How do I send long PUT data in libcurl without using file pointers?

c++, curl, http, libcurl, post

Solution

You will still want to use `CURLOPT_READDATA`, however if you use `CURLOPT_READFUNCTION`, it can be any user-specified pointer. You can create a simple structure like:

struct put_data
{
  char *data;
  size_t len;
};

where `data` is the PUT data and `len` is the length (remaining).

Then, set `CURLOPT_READDATA` to a pointer to an initialized instance of that structure. You will be passed it in `CURLOPT_READFUNCTION` as `userdata`. In that function, do something like:

size_t curl_size = nmemb * size;
size_t to_copy = (userdata->len < curl_size) ? userdata->len : curl_size;
memcpy(ptr, userdata->data, to_copy);
userdata->len -= to_copy;
userdata->data += to_copy;
return to_copy;

This basically figures out the amount to copy, copies it, then updates the length and pointer. On the `to_copy` line, we calculate the minimum because were are bounded by both the amount remaining and the size of curl's buffer. Finally, we return the number of bytes copied, as required by curl. When you're at the end of content `user_data->len` (and thus `to_copy`) will be 0. Nothing will be copied and returning 0 ends the transfer.

Problem

I'm trying to interface with the Google Contact API, and in order to update a contact on the remote side, I need to use a PUT request instead of POST. Since the data I want to PUT is already in memory, I really don't want to deal with file pointers, which seems to be the default behavior of CURLOPT_READDATA. Thus, I understand I need to use CURLOPT_READFUNCTION in addition to that but I'm failing to understand the logic. Here is the relevant doc: http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTREADFUNCTION Printing the variables passed in, I see `nmemb=16372` and `size=1`. If my data is going to be of size 100K, for example, what is the logic that I need to implement? Thank you.

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