Client and Server send() and recv() in C

Question

This is my first time socket programming and have a question about send and recv. I'm sending a file from server to client which works fine. But, when I want to continue my program with more send() and recv() calls (the commented code at the end of client and server), the client receives this data and writes it to newfile.txt.

So, my question is, how do I call send() from the server to only send data from test.txt, and then recv() on the client to only receive and write data from test.txt to newfile.txt? After this, I would want to continue with my program with more send() and recv() calls which dont get mixed up with the file transfer code.

Client:

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

int main(int argc, char *argv[]){
    int port = 8001;
    int client_socket;
    struct sockaddr_in server_addr;
    int connect_status;

    client_socket = socket(AF_INET, SOCK_STREAM, 0);    //SOCK_STREAM for TCP
    if (client_socket < 0)
    {
        printf("Error creating socket\n");
        exit(1);
    }
    printf("Socket created\n");

    //address for socket to connect to
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);             //8001 arbitrary port
    server_addr.sin_addr.s_addr = INADDR_ANY;

    //connect to address of socket
    connect_status = connect(client_socket, (struct sockaddr *) &server_addr, sizeof(server_addr));
    if(connect_status == -1)
    {
        printf("Error connecting to server\n");
        exit(1);
    }
    printf("Connected to Server\n");




    FILE* fp = fopen( "newfile.txt", "w");
    char data[512];
    int b;

    while((b=recv(client_socket, data, 512,0))>0){
        fwrite(data, 1, b, fp);
    }
    fclose(fp);


    //recv(client_socket, data, 512,0);
    //printf("client buffer: %s\n", data);

    close(client_socket);
    return 0;

}

Server:

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

int main(int argc, char *argv[]){
    int port = 8001;
    int server_socket;
    struct sockaddr_in server_addr;
    int client_socket, client_addr;
    int bind_status, listen_status;


    server_socket = socket(AF_INET, SOCK_STREAM, 0);    //SOCK_STREAM for TCP
    if (server_socket < 0)
    {
        printf("Error creating socket\n");
        exit(1);
    }
    printf("Socket created\n");

    //address for socket to connect to
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    server_addr.sin_addr.s_addr = INADDR_ANY;

    //bind socket to the IP and port
    bind_status = bind(server_socket, (struct sockaddr *) &server_addr, sizeof(server_addr));
    if (bind_status < 0)
    {
        printf("Error binding to socket\n");
        exit(1);
    }
    printf("Socket binded\n");

    //listen for client connections
    listen_status = listen(server_socket, 10);
    if (listen_status == -1)
    {
        printf("Error listening to socket\n");
        exit(1);
    }
    printf("Listening\n");

    //accept client connection
    socklen_t addr_len;
    addr_len = sizeof(client_socket);
    client_socket = accept(server_socket, (struct sockaddr *) &client_addr, &addr_len);




    FILE *fp = fopen("test.txt", "r");
    char data[512];
    int b;

    while((b = fread(data, 1, 512, fp))>0){
        send(client_socket, data, b, 0);
    }
    fclose(fp);


   // strcpy(data,"test message");
    //printf("server buffer: %s\n", data);
    //send(client_socket, data, 512, 0);

    close(server_socket);
    return 0;
}

Answer 1

You need some way to indicate the server that you have finished sending a file, and now you want to send another thing.

While the socket abstraction seems to show you that the recv and send calls are somehow synchronized, this means that the data you send from the client in one call to send is recv'd in the server with exactly one recv, that is not true, due fundamentally to how the sockets are implemented (the client tcp can decide to split your transfer in several packets, and the unattending of the server can make all those packets to buffer n the receiver before the server receives part of them in one call to recve and others in the text call.

The only thing sure is that a byte that has been sent before, is receive before, and no repeated or missing bytes in between. But the number of bytes received at some recve call is dictated only by the amount of buffered data that one side of the connection has.

This means that, for telling the server that you are finished with your file and some other thing is to be sent, you must do something that allows the server to recognize that the data has ended and more data on a different thing is about to come.

There are several approaches here... you can send an inband sequence (some control sequence) that, wen read in the other side, will be recognized as the end of a block of data and the beginning of the next. Some systems use a packet encoding that simply prepends each block with a number of bytes to follow, and an empty packet (e.g. a single number 0, meaning 0 bytes to follow) can represent this sequence. Another way, can be to use a specific sequence you know is improbable to occur in the data (e.g. \n.\m, two newlines with one dot interspersed ---this has been used many times in unix's ed editor, for example, or the post office protocol uses it.) and if it is the case that such a sequence happens to be in the file, just double the dot, to indicate that it is not the separator sequence. And both ends must agree on this (so called) protocol.

Other mechanisms are valid, you can close the connection and use another socket for a new data exchange.... for example, FTP protocol uses this approach by using one control connection for FTP commands, while using other connections for data transfers.

I have not included code because there are plenty of examples in the literature. Try to get access to the book "Unix Network Programming" of Richard W. Stevens. That's a very good reference to get initiated on socket protocols and how to deal with all these things.