CURLOPT_WRITEDATA man page
CURLOPT_WRITEDATA — custom pointer passed to the write callback
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_WRITEDATA, void *pointer);
A data pointer to pass to the write callback. If you use the CURLOPT_WRITEFUNCTION(3) option, this is the pointer you'll get in that callback's 4th argument. If you don't use a write callback, you must make pointer a 'FILE *' (cast to 'void *') as libcurl will pass this to fwrite(3) when writing data.
The internal CURLOPT_WRITEFUNCTION(3) will write the data to the FILE * given with this option, or to stdout if this option hasn't been set.
If you're using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITEFUNCTION(3) if you set this option or you will experience crashes.
By default, this is a FILE * to stdout.
Used for all protocols.
A common technique is to use the write callback to store the incoming data into a dynamically growing allocated buffer, and then this CURLOPT_WRITEDATA(3) is used to point to a struct or the buffer to store data in. Like in the getinmemory example: https://curl.haxx.se/libcurl/c/getinmemory.html
Available in all libcurl versions. This option was formerly known as CURLOPT_FILE, the name CURLOPT_WRITEDATA(3) was introduced in 7.9.7.
This will return CURLE_OK.
curl_easy_setopt(3), CURLOPT_INTERLEAVEFUNCTION(3), CURLOPT_READDATA(3), CURLOPT_WRITEFUNCTION(3), libcurl-tutorial(3).