Implement OAuth 2.0 authorization to access Google Drive API in C programming

main.c

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

#include <curl/curl.h>

struct MemoryStruct {
    char *memory;
    size_t size;
};

static size_t WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp)
{
    size_t realsize = size * nmemb;
    struct MemoryStruct *mem = (struct MemoryStruct *)userp;
    
    char *ptr = realloc(mem->memory, mem->size + realsize + 1);
    if(ptr == NULL) {
        /* out of memory! */
        printf("not enough memory (realloc returned NULL)\n");
        return 0;
    }
    
    mem->memory = ptr;
    memcpy(&(mem->memory[mem->size]), contents, realsize);
    mem->size += realsize;
    mem->memory[mem->size] = 0;
    
    return realsize;
}


int main(void)
{
    CURL *curl_handle, *curl_handle2, *curl_handle3, *curl_handle4;
    CURLcode res, res2, res3, res4;
    
    struct MemoryStruct chunk, chunk2, chunk3, chunk4;
    
    chunk.memory = malloc(1);  /* will be grown as needed by the realloc above */
    chunk.size = 0;    /* no data at this point */
    
    char user_code[21];
    char device_code[200];
    char access_token[200];
    
    curl_global_init(CURL_GLOBAL_ALL);
    
    /* init the curl session */
    curl_handle = curl_easy_init();
    
    /* specify URL to get */
    curl_easy_setopt(curl_handle, CURLOPT_URL, "https://accounts.google.com/o/oauth2/device/code");
    
    /* Now specify the POST data */
    /* Step 1: Request device and user codes */
    curl_easy_setopt(curl_handle, CURLOPT_POSTFIELDS, "client_id=603382496792-5ac366vug85fs77v13lh1550cjem0fs5.apps.googleusercontent.com&scope=https://www.googleapis.com/auth/drive.file");
    
    /* send all data to this function  */
    curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
    
    /* we pass our 'chunk' struct to the callback function */
    curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void *)&chunk);
    
    /* some servers don't like requests that are made without a user-agent
     field, so we provide one */
    curl_easy_setopt(curl_handle, CURLOPT_USERAGENT, "libcurl-agent/1.0");
    
    /* get it! */
    res = curl_easy_perform(curl_handle);
    
    /* Step 2: Handle the authorization server response */
    /* check for errors */
    if(res != CURLE_OK) {
        fprintf(stderr, "curl_easy_perform() failed: %s\n",
                curl_easy_strerror(res));
    }
    
    else {
        char *expire_result = strstr(chunk.memory, "\"expires_in");
        int expire_position = (int)(expire_result - chunk.memory);
        
        char *user_result = strstr(chunk.memory, "\"user_code");
        int user_position = (int)(user_result - chunk.memory);
        int uc_length = expire_position - user_position - 19;
        memcpy(user_code, &chunk.memory[user_position] + 14, uc_length);
        user_code[uc_length] = '\0';
        
        char *device_result = strstr(chunk.memory, "device_code");
        int device_position = (int)(device_result - chunk.memory);
        int dc_length = user_position - device_position - 20;
        memcpy(device_code, &chunk.memory[device_position] + 15, dc_length);
        device_code[dc_length] = '\0';
    }
    curl_easy_cleanup(curl_handle);
    
    free(chunk.memory);
    
    /* Step 3: Display the user code */
    printf("Enter below code on 'https://www.google.com/device' :\n");
    printf("%s\n", user_code);
    printf("Press enter when finish\n");
    while( getchar() != '\n' );
    
    /* init the curl session */
    curl_handle2 = curl_easy_init();
    
    /* specify URL to get */
    curl_easy_setopt(curl_handle2, CURLOPT_URL, "https://www.googleapis.com/oauth2/v4/token");
    
    char url[300];
    strcat(url, "client_id=603382496792-5ac366vug85fs77v13lh1550cjem0fs5.apps.googleusercontent.com&client_secret=6kVOv06vdWFKYwMixUFioF5d&code=");
    strcat(url, device_code);
    strcat(url, "&grant_type=http://oauth.net/grant_type/device/1.0");
    
    curl_easy_setopt(curl_handle2, CURLOPT_POSTFIELDS, url);
    
    curl_easy_setopt(curl_handle2, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
    
    curl_easy_setopt(curl_handle2, CURLOPT_WRITEDATA, (void *)&chunk2);
    
    curl_easy_setopt(curl_handle2, CURLOPT_USERAGENT, "libcurl-agent/1.0");
    
    res2 = curl_easy_perform(curl_handle2);
    
    if(res2 != CURLE_OK) {
        fprintf(stderr, "curl_easy_perform() failed: %s\n",
                curl_easy_strerror(res2));
    }
    else {
        //printf("%s\n", chunk2.memory);
        char *ei2_result = strstr(chunk2.memory, "\"expires_in");
        int ei2_position = (int)(ei2_result - chunk2.memory);
        
        char *at_result = strstr(chunk2.memory, "\"access_token");
        int at_position = (int)(at_result - chunk2.memory);
        int at_length = ei2_position - at_position - 22;
        memcpy(access_token, &chunk2.memory[at_position] + 17, at_length);
        access_token[at_length] = '\0';
        
        /* init the curl session */
        curl_handle3 = curl_easy_init();
        
        char token[200];
        strcat(token, "https://www.googleapis.com/drive/v2/files?access_token=");
        strcat(token, access_token);

        curl_easy_setopt(curl_handle3, CURLOPT_URL, token);

        curl_easy_setopt(curl_handle3, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);

        curl_easy_setopt(curl_handle3, CURLOPT_WRITEDATA, (void *)&chunk3);

        curl_easy_setopt(curl_handle3, CURLOPT_USERAGENT, "libcurl-agent/1.0");

        res3 = curl_easy_perform(curl_handle3);
        if(res3 != CURLE_OK){
            fprintf(stderr, "curl_easy_perform() failed: %s\n",
                    curl_easy_strerror(res3));
        }
        else{
            printf("%s\n", "Authorization sucessed");
        }
        
        curl_easy_cleanup(curl_handle3);
        free(chunk3.memory);
    }
    /* cleanup curl stuff */
    curl_easy_cleanup(curl_handle2);
    
    free(chunk2.memory);