CURLOPT_RESUME_FROM - Man Page

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_RESUME_FROM, long from);

Pass a long as parameter. It contains the offset in number of bytes that you want the transfer to start from. Set this option to 0 to make the transfer start from the beginning (effectively disabling resume). For FTP, set this option to -1 to make the transfer start from the end of the target file (useful to continue an interrupted upload).

When doing uploads with FTP, the resume position is where in the local/source file libcurl should try to resume the upload from and it then appends the source file to the remote target file.

If you need to resume a transfer beyond the 2GB limit, use CURLOPT_RESUME_FROM_LARGE(3) instead.

0, not used

This functionality affects all supported protocols

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    long size_of_file;

    curl_easy_setopt(curl, CURLOPT_URL, "ftp://example.com");

    /* resume upload at byte index 200 */
    curl_easy_setopt(curl, CURLOPT_RESUME_FROM, 200L);

    /* ask for upload */
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);

    /* set total data amount to expect */
    curl_easy_setopt(curl, CURLOPT_INFILESIZE, size_of_file);

    /* Perform the request */
    curl_easy_perform(curl);
  }
}

Added in curl 7.1

Returns CURLE_OK

CURLOPT_INFILESIZE(3), CURLOPT_RANGE(3), CURLOPT_RESUME_FROM_LARGE(3)

curl_easy_setopt(3), CURLOPT_APPEND(3), CURLOPT_RANGE(3), CURLOPT_RESUME_FROM_LARGE(3), libcurl-symbols(3).