Variable argument passing from C to assembly function

MDK35233

Prodigy 720 points

Hello,

I need to modify an assembly function because of the following changes done to the prototype.

Previously the function prototype is as follows:-

int foo(int a, int b, int c, int d)

and is changed to

int foo(int a, int b, int c, int d, ...)

I need to know how to handle variable argument passing in the assembly code. If you have

any sample code, please post it here. Any document that explains this with example will help

as well.

Thanks.

Murali

over 15 years ago

0 one and zero over 15 years ago

TI__Mastermind 18256 points

Hi Murali,

which DSP/MCU are you using?

Kind regards,

one and zero

0 Lori Heustess over 15 years ago in reply to one and zero

TI__Guru* 93400 points

If the processor is a C28x MCU then there is information on this here:

http://processors.wiki.ti.com/index.php/C28x_Code_Generation_Tips_and_Tricks#Q:_I_have_a_function_in_assembly_and_I_want_to_call_it_from_C_-_How_can_I_access_the_arguments_inside_the_function.3F

-Lori

0 one and zero over 15 years ago in reply to Lori Heustess

TI__Mastermind 18256 points

... in case of C6000:

"For a function declared with an ellipsis indicating that it is called with varying numbers of arguments,
the convention is slightly modified. The last explicitly declared argument is passed on the stack, so that
its stack address can act as a reference for accessing the undeclared arguments."

as in SPRU187Q from page 181 onwards ...

Kind regards,

one and zero

0 Archaeologist over 15 years ago in reply to one and zero

TI__Guru* 84285 points

I recommend writing a wrapper function "foo" as C code to intercept and handle the variadic arguments, and then call the original "foo" written in assembly code by a different name. Handling variadic arguments in assembly code is tedious and therefore error-prone.

0 George Mock over 15 years ago

TI__Guru**** 252320 points

mdk36128 said:

I need to know how to handle variable argument passing in the assembly code. If you have

any sample code, please post it here.

There isn't any sample code, because no one does that. To get an idea of the difficulty involved, look at the macros va_start and va_arg in the RTS header file stdarg.h. That's hard to write in C! Doing it in hand coded assembly boggles the mind.

I like the suggestion of writing a wrapper function in C that is dedicated to collecting the arguments.

Thanks and regards,

-George

Code Composer Studio™︎

Code Composer Studio forum

Variable argument passing from C to assembly function