A C socket server that runs a command and returns the response, socket input is ignored

server.c

// A simple C socket server cobbled of examples that runs a command
// and returns the response over the socket, socket input is ignored.

// MIT LICENSE / NO WARRANTY OR GUARANTEE OF FITNESS FOR ANY PURPOSE

// ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !
// Please note and fill in the defines below
// ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

#define CMD      "date"    // The command to run
#define BUFSIZE  4096      // CMD stdout buffer, max length
#define PORT     9550      // The port to listen on
#define SLEEP_MS 500       // Intentional rate limiting / DDOS

#include <unistd.h>
#include <stdio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <string.h>

void start_listener();

int main(int argc, char const *argv[])
{
  start_listener();
  return 0;
}

char buf[BUFSIZE];
char *cmd = CMD;

char* run_command(void)
{
  FILE *fp;

  if ((fp = popen(cmd, "r")) == NULL) {
    printf("Error opening pipe!\n");
    return NULL;
  }

  int idx = 0;
  char ch;
  while (EOF != (ch = fgetc(fp))) {
    buf[idx++] = ch;
    if (idx >= BUFSIZE) return NULL; // cmd buffer overflow
  }

  if (pclose(fp))  {
    printf("Command not found or exited with error status\n");
    return NULL;
  }

  return buf;
}

void start_listener()
{
  int server_fd, new_socket, valread;
  struct sockaddr_in address;
  int opt = 1;
  int addrlen = sizeof(address);

  // Creating socket file descriptor
  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 );

  // Forcefully attaching socket to the port 8080
  if (bind(server_fd, (struct sockaddr *)&address,
           sizeof(address))<0)
    {
      perror("bind failed");
      exit(EXIT_FAILURE);
    }
  if (listen(server_fd, 3) < 0)
    {
      perror("listen");
      exit(EXIT_FAILURE);
    }

  while (1) {

    if ((new_socket = accept(server_fd, (struct sockaddr *)&address,
                             (socklen_t*)&addrlen))<0)
      {
        perror("accept");
        exit(EXIT_FAILURE);
      }

    char* result = run_command();
    if (result) {
      send(new_socket , result , strlen(result) , 0 );
      printf("Result sent\n");
    }

    close(new_socket);

    usleep(SLEEP_MS * 1000); // Throttle connections by SLEEP_MS milliseconds
  }

}