(buffer, stdout) == EOF)
        {
            perror("fputs error"); /* write daytime on stdout */
            return -1;
        }
    }

    close(sock_fd);
    exit(0);
}

Nozioni di base sui socket

Sono disponibili quattro tipi di socket in API POSIX: TCP, UDP, UNIX e (facoltativamente) RAW. I socket di dominio Unix possono agire come socket di flusso o come socket di datagrammi.

Alcuni tipi di endpoint:

1. struct sockaddr - tipo di endpoint universale. In genere, altri tipi di endpoint concreti vengono convertiti in questo tipo solo nelle chiamate posix.
2. struct sockaddr_in - Endpoint IPv4

struct sockaddr_in {
    sa_family_t sin_family;
    in_port_t sin_port;            /* Port number.  */
    struct in_addr sin_addr;       /* Internet address.  */
};
struct in_addr {
    in_addr_t s_addr;
};

3. struct sockaddr_in6 - Endpoint IPv6

struct sockaddr_in6 {
    sa_family_t sin6_family;
    in_port_t sin6_port;    /* Transport layer port # */
    uint32_t sin6_flowinfo;    /* IPv6 flow information */
    struct in6_addr sin6_addr;    /* IPv6 address */
    uint32_t sin6_scope_id;    /* IPv6 scope-id */
};

4. struct sockaddr_un .

struct sockaddr_un {
    sa_family_t sun_family;               /* AF_UNIX */
    char        sun_path[108];            /* pathname */
};

Intero programma

#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <unistd.h>

#define DESIRED_ADDRESS "127.0.0.1"
#define DESIRED_PORT 3500
#define BUFSIZE 512

int main()
{
    // ADDRESS PART
    // MAIN PART
    close(sock);
    return EXIT_SUCCESS;
}

Creazione di endpoint IPv4

struct sockaddr_in addr = {0};
addr.sin_family = AF_INET;
addr.sin_port = htons(DESIRED_PORT); /*converts short to
                                       short with network byte order*/
addr.sin_addr.s_addr = inet_addr(DESIRED_ADDRESS);

Snippet del server TCP

int sock = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (sock == -1) {
    perror("Socket creation error");
    return EXIT_FAILURE;
}

if (bind(sock, (struct sockaddr*) &addr, sizeof(addr)) == -1) {
    perror("Bind error");
    close(sock);
    return EXIT_FAILURE;
}

if (listen(sock, 1/*length of connections queue*/) == -1) {
    perror("Listen error");
    close(sock);
    return EXIT_FAILURE;
}

socklen_t socklen = sizeof addr;
int client_sock = accept(sock, &addr, &socklen); /* 2nd and 3rd argument may be NULL. */
if (client_sock == -1) {
    perror("Accept error");
    close(sock);
    return EXIT_FAILURE;
}

printf("Client with IP %s connected\n", inet_ntoa(addr.sin_addr));

char buf[BUFSIZE];
if (send(sock, "hello", 5, 0) == -1) {
    perror("Send error");
    close(client_sock);
    close(sock);
    return EXIT_FAILURE;
}

ssize_t readden = recv(sock, buf, BUFSIZE, 0);
if (readden < 0) {
    perror("Receive error");
    close(client_sock);
    close(sock);
    return EXIT_FAILURE;
}
else if (readden == 0) {
  fprintf(stderr, "Client orderly shut down the connection.\n");
}
else {readden > 0) {
    if (readden < BUFSIZE)
    {
      fprintf(stderr, "Received less bytes (%zd) then requested (%d).\n", 
        readden, BUFSIZE);
    }

    write (STDOUT_FILENO, buf, readden);
}

Snippet del client TCP

int sock = socket (AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (sock == -1) {
    perror("Socket creation error");
    return EXIT_FAILURE;
}
if (connect(sock, (struct sockaddr*) &addr, sizeof(addr)) == -1) {
    perror("Connection error");
    close(sock);
    return EXIT_FAILURE;
}

char buf[BUFSIZE];
if (send(sock, "hello", 5, 0); /*write may be also used*/ == -1) {
    perror("Send error");
    close(client_sock);
    close(sock);
    return EXIT_FAILURE;
}

ssize_t readden = recv(sock, buf, BUFSIZE, 0); /*read may be also used*/
if (readden < 0) {
    perror("Receive error");
    close(client_sock);
    close(sock);
    return EXIT_FAILURE;
}
else if (readden == 0)
{
    fprintf(stderr, "Client orderly shut down the connection.\n");
}
else /* if (readden > 0) */ {
    if (readden < BUFSIZE)
    {
      fprintf(stderr, "Received less bytes (%zd) then requested (%d).\n", 
        readden, BUFSIZE);
    }

    write (STDOUT_FILENO, buf, readden);
}  

Snippet del server UDP

int sock = socket (AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (sock == -1) {
    perror("Socket creation error");
    return EXIT_FAILURE;
}
if (bind(sock, (struct sockaddr*) &addr, sizeof(addr)) == -1) {
    perror("Bind error");
    close(sock);
    return EXIT_FAILURE;
}

char buf[BUFSIZE];
ssize_t readden = recvfrom(sock, buf, BUFSIZE, 0, &addr, sizeof(addr));
if (readden > 0) {
    printf("Client with IP %s sent datagram\n", inet_ntoa(addr.sin_addr));
    write (STDOUT_FILENO, buf, readden);
}
sendto(sock, "hello", 5, 0, &addr, sizeof(addr));

