From: Pierre NGUYEN-TUONG <pierre.nguyen-tuong@lip6.fr>
To: help-gcc@gnu.org
Subject: Constructing Function Calls
Date: Wed, 10 Oct 2001 11:26:00 -0000 [thread overview]
Message-ID: <3BC492D8.9020601@lip6.fr> (raw)
Hello,
I have a slight problem. Consider two functions, function_A and
function_B. I want to call function_B
inside function_A, with the same arguments of function_A, but without
knowing the prototype of
function_B, ie passing all arguments of funtion_A to function_B (I don't
want to use va_list in function_B).
A set of builtin functions allows this :
__builtin_apply_args()
__builtin__apply(void (*function)(),void *arguments,size_t size)
As you can see, you need to know the size of the stack arguments passed
to the function.
Let see the code. The function_B is
---------------------------------------------------------
void function_B(char *name,void *function,int a,long b,float *c,int d)
{
printf("Function B : printing\n") ