CURLcode curl_easy_setopt(CURL *handle, CURLOPT_UPLOAD_BUFFERSIZE, long size);
This is just treated as a request, not an order. You cannot be guaranteed to actually get the given size.
The upload buffer size is by default 64 kilobytes. The maximum buffer size allowed to be set is 2 megabytes. The minimum buffer size allowed to be set is 16 kilobytes.
Since curl 7.61.1 the upload buffer is allocated on-demand - so if the handle isn't used for upload, this buffer will not be allocated at all.
CURL *curl = curl_easy_init(); if(curl) { curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com/foo.bin"); /* ask libcurl to allocate a larger upload buffer */ curl_easy_setopt(curl, CURLOPT_UPLOAD_BUFFERSIZE, 120000L); ret = curl_easy_perform(curl); curl_easy_cleanup(curl); }