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Re: CURLOPT_WRITEFUNCTION issues - warning: curl_easy_setopt expects a curl_write_callback argument

From: Timothe Litt <>
Date: Fri, 21 Jan 2022 08:15:14 -0500

I'm confused about what you're trying to do.

outstream is an input parameter.  It will contain whatever you set in

If it's a json structure that's input to your callback, overwriting the
pointer with json_loadb doesn't make sense.

In fact, the value of outstream, now in json isn't used - loadb simply
overwrites it.

Further, your callback may be called many times with pieces of the json
string.  You need to get the entire response before calling json_loadb,
or use another call.

You can use "outstream" as a pointer to a buffer that accumulates the
response, or to a struct with the next write pointer and remaining count
in that buffer, or ...

You need to re-architect this code.

Timothe Litt
ACM Distinguished Engineer
This communication may not represent the ACM or my employer's views,
if any, on the matters discussed.

On 21-Jan-22 07:34, Gavin Henry via curl-library wrote:
> Thanks. Switched it to:
> static size_t curl_to_jansson_to_version(void *buffer, size_t size,
> size_t nmemb, void *outstream)
> {
> json_t *json = outstream;
> json = json_loadb(buffer, size * nmemb, 0, NULL);
> assert_non_null(json);
> then my CLion moans, but is OK :-)
> "The value is never used". I'll leave gcc and clang to sort.

Received on 2022-01-21