curl aternative to InternetReadFile - c

Question

I wanted to check whether curl has any alternative like InternetReadFile which returns the content with size specified in the buffer size.
I have used:
curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, Read_Cb);
curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, &ReadBuffer);
curl_easy_perform(curl_handle);
But my Read_Cb gets called back multiple time (which is documented behaviour) and that is fine.
I want curl_easy_perform to return when my buffer size is reached. I explored CURLOPT_BUFFERSIZE, but that doesn't seem to help here.
CURLE_WRITE_ERROR is a problem becuase it aborts the transfer. I could have returned something from my callback which will gracefully tell curl to return curl_easy_perform.

Answer 1

Does CURLOPT_RANGE help?
CURL *curl = curl_easy_init();
if (curl)
{
size_t from = 0, to = 1024;
char range[64];
snprintf(range, sizeof range, "%zu-%zu", from, to);
curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
curl_easy_setopt(curl, CURLOPT_RANGE, range);
curl_easy_perform(curl);
}

Answer 2

If the server supports range requests, use HTTP Range:
CURL* curl = curl_easy_init();
if (curl) {
char range[32];
const size_t size = 65536;
snprintf(range, sizeof(range), "0-%zu", size - 1);
curl_easy_setopt(curl, CURLOPT_URL, "http://httpbin.org/range/65536");
curl_easy_setopt(curl, CURLOPT_RANGE, range);
curl_easy_perform(curl);
}
See CURLOPT_RANGE for more details.
If the server does not support range requests, use a progress or a write function to terminate the request if the size is reached.
const size_t size = 65535;
struct memory {
char* response;
size_t size;
};
static size_t cb(void* data, size_t size, size_t nmemb, void* userp) {
size_t realsize = size * nmemb;
struct memory* mem = (struct memory*)userp;
if (mem->size + realsize > size)
realsize = size - mem->size;
char* ptr = realloc(mem->response, mem->size + realsize);
if (ptr == NULL)
return 0; /* out of memory! */
mem->response = ptr;
memcpy(&(mem->response[mem->size]), data, realsize);
mem->size += realsize;
/* if realsize < size * nmemb, this will cause the transfer to get
aborted and curl_easy_perform will return URLE_WRITE_ERROR */
return realsize;
}
struct memory chunk = {0};
CURL* curl = curl_easy_init();
if (curl) {
curl_easy_setopt(curl, CURLOPT_URL, "http://httpbin.org/stream-bytes/131072");
/* send all data to this function */
curl_easy_setopt(curl_handle, CURLOPT_WRITEFUNCTION, cb);
/* we pass our 'chunk' struct to the callback function */
curl_easy_setopt(curl_handle, CURLOPT_WRITEDATA, (void*)&chunk);
curl_easy_perform(curl);
}
If URLE_WRITE_ERROR is not desired, you can use curl_multi_perform(multi, &running_handles) and remove handle from multi if size reached. See curl_multi_remove_handle for more details.