Curling some JSON for fun!

curl_json_test.cpp

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

#define CURL_STATICLIB

#include <curl/curl.h>
#include "picojson.h"

using namespace std;

/**
 * @link https://github.com/kazuho/picojson
 * @link https://stackoverflow.com/questions/2329571/c-libcurl-get-output-into-a-string#answer-2329792
 * @link http://xdev.me/en/article/C++_download_a_URL_with_libcurl
 *
 * Compiling with `-static` makes libcurl blow up, we use `#define CURL_STATICLIB` in the page (or it can be set via compiler flag.)
 * `g++ -std=c++11 -Wall curl_test.cpp -lcurl -lwsock32 -lidn -lwldap32 -lssh2 -lrtmp -lcrypto -lz -lws2_32 -lwinmm -lssl`
 */

typedef struct {
    char *ptr;
    size_t len;
} string_t;

void init_string(string_t *s) {
    s->len = 0;
    s->ptr = (char*)malloc(s->len + 1);

    if (s->ptr == NULL) {
        fprintf(stderr, "malloc() failed\n");

        exit(EXIT_FAILURE);
    }

    s->ptr[0] = '\0';
}

size_t write_fn(void *ptr, size_t size, size_t nmemb, string_t *s) {
    size_t new_len = s->len + (size * nmemb);
    s->ptr = (char*)realloc(s->ptr, new_len + 1);

    if (s->ptr == NULL) {
        fprintf(stderr, "realloc() failed\n");

        exit(EXIT_FAILURE);
    }

    memcpy(s->ptr+s->len, ptr, size * nmemb);
    s->ptr[new_len] = '\0';
    s->len = new_len;

    return size * nmemb;
}

int main(void)
{
    CURL *curl;
    CURLcode res;

    string_t s;
    init_string(&s);

    curl = curl_easy_init();

    if(curl) {
        curl_easy_setopt(curl, CURLOPT_URL, "http://localhost/json-test.php");

        // Tell libcurl to follow redirection
        curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_fn);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &s);

        // Perform the request, res will get the return code
        res = curl_easy_perform(curl);

        // Check for errors
        if (res != CURLE_OK) {
            fprintf(stderr, "curl_easy_perform() failed: %s\n",
            curl_easy_strerror(res));
        }

        const char *json = (const char*)s.ptr;

        picojson::value v;
        string err = picojson::get_last_error();
        picojson::parse(v, json, json + strlen(json), &err);

        if (!err.empty()) {
            cerr << err << endl;
        }

        // check if the type of the value is "object"
        if (! v.is<picojson::object>()) {
            cerr << "JSON is not an object" << endl;
            exit(2);
        }

        // obtain a const reference to the map, and print the contents
        const picojson::value::object& obj = v.get<picojson::object>();

        for (picojson::value::object::const_iterator i = obj.begin(); i != obj.end(); ++i) {
            cout << i->first << ": " << i->second.to_str() << endl;
        }

        // Always cleanup
        curl_easy_cleanup(curl);
    }

    return 0;
}