Why does this simple server fail on very low concurrent request? - c++

Question

#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <unistd.h>
#include <iostream>
#include <pthread.h>
int f(int x) {
if (x == 0) return 1;
return x % 100;
}
void *deal(void *arg) {
int fd = * (int *) arg;
int x;
int n = read(fd, &x, sizeof x);
if (n < 0) std::cout << "error reading" << std::endl;
std::cout << "request " << x << " received" << std::endl;
int y = f(x);
write(fd, &y, sizeof y);
std::cout << "response to " << x << ": " << y << std::endl;
close(fd);
return NULL;
}
int main(int argc, char *argv[]) {
int port = atoi(argv[1]);
int socket_fd = socket(AF_INET, SOCK_STREAM, 0);
sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = INADDR_ANY;
addr.sin_port = htons(port);
bind(socket_fd, (sockaddr *) &addr, sizeof addr);
listen(socket_fd, 20);
std::cout << "listen on " << port << std::endl;
sockaddr remote_addr;
socklen_t len;
for (; ;) {
int fd = accept(socket_fd, &remote_addr, &len);
pthread_t p;
pthread_create(&p, NULL, deal, (void *) &fd);
}
return 0;
}
This is the code of the server. It simply create a new thread for each connection and then read an integer, then reply the integer % 100.
It works well when there are no concurrent requests. However, it should also work on concurrent. But it failed on just 10 concurrent requests.

Answer 1

You are using the memory for fd time and time again in different threads. I.e. it is getting overwritten on each accept.
Change
pthread_create(&p, NULL, deal, (void *) &fd);
to
pthread_create(&p, NULL, deal, (void *) fd);
And
void *deal(void *arg) {
int fd = * (int *) arg;
to
void *deal(void *arg) {
int fd = (int) arg;