udp server sample using libuv

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "uv.h"

#define CHECK(r, msg)                                       \
    if (r<0) {                                              \
        fprintf(stderr, "%s: %s\n", msg, uv_strerror(r));   \
        exit(1);                                            \
    }
  
static uv_loop_t *uv_loop;
static uv_udp_t   server;
 
static void on_recv(uv_udp_t* handle, ssize_t nread, const uv_buf_t* rcvbuf, const struct sockaddr* addr, unsigned flags) {
    if (nread > 0) {
        printf("%lu\n",nread);
        printf("%s",rcvbuf->base);
    }
    printf("free  :%lu %p\n",rcvbuf->len,rcvbuf->base);
    free(rcvbuf->base);
}
 
static void on_alloc(uv_handle_t* client, size_t suggested_size, uv_buf_t* buf) {
    buf->base = malloc(suggested_size);
    buf->len = suggested_size;
    printf("malloc:%lu %p\n",buf->len,buf->base);
}

int main(int argc,char *argv[]) {
    int status;
    struct sockaddr_in addr;
    uv_loop = uv_default_loop();
    
    status = uv_udp_init(uv_loop,&server);
    CHECK(status,"init");
    
    uv_ip4_addr("0.0.0.0", 11000, &addr);

    status = uv_udp_bind(&server, (const struct sockaddr*)&addr,0);
    CHECK(status,"bind");
    
    status = uv_udp_recv_start(&server, on_alloc, on_recv);
    CHECK(status,"recv");
    
    uv_run(uv_loop, UV_RUN_DEFAULT);
    
    return 0;
}

Makefile

LIBUV_HOME := $(HOME)/src/github.com/joyent/libuv
LIBUV := $(LIBUV_HOME)/build/Release/libuv.a
INC = $(LIBUV_HOME)/include

all:
	gcc -o main -I$(INC) -Wall main.c $(LIBUV)

PHONY: all

It's better to clean memory with bzero() after calling malloc() on on_alloc() callback, or directly use calloc(1, suggested_size), otherwise you'll risk to read unterminated string when you printf() on rcvbuf->base in on_recv() callback. This is a security and stability issue.
The rest of the file seems OK. Thank you.

Is there a way to get the IP address of the interface which received the frame in on_recv?

You can use uv_ip4_name() on the returned addr parameter.

char sender[17] = { 0 };
uv_ip4_name((const struct sockaddr_in*) addr, sender, 16);
printf("Recv from %s\n", sender);