curlopt_WRITEFUNCTION

CURLOPT_WRITEFUNCTION is a callback function used to write data received during a transfer to a user-defined buffer.

When performing an HTTP request using libcurl, the data received from the server is stored in a buffer. This buffer can be either pre-allocated or dynamically allocated by libcurl. By default, libcurl will write the received data to the standard output stream (stdout). However, in most cases, users will want to store this data in a custom buffer to process it later.

To achieve this, users can provide a callback function to CURLOPT_WRITEFUNCTION. This function is called by libcurl every time it receives data from the server. The callback function should have the following prototype:

size_t write_callback(char *ptr, size_t size, size_t nmemb, void *userdata);

where:

- `ptr`: A pointer to the received data.
- `size`: The size of each data element.
- `nmemb`: The number of data elements received.
- `userdata`: A pointer to user-defined data.

The callback function should return the number of bytes written to the user-defined buffer. If the function returns a value different from `(size * nmemb)`, libcurl will consider the transfer as failed, and it will return an error code.