server.c

#include <stdio.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc, char **argv) {
    int MasterSocket = socket(
        AF_INET /*IPv4*/,
        SOCK_STREAM /*TCP*/,
        IPPROTO_TCP);

    struct sockaddr_in SockAddr;
    SockAddr.sin_family = AF_INET;
    SockAddr.sin_port = htons(12345);
    SockAddr.sin_addr.s_addr = htonl(INADDR_ANY); //0.0.0.0
    bind(MasterSocket, (struct sockaddr *)(&SockAddr), sizeof(SockAddr));

    listen(MasterSocket, SOMAXCONN);

    while(1) {
        int SlaveSocket = accept(MasterSocket, 0, 0);

        int Buffer[5] = {0, 0, 0, 0, 0};

        unsigned int counter = 0;
        while(counter < 4) {
            int res = recv(SlaveSocket, Buffer + 4 - counter, counter, MSG_NOSIGNAL);
            if(res > 0) counter += res;
        }

        send(SlaveSocket, Buffer, 4, MSG_NOSIGNAL);
        shutdown(SlaveSocket, SHUT_RDWR);
        close(SlaveSocket);

        printf("%s\n", Buffer);
    }
    return 0;
}