CURLOPT_READFUNCTION.md: mention the seek callback

Brought-up-by: Jeroen Ooms
URL: https://curl.se/mail/lib-2025-05/0030.html
Closes #17455

diff --git a/docs/libcurl/opts/CURLOPT_READFUNCTION.md b/docs/libcurl/opts/CURLOPT_READFUNCTION.md
index 9e3c3d1f85494fb7004adee63890989f21cfff3c..71ad2e9ca0fd0f6e5ec1085b4deafd1c50a90625 100644 (file)
--- a/docs/libcurl/opts/CURLOPT_READFUNCTION.md
+++ b/docs/libcurl/opts/CURLOPT_READFUNCTION.md
@@ -71,6 +71,11 @@ CURLOPT_INFILESIZE_LARGE(3) or CURLOPT_POSTFIELDSIZE_LARGE(3),
 depending on the type of transfer. For some transfer types it may be required
 and it allows for better error checking.
 
+When this option is used in combination with telling libcurl to follow
+redirects with CURLOPT_FOLLOWLOCATION(3), the data might need to be rewound
+and sent again. The CURLOPT_SEEKFUNCTION(3) can then be invoked for that
+rewind operation.
+
 # DEFAULT
 
 fread(3)