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 10:31:37 -0500

This is wandering off topic for curl.

json_loadb wants the full JSON string.

You can use the example previously cited, and call json_loadb where the
transfer is complete.  Something like.

https://curl.se/libcurl/c/getinmemory.html

  *else* {
     //* * Now, our chunk.memory points to a memory block that is chunk.size
* bytes big and contains the remote file. * * Do something nice with it! *//
   json_t response;
    json_error_t error;

     printf(*"%lu bytes retrieved at address %p\n"*, (*unsigned* *long*)chunk.size, chunk.memory);

         response = json_loadb(chunk.memory, chunk.size, 0, &error);

        if( !response ) {

         fprintf( stderr, "Response error: %s at line %d, column %d\n",
error.text? error.text: "unspecified", error.line, error.column ); exit(1);

}

jjson_t * val = json_object_get( json, ...); const char *ver =
json_string_value( val );

janson_decref(json); json = NULL;

And free the chunk.memory before exiting.

This is a long way from a compiler error - but note that every error you
reported was a legitimate error in your code.

You need to be very, very careful about ignoring errors - while they
don't catch the architectural errors, they almost always indicate a real
problem - functional or portability.

Have fun.

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 10:00, Gavin Henry via curl-library wrote:
> Hi Timothe,
>
>> I'm confused about what you're trying to do.
> It seems I am too as I have fully understood the callback.
>
>> outstream is an input parameter. It will contain whatever you set in the CURLOPT_WRITEDATA option.
>>
>> If it's a json structure that's input to your callback, overwriting the pointer with json_loadb doesn't make sense.
> No, it doesn't now. What I'm trying to do is save the result of this,
> then pull out the "version" and compare it to the PACKAGE_VERSION of
> SentryPeer in my test suite. So if it was in bash or on the CLI:
>
> curl -H "Content-Type: application/json" http://127.0.0.1:8082/health-check { "status": "OK",
> "message": "Hello from SentryPeer!",
> "version": "0.0.6"
> }
>
>
>> 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.
> It's my misunderstanding of where to put json_loadb(). It's not
> supposed to be done in the callback, but filling up a json buffer in
> there and processing wherever I've set CURLOPT_WRITEDATA, correct?
>
> As the callback, as you say, may not parse all of the above json in
> one go or be given it from the API endpoint.
>
> My goal is to be able save/process the returned JSON and pull out the
> version string to do:
>
> json_t *sentrypeer_version_json = json_object_get(json, "version");
> assert_string_equal(json_string_value(sentrypeer_version_json),
> PACKAGE_VERSION);
>
> Thanks.

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Received on 2022-01-21