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Re: Please help me understand va_alist
From: |
Bruno Haible |
Subject: |
Re: Please help me understand va_alist |
Date: |
Thu, 06 Feb 2020 13:30:11 +0100 |
User-agent: |
KMail/5.1.3 (Linux/4.4.0-171-generic; KDE/5.18.0; x86_64; ; ) |
Hi,
> after several trials and errors I was able to get ffcallback to work.
> However I don't understand how to use va_alist and how to adapt it for
> my own use case beyond the basic examples that I've copy-pasted.
> Consider this snippet, for example, taken from "ffcall/callback/tests.c":
>
> void i_i_simulator (void* data, va_alist alist)
> {
> va_start_int(alist);
> {int a = va_arg_int(alist);
> int r=a+1;
> fprintf(out,"int f(int):(%d)",a);
> fflush(out);
> va_return_int(alist, r);
> }}
>
> what do va_start_int and va_return_int do? The manpage has them defined like
> this:
>
> va_start_type(alist[, return_type]);
> arg = va_arg_type(alist[, arg_type]);
> va_return_type(alist[[, return_type], return_value]);
>
> but what are all those optional return_type/return_value arguments and what
> are they used for?
As written in callback.html:
arg = va_arg_type(alist[, arg_type]);
fetches the next argument from the argument list.
va_return_type(alist[[, return_type], return_value]);
ends the walk through the argument list and specifies the return value.
int a = va_arg_int(alist);
fetches the next argument, assuming it is of type 'int'.
va_return_int(alist, r);
stores r as the return value of the call, assuming the return type is of
type 'int'.
> Finally, what if one of my functions accepts a "struct" as argument insted of
> int? Should I use "va_start_struct_mystruct"?
Yes, to fetch a 'struct mystruct' argument, you'll use
struct mystruct a = va_arg_struct (alist, struct mystruct);
Bruno