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

From: Timothe Litt <litt_at_acm.org>
Date: Fri, 21 Jan 2022 06:35:56 -0500

Casting isn't the answer.  Read the error carefully.  You're returning a
size_t (integer) from a function declared to return a function pointer. 
The pointer is supposed to be to a function that returns a size_t.

multiplying 2 size_t variables results in a size_t, not a function pointer.

curl_write_callback declares a pointer to the callback function. Your
function (as in the example) needs to be the actual function - I assumed
that was obvious.  Sorry that I wasn't explicit.

So your callback wants to be a

typedef size_t (curl_write_callback_action)(char *buffer,
                                       size_t size,
                                       size_t nitems,
                                       void *outstream);

note the omitted "*" vs. the original.

Perhaps curl.h should include typedefs for the callback actions as well
as for the pointers...


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 06:13, Gavin Henry via curl-library wrote:
>> It enables link-time optimizations. The compiler/linker can see further into to the source, and detect errors not seen at the module level.
>>
>> It's saying that your callback routine must be declared/typedefed as a "curl_write_callback".
> Thanks Tim.
>
> Which is actually:
>
> https://curl.se/libcurl/c/getinmemory.html
> as per curl.h and is the same as the example code:
>
> https://curl.se/libcurl/c/getinmemory.html
>
> so if I switch to:
>
>
> static curl_write_callback curl_to_jansson_to_version(void *buffer, size_t size,
> size_t nmemb, json_t *json)
> {
>
>
> I get:
>
> Incompatible integer to pointer conversion returning 'unsigned long'
> from a function with result type 'curl_write_callback' (aka 'unsigned
> long (*)(char *, unsigned long, unsigned long, void *)')
>
> error: returning ‘size_t’ {aka ‘long unsigned int’} from a function
> with return type ‘curl_write_callback’ {aka ‘long unsigned int
> (*)(char *, long unsigned int, long unsigned int, void *)’} makes
> pointer from integer without a cast [-Werror=int-conversion]
> 43 | return size * nmemb;
> | ~~~~~^~~~~~~
>
>
> so I'd need to cast that.
>
>
> Gavin Henry.
> https://sentrypeer.org

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Received on 2022-01-21