UDP echo server sample.

client.c

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

struct udp_socket {
  int fd;
  struct sockaddr_in addr;
};

struct sockaddr_in create_sockaddr_in(const char *target, const int port) {
  struct sockaddr_in addr = { .sin_family = AF_INET };
  addr.sin_port = htons(port);
  addr.sin_addr.s_addr = inet_addr(target);
  return addr;
}

struct udp_socket connect_udp(const char *target, const int port) {
  struct udp_socket sock;
  sock.fd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock.fd < 0) {
    perror("sock");
    return sock;
  }
  sock.addr = create_sockaddr_in(target, port);

  static const int nonblocking = 1;
  ioctl(sock.fd, FIONBIO, &nonblocking);

  return sock;
}

int send_by_udp (const struct udp_socket sock, char *msg, size_t length, size_t offset) {
  return sendto(sock.fd, msg, length, offset, (struct sockaddr *)&sock.addr, sizeof(sock.addr));
}

int recv_by_udp (const struct udp_socket sock, char *buf, size_t length, size_t offset) {
  socklen_t addrlen = sizeof(sock.addr);
  return recvfrom(sock.fd, buf, length, offset, (struct sockaddr *)&sock.addr, &addrlen);
}

int main() {
  const struct udp_socket sock = connect_udp("127.0.0.1", 5000);
  if (sock.fd < 0) {
    perror("failed to connect. udp://127.0.0.1:5000");
    return -1;
  }

  int loop = 1;
  while (loop) {
    int length = 0;
    char buf[4096];

    printf("send: ");
    if (fgets(buf, 4096, stdin) == NULL) {
      continue;
    }

    send_by_udp(sock, buf, strlen(buf), 0);
    while ((length = recv_by_udp(sock, buf, 4096, 0)) < 0);
    printf("recv: %.*s", length, buf);
  }

  close(sock.fd);
  return 0;
}

server.c

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

struct udp_socket {
  int fd;
  struct sockaddr_in addr;
};

struct sockaddr_in create_sockaddr_in(const char *target, const int port) {
  struct sockaddr_in addr = { .sin_family = AF_INET };
  addr.sin_port = htons(port);
  addr.sin_addr.s_addr = inet_addr(target);
  return addr;
}

struct udp_socket listen_udp(const char *target, const int port) {
  struct udp_socket sock;
  sock.fd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock.fd < 0) {
    perror("sock");
    return sock;
  }

  const struct sockaddr_in addr = create_sockaddr_in(target, port);
  bind(sock.fd, (struct sockaddr *)&addr, sizeof(addr));

  static const int nonblocking = 1;
  ioctl(sock.fd, FIONBIO, &nonblocking);

  return sock;
}

int send_by_udp (const struct udp_socket *sock, char *msg, size_t length, size_t offset) {
  return sendto(sock->fd, msg, length, offset, (struct sockaddr *)&sock->addr, sizeof(sock->addr));
}

int reply_by_udp (const struct udp_socket *sock, char *msg, size_t length, size_t offset) {
  return sendto(sock->fd, msg, length, offset, (struct sockaddr *)&sock->addr, sizeof(sock->addr));
}

int recv_by_udp (const struct udp_socket *sock, char *buf, size_t length, size_t offset) {
  socklen_t addrlen = sizeof(sock->addr);
  return recvfrom(sock->fd, buf, length, offset, (struct sockaddr *)&sock->addr, &addrlen);
}

int main() {
  const struct udp_socket sock = listen_udp("127.0.0.1", 5000);
  if (sock.fd < 0) {
    perror("failed to connect. udp://127.0.0.1:5000");
    return -1;
  }

  int loop = 1;
  while (loop) {
    int offset = 0;
    int length = 0;
    char buf[4096];
    while ((length = recv_by_udp(&sock, buf, 4096, offset)) > 0) {
      send_by_udp(&sock, buf, length, 0);
      printf("recv: %.*s", length, buf);
      offset += length;
    }
  }

  close(sock.fd);
  return 0;
}

server only work on localhost (tested with netcat).