http-client.c

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

static void on_close(uv_handle_t* handle);
static void on_connect(uv_connect_t* req, int status);
static void on_write(uv_write_t* req, int status);

static void on_alloc(uv_handle_t* handle, size_t suggested_size, uv_buf_t* buf)
{
  void *ptr;
  printf("on_alloc\n");
  ptr = malloc(suggested_size);
  buf->base = ptr;
  buf->len = suggested_size;
}


static void on_close(uv_handle_t* handle)
{
  printf("on_close\n");
}

static void on_write(uv_write_t* req, int status)
{
  printf("on_write\n", status);
}

static void on_read(uv_stream_t *tcp, ssize_t nread, const uv_buf_t *buf)
{
  if(nread > 0) {
    printf("on_read\n");
    fwrite(buf->base, nread, 1, stdout);
  }
  else {
    /* EOF */
    uv_close((uv_handle_t*)tcp, on_close);
  }

  free(buf->base);
}

static void on_connect(uv_connect_t* connection, int status)
{

  uv_stream_t *stream = connection->handle;
  uv_write_t request;
  uv_buf_t http = {
    .base = "GET / HTTP/1.0\r\n\r\n",
    .len = 18,
  };

  printf("connected.\n");

  uv_write(&request, stream, &http, 1, on_write);
  uv_read_start(stream, on_alloc, on_read);
}

int main(int argc, char **argv)
{
  uv_loop_t *loop = uv_default_loop();
  uv_tcp_t socket;
  struct sockaddr_in addr;
  uv_connect_t connect;

  uv_tcp_init(loop, &socket);
  uv_tcp_keepalive(&socket, 1, 60);
  uv_ip4_addr("127.0.0.1", 80, &addr);
  uv_tcp_connect(&connect, &socket, (const struct sockaddr *) &addr, on_connect);

  uv_run(loop, UV_RUN_DEFAULT);

  return 0;
}

You are allocating the uv_write_t on the stack, but it must remain valid until the write callback is called, otherwise bad things will happen.