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 07:30:22 -0500

That's right.  You can assign a json_t * to the parameter inside your
function, but you can't change the prototype.

e.g.

static size_t curl_to_jansson_to_version(void *buffer, size_t size,
                                     size_t nmemb, void *outstream) {
   json_t *json = outstream;
...
}

Some coding standards would suggest/require a cast to json_t on outstream to make it explicit (and compatible with c++).

In any case, don't worry about introducing the extra variable - the compiler will optimize it away (on most architectures).

Glad things worked out.

P.S. "Tim" is fine.

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 07:17, Gavin Henry via curl-library wrote:
> On Fri, 21 Jan 2022 at 11:36, Timothe Litt via curl-library
> <curl-library_at_lists.haxx.se> wrote:
>> 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...
> Thanks for time here Timothe (sorry, I shortened to TIm before).
>
> So confirm, the only difference in the example vs my usage is the last
> argument to my callback isn't a pointer to void, but to json_t.
> Switching that to void removes the error:
>
> This is fine, which made me look extra hard at my function:
>
> gcc -Wall -Werror -Wextra -Wpedantic -pedantic -Wformat=2
> -Wno-unused-parameter -Wshadow -Wwrite-strings -Wstrict-prototypes
> -Wold-style-definition -Wredundant-decls -Wnested-externs
> -Wmissing-include-dirs -std=c18 -D_FORTIFY_SOURCE=2 -fpie -fpic -g3
> -O2 -fstack-protector-strong -grecord-gcc-switches
> -Werror=format-security -Werror=implicit-function-declaration
> -Wmisleading-indentation -O2 -flto=auto -ffat-lto-objects
> -fexceptions -g -grecord-gcc-switches -pipe -Wall
> -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2
> -Wp,-D_GLIBCXX_ASSERTIONS
> -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
> -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1
> -m64 -mtune=generic -fasynchronous-unwind-tables
> -fstack-clash-protection -fcf-protection -Wl,-z,relro -Wl,--as-needed
> -Wl,-z,now -specs=/usr/lib/rpm/redhat/redhat-hardened-ld
> -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1 -g -O2 -o gitmemory
> gitmemory.c -lcurl
>
>
> Mine was:
>
> static size_t curl_to_jansson_to_version(void *buffer, size_t size,
> size_t nmemb, json_t *json)
> {
>
> and is now:
>
> static size_t curl_to_jansson_to_version(void *buffer, size_t size,
> size_t nmemb, void *json)
> {
>
> so the error has gone.
>
> Thanks for explaining.
>
> Gavin.
>

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Received on 2022-01-21