client_0: 4
client_1: 8
client_2: 5
client_4: 7

#define PORT 60000

#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <vector>
#include <string>
#include <iostream>
#include <mutex>

using namespace std;

vector<int> inputQueue; //<--------!
mutex mtx; //<---------------------!

void printVector(vector<int> input) {
    cout << "inputQueue: [";
    for (unsigned int i = 0; i < input.size(); i++ ) {
        if (i != input.size() - 1)
            cout << input[i] << ", ";
        else
            cout << input[i];
    }
    cout << "]." << endl;
}

int main(int argc, char const *argv[])
{
    int server_fd, client_fd;
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);

    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons( PORT );
    if (bind(server_fd, (struct sockaddr *)&address, sizeof(address))<0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    if (listen(server_fd, 10) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    while(1) {
        char buffer[4];
        if ((client_fd = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen))<0) {
            perror("accept");
            exit(EXIT_FAILURE);
        }
        if (!fork()) {
            recv(client_fd, buffer, 4, MSG_WAITALL);
            int receivedInt = int(
                    (unsigned char)(buffer[0]) << 24 |
                    (unsigned char)(buffer[1]) << 16 |
                    (unsigned char)(buffer[2]) << 8 |
                    (unsigned char)(buffer[3])
            );
            mtx.lock(); //<-------------------------------------!
            inputQueue.push_back(receivedInt); //<--------------!
            cout << "Client context. Integer registered: " << receivedInt << ": inputQueue length is " << inputQueue.size() << endl;
            printVector(inputQueue); //<------------------------!
            mtx.unlock(); //<-----------------------------------!
            close(server_fd); close(client_fd);
        }
        cout << "Server context: inputQueue length is " << inputQueue.size() << endl;
        printVector(inputQueue);
    }
    return 0;
}

Server context: inputQueue length is 0
inputQueue: [].
Client context. Integer registered: 4: inputQueue length is 1
inputQueue: [4].
Server context: inputQueue length is 1
inputQueue: [4].
Server context: inputQueue length is 0
inputQueue: [].
Client context. Integer registered: 8: inputQueue length is 1
inputQueue: [8].
Server context: inputQueue length is 0
inputQueue: [].
Server context: inputQueue length is 1
inputQueue: [8].
Client context. Integer registered: 5: inputQueue length is 1
inputQueue: [5].
Server context: inputQueue length is 1
inputQueue: [5].
Server context: inputQueue length is 0
inputQueue: [].
Client context. Integer registered: 7: inputQueue length is 1
inputQueue: [7].
Server context: inputQueue length is 1
inputQueue: [7].