Curl request and JSON Parsing in C

build.sh

#!/bin/sh

set -xe

cc -Wall -Werror -std=c11 -pedantic -lcurl -ljson-c -o main main.c

main.c

#include <stdio.h>
#include <string.h>

#include <curl/curl.h>
#include <json-c/json.h>

struct ResponseBuffer {
  char *payload;
  size_t size;
};

int curl_callback(void *data, size_t size, size_t nmemb, void *buffer_in) {
  size_t real_size = size * nmemb;

  struct ResponseBuffer *p = (struct ResponseBuffer *) buffer_in;

  char *temp = realloc(p->payload, p->size + real_size + 1);

  if (temp == NULL) {
    fprintf(stderr, "ERROR: Failed to expand buffer in curl_callback.\n");
    free(p->payload);
    return 1;
  }

  p->payload = temp;

  memcpy(&(p->payload[p->size]), data, real_size);

  p->size += real_size;

  p->payload[p->size] = 0;

  return real_size;
}

CURLcode make_curl_request(char *url, struct ResponseBuffer *rb) {
  CURL *curl;

  struct curl_slist *headers = NULL;
  headers = curl_slist_append(headers, "Accept: application/json");
  headers = curl_slist_append(headers, "charset: utf-8");

  curl_global_init(CURL_GLOBAL_ALL);

  curl = curl_easy_init();

  if (curl == NULL) {
    fprintf(stderr, "ERROR: Could not initialize curl.\n");
    return 1;
  }

  curl_easy_setopt(curl, CURLOPT_URL, url);
  curl_easy_setopt(curl, CURLOPT_HTTPGET, 1);
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
  curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, curl_callback);
  curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *) rb);
  curl_easy_setopt(curl, CURLOPT_USERAGENT, "libcurl-agent/1.0");
  curl_easy_setopt(curl, CURLOPT_TIMEOUT, 15);
  curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1);
  curl_easy_setopt(curl, CURLOPT_MAXREDIRS, 1);

  CURLcode response_code =  curl_easy_perform(curl);

  curl_easy_cleanup(curl);
  curl_slist_free_all(headers);
  curl_global_cleanup();

  return response_code;
}

int main(void) {
  char url[] = "https://api.myip.com";

  struct ResponseBuffer response_buffer;
  response_buffer.payload = (char *) calloc(1, sizeof(response_buffer.payload));

  if (response_buffer.payload == NULL) {
    fprintf(stderr, "ERROR: Failed to allocate response buffer.\n");
    return CURLE_FAILED_INIT;
  }

  response_buffer.size = 0;

  CURLcode response_code = make_curl_request(url, &response_buffer);

  if (response_code != CURLE_OK) {
    fprintf(stderr, "ERROR: Request failed: %s\n", curl_easy_strerror(response_code));
    return 1;
  } else {
    printf("DEBUG: Raw response: %d\n", response_code);
  }

  if (response_buffer.payload == NULL) {
    fprintf(stderr, "ERROR: Failed to populate response buffer.");
    return 1;
  }

  enum json_tokener_error jerr = json_tokener_success;

  json_object *json = json_tokener_parse_verbose(response_buffer.payload, &jerr);

  printf("DEBUG: Size - %zu\n", response_buffer.size);
  printf("DEBUG: Payload - %s\n", response_buffer.payload);
  printf("DEBUG: JSON parsed - %s\n", json_object_to_json_string(json));

  free(response_buffer.payload);

  struct json_object *ip;
  struct json_object *country;
  struct json_object *cc;

  json_object_object_get_ex(json, "ip", &ip);
  json_object_object_get_ex(json, "country", &country);
  json_object_object_get_ex(json, "cc", &cc);

  printf("INFO: Result: \n");
  printf("  My ip: %s\n", json_object_get_string(ip));
  printf("  My country: %s\n", json_object_get_string(country));
  printf("  My country code: %s\n", json_object_get_string(cc));

  json_object_put(json);

  return 0;
}