Georg Lippitsch brought CURLOPT_SEEKFUNCTION and CURLOPT_SEEKDATA to allow

libcurl to seek in a given input stream. This is particularly important when
doing upload resumes when there's already a huge part of the file present
remotely. Before, and still if this callback isn't used, libcurl will read
and through away the entire file up to the point to where the resuming
begins (which of course can be a slow opereration depending on file size,
I/O bandwidth and more). This new function will also be preferred to get
used instead of the CURLOPT_IOCTLFUNCTION for seeking back in a stream when
doing multi-stage HTTP auth with POST/PUT.

include/curl/curl.h

@@ -244,6 +244,10 @@ typedef size_t (*curl_write_callback)(char *buffer,
 /* This is a return code for the read callback that, when returned, will
    signal libcurl to pause sending data on the current transfer. */
 #define CURL_READFUNC_PAUSE 0x10000001
+typedef int (*curl_seek_callback)(void *instream,
+                                  curl_off_t offset,
+				  int origin); /* 'whence' */
+
 typedef size_t (*curl_read_callback)(char *buffer,
                                       size_t size,
                                       size_t nitems,
@@ -1180,6 +1184,10 @@ typedef enum {
   /* set transfer mode (;type=<a|i>) when doing FTP via an HTTP proxy */
   CINIT(PROXY_TRANSFER_MODE, LONG, 166),
 
+  /* Callback function for seeking in the input stream */
+  CINIT(SEEKFUNCTION, FUNCTIONPOINT, 167),
+  CINIT(SEEKDATA, OBJECTPOINT, 168),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;