CURLOPT_UPLOAD_BUFFERSIZE - upload buffer size

Added in curl 7.62.0

65536 bytes

Pass a long specifying your preferred size (in bytes) for the upload buffer in libcurl. It makes libcurl uses a larger buffer that gets passed to the next layer in the stack to get sent off. In some setups and for some protocols, there is a huge performance benefit of having a larger upload buffer. 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. The upload buffer is allocated on-demand - so if the handle is not used for upload, this buffer is not allocated at all. DO NOT set this option on a handle that is currently used for an active transfer as that may lead to unintended consequences.

int main(void) { CURL *curl = curl_easy_init(); if(curl) { CURLcode res; 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); res = curl_easy_perform(curl); curl_easy_cleanup(curl); } }

CURLOPT_UPLOAD_BUFFERSIZE - upload buffer size

This functionality affects all supported protocols

curl_easy_setopt(3) returns a CURLcode indicating success or error. CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3).

CURLOPT_BUFFERSIZE(3), CURLOPT_READFUNCTION(3), CURLOPT_TCP_NODELAY(3) libcurl 2025-06-16 CURLOPT_UPLOAD_BUFFERSIZE(3)

#include <curl/curl.h> CURLcode curl_easy_setopt(CURL *handle, CURLOPT_UPLOAD_BUFFERSIZE, long size);