Accettare le connessioni su un socket di blocco

Il programma AC che desidera accettare le connessioni di rete (funge da "server") deve prima creare un socket associato all'indirizzo "INADDR_ANY" e richiamare l' listen su di esso. Quindi, può chiamare accept sul socket del server per bloccare fino a quando un client si connette.

//Create the server socket
int servsock = socket(AF_INET, SOCK_STREAM, 0);
if(servsock < 0) perror("Failed to create a socket");

int enable = 1;
setsockopt(servsock, SOL_SOCKET, SO_REUSEADDR, (char*)&enable, sizeof(int));

//Bind to "any" address with a specific port to listen on that port
int port = 12345;
sockaddr_in serv_addr;
memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(port);

if(bind(servsock, (sockaddr*)&serv_addr, sizeof(serv_addr)) < 0)
    perror("Error binding to socket");

listen(servsock, 5);

//Accept a client
struct sockaddr_storage client_addr_info;
socklen_t len = sizeof client_addr_info;

int clientsock = accept(servsock, (struct sockaddr*)&client_addr_info, &len);

//Now you can call read, write, etc. on the client socket

La struttura sockaddr_storage che viene passata ad accept può essere utilizzata per recuperare informazioni sul client connesso. Ad esempio, ecco come determinare l'indirizzo IP del client:

char client_ip_str[INET6_ADDRSTRLEN + 1];
if(client_addr_info.ss_family == AF_INET) {
    // Client has an IPv4 address
    struct sockaddr_in *s = (struct sockaddr_in *)&client_addr_info;
    inet_ntop(AF_INET, &s->sin_addr, client_ip_str, sizeof(client_ip_str));
} else {  // AF_INET6
    // Client has an IPv6 address
    struct sockaddr_in6 *s = (struct sockaddr_in6 *)&client_addr_info;
    inet_ntop(AF_INET6, &s->sin6_addr, client_ip_str, sizeof(client_ip_str));
}

Connessione a un host remoto

char * server = "www.example.com";

int sock = socket(AF_INET, SOCK_STREAM, 0);
if(sock < 0) 
  perror("Failed to create a socket");

hostent *server = gethostbyname(servername);
if (server == NULL) 
  perror("Host lookup failed");

char server_ip_str[server->h_length];
inet_ntop(AF_INET, server->h_addr, server_ip_str, server->h_length);

sockaddr_in serv_addr;
memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(port);
memcpy(&serv_addr.sin_addr.s_addr, server->h_addr, server->h_length);

if (connect(sock, (sockaddr*)&serv_addr, sizeof(serv_addr)) < 0)
    perror("Failed to connect");

// Now you can call read, write, etc. on the socket.

close(sock);

Leggere e scrivere su una presa di blocco

Anche quando i socket sono in modalità "blocking", le operazioni di read e write su di essi non necessariamente leggono e scrivono tutti i dati disponibili per essere letti o scritti. Per scrivere un intero buffer in un socket o leggere una quantità nota di dati da un socket, devono essere richiamati in un ciclo.

/*
 * Writes all bytes from buffer into sock. Returns true on success, false on failure.
 */
bool write_to_socket(int sock, const char* buffer, size_t size) {
    size_t total_bytes = 0;
    while(total_bytes < size) {
        ssize_t bytes_written = write(sock, buffer + total_bytes, size - total_bytes);
        if(bytes_written >= 0) {
            total_bytes += bytes_written;
        } else if(bytes_written == -1 && errno != EINTR) {
            return false;
        }
    }
    return true;
}

/*
 * Reads size bytes from sock into buffer. Returns true on success; false if
 * the socket returns EOF before size bytes can be read, or if there is an
 * error while reading.
 */
bool read_from_socket(int sock, char* buffer, size_t size) {
    size_t total_bytes = 0;
    while(total_bytes < size) {
        ssize_t new_bytes = read(sock, buffer + total_bytes, size - total_bytes);
        if(new_bytes > 0) {
            total_bytes += new_bytes;
        } else if(new_bytes == 0 || (new_bytes == -1 && errno != EINTR)) {
            return false;
        }
    }
    return true;
}