how to implement Transfer complete callback in esp_idf_webserver

[noweare]

Getting this error when trying to use "transfer complete" callback in my code:
In function 'esp_err_t echo_handler(httpd_req_t*)':
src/main.cpp:245:44: error: expression list treated as compound expression in functional cast [-fpermissive]
transfer_complete_cb(ret, socketID, msg);

in the esp_htp_sever.h there is a callback prototype

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typedef void (*transfer_complete_cb)(esp_err_t err, int socket, void *arg);

in my code i am calling it by:

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char msg[20] = "transfer complete";
 transfer_complete(ret, socketID, msg);

I am implementing the callback function like this:

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void transfer_complete(esp_err_t err, int socket, void *arg)
{
  char *myMsg = (char *)arg;
  log_i("transfer complete skt %d, arg %s with error %d", socket, myMsg, err);
  return;
}

I have looked at other invocations of callbacks in the example code of different componants and
sometimes the callback is registered like in events but mostly nothing special is done just drop the typedef and the * and change
the name of the function and it works but not in this case.

[noweare]

Now the same code compiles and it works. So I don't know why that is.

But I do have a question:
What is the advantage of using that callback rather than just writing my own function?
There is no system setup to use this function as a callback in any automated way.

[MicroController]

transfer_complete_cb(ret, socketID, msg);

transfer_complete_cb is the type of the callback function pointer, while your function's name is/was transfer_complete, without the _cb. So the compiler read it as you trying to cast the compound expression "ret, socketId, msg" to a value of type "transfer_complete_cb".