Question

I just began learning network programming. My first program is very simple: once connected, the server program displays what the client program sends (characters). Everything works fine until they get connected. At first, I did the sending loop using putc:

while((c = getc(stdin)) != '\n')
    putc(c, (FILE *) connection);

and the receiving one with getc:

while((c = getc((FILE *) connection)) != '\n')
    putc(c, stdout);

Once connected, I get a segmentation fault. Then I tried using send:

while(1) {
    memset(str, null, strlen(memset));

    while(((c = getc(stdin)) != '\n') && sizeof(str) <= 100) {
        strcat(str, (char *) c);       // cast to char * is to make gcc happy
    }

    send(connection, (void *) str, sizeof(str), (int) NULL);
}

and recv:

while((stat = recv(connection,str,100,0)) != 0) {
    printf("\nReceived %d bytes of data from %s: %s", stat, client, str);
}

Now recv() keeps returning -1 and errno is set to 107. What does this mean? Where am I doing wrong? By the way I’m using Linux with gcc.

Thanks a lot for your help! 🙂

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EDIT: Even if I used getc() and putc(), I get connection with socket(), then I connect() (client). The server, after obtaining connection (with socket()) bind()s, then listen()s and accept()s. Sorry if I ometted.

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Here’s the server code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <errno.h>
#include <arpa/inet.h>
#include <netdb.h>

void main(int argc, char *argv[])
{
    struct addrinfo hint;
    struct addrinfo *servinfo, *count;
    struct sockaddr_storage conn_addr;
    struct sockaddr host;

    int connessione;
    int stat;
    int conn_sock;
    int conn_size;
    int success;
    int c;

    char t[INET_ADDRSTRLEN];
    char str[100];
    char *client;

    if(argc != 1 && strcmp(argv[1], "aiuto") == 0) {
        printf("%s(porta)\n",argv[0]);
        printf("porta: specifica su quale porta installarsi\n");
        exit(-1);
    }

    memset(&hint,0,sizeof hint);


    hint.ai_family = AF_INET;
    hint.ai_socktype = SOCK_STREAM;
    hint.ai_flags = AI_PASSIVE;

    if((stat = getaddrinfo(NULL, argv[1], &hint, &servinfo)) != 0)  {
        printf("\n[FATAL]: getaddrinfo: %s",gai_strerror(stat));
        exit(-1);
    }

    success = 0;

    for(count = servinfo; count != NULL; count = count->ai_next) {
        if((connessione = socket(count->ai_family, count->ai_socktype, count->ai_protocol)) <= 0)
            continue;

        if((stat = bind(connessione, count->ai_addr, count->ai_addrlen)) == 0) {
            success = 1;
            break;
        }
        else {
            continue;
        }
    }

    if(success == 0) {
        printf("\n[FATAL]: Unable to bind()!\n");
        exit(-1);
    }
    else {
        printf("\n[DEBUG]: %s: listening...", argv[0]);
    }

    servinfo = count;
    freeaddrinfo(count);

    if(listen(connessione, 20) == -1) {
        printf("\n[FATAL]: listen: %s (%d)\n", gai_strerror(errno), errno);
        close(connessione);
        exit(-1);
    }

    conn_size = sizeof(conn_addr);

    if(accept(connessione, (struct sockaddr *) &conn_addr, &conn_size) == -1) {
        printf("\n[FATAL]: accept: %s", gai_strerror(errno));
        close(connessione);
        exit(-1);
    }

    client = (char *) malloc(INET_ADDRSTRLEN * sizeof(char));
    client = inet_ntop(servinfo->ai_family, &conn_addr, t, INET_ADDRSTRLEN);

    printf("\n[DEBUG]: %s: connection accepted: %s", argv[0], client);

    while((stat = recv(connessione,str,100,0)) != 0) {
        printf("\nReceived %d bytes of data from %s: %s", stat,client, str);
    }

    printf("\n[DEBUG]: connection closed by %s\n",client);
}

And the client’s code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <errno.h>
#include <arpa/inet.h>
#include <netdb.h>

void main(int argc, char *argv[])
{
    struct addrinfo hint;
    struct addrinfo *servinfo, *count;
    struct sockaddr_storage conn_addr;
    struct sockaddr host;

    int connessione;
    int stat;
    int conn_sock;
    int conn_size;
    int success;
    int c;

    char t[INET_ADDRSTRLEN];
    char *indirizzo;
    char str[100];

    memset(&hint,0,sizeof hint);

    host.sa_family = AF_INET;
    if(inet_pton(host.sa_family, argv[1], (void *) host.sa_data) == -1) {
        printf("\n[FATAL]: pton: %s (%d)\n", gai_strerror(errno), errno);
        exit(-1);
    }

    hint.ai_family = AF_INET;
    hint.ai_socktype = SOCK_STREAM;
    hint.ai_flags = AI_PASSIVE;
    hint.ai_addr = &host;

    if((stat = getaddrinfo(argv[1], argv[2], &hint, &servinfo)) != 0)  {
        printf("[FATAL]: getaddrinfo: %s\n",gai_strerror(stat));
        exit(-1);
    }

    success = 0;
    for(count = servinfo; count != NULL; count = count->ai_next) {
        if((connessione = socket(count->ai_family, count->ai_socktype, count->ai_protocol)) == -1)
            continue;

        if(connect(connessione, count->ai_addr, count->ai_addrlen) != -1) {
            success = 1;
            break; 
        }
    }

    if(success == 0) {
        printf("\n[FATAL]: impossibile trovare l'host specificato\n");
        exit(-1);
    }

    servinfo = count;
    freeaddrinfo(count);

    indirizzo = (char *) malloc(INET_ADDRSTRLEN * sizeof(char));
    indirizzo = inet_ntop(servinfo->ai_family, &conn_addr, t, INET_ADDRSTRLEN);

    printf("\n[DEBUG]: %s: connesso a: %s\n",argv[0], indirizzo);

    while(1) {
        strcpy(str,"buffer for data to send");

        while(((c = getc(stdin)) != '\n') && sizeof(str) <= 100) {
            strcat(str, (char *) c);
        }

        send(connessione, (void *) str, sizeof(str), (int) NULL);
    }
}

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At the end…

It works perfectly! Thanks everyone. 🙂

Answer 1

Normally you don’t use a FILE * handle to do network programming. Rather, you need a socket descriptor returned by the socket(2) system call. Once you have it, you can either connect(2) it (the client) or bind(2) it (the server). The ongoing communication can be performed using recv(2) and send(2).

If you really want to use a FILE * handle to do network programming, take a look at the fdopen(3) library function. It takes a descriptor that could be returned by socket(2) and returns a FILE * handle that is compatible with fread / fwrite / fprintf.

After posting your full code, the most significant error that I could see was:

if (accept(connessione, (struct sockaddr *) &conn_addr, &conn_size))

accept(2) returns a socket which you have to use with recv(2) / send(2), and you discard it, using the server socket instead.