[llvm-dev] Fwd: about clang compile c to wasm lost argument

[Petr Penzin via llvm-dev]

Hi,

The reason this happens is that Wasm functions can take a predefined 
number of parameters only, they cannot be truly variadic. Variadic 
functions are emulated by putting the extra arguments in memory and 
passing a pointer ("memory index" in wasm) to the first one instead of 
`...` part of the argument list. That's why printf takes two arguments - 
first one is format string, the second is the pointer to the rest of the 
arguments.

Best,
Petr


On 12/19/21 02:48, shankusu2017 via llvm-dev wrote:
>
>
> ---------- Forwarded message ---------
> 发件人： *shankusu2017* <shankusu2017 at gmail.com>
> Date: 2021年12月19日周日 18:17
> Subject: Fwd: about clang compile c to wasm lost argument
> To: <llvm-admin at lists.llvm.org>
>
>
>
>
> ---------- Forwarded message ---------
> 发件人： *shankusu2017* <shankusu2017 at gmail.com>
> Date: 2021年12月19日周日 18:15
> Subject: about clang compile c to wasm lost argument
> To: <surma at surma.dev>
>
>
> hi:
>    I use clang to compile the c code into wasm, and I found that 
> clang13/14 regards the variable reference in the c language as a 
> parameter, such as the "..." part in int printf(const char *format, 
> ...); In this way, when I implemented my own printf function in the 
> host environment, wasm only passed 2 parameters to the host when 
> calling prinf, but I actually passed more than 2 parameters. This will 
> cause the printf function I implemented by myself to receive only 2 
> parameters, and thus lose one parameter. The attachment contains my 
> original c code and compiled wasm and wat codes. The green box part 
> shows that the printf function only accepts 2 input parameters. I 
> don't know how to make clang understand that the number of parameters 
> received by printf is uncertain. Is there any solution?
>    follow build cmd:
> clang -I ../../src/ --target=wasm32 -nostdlib  -Wl,--no-entry 
> -Wl,--export-all -Wl,--allow-undefined --sysroot=/usr printf.c -o 
> ../.././build/printf.wasm
> wabt
>
> wasm2wat printf.wasm -o printf.wat
>
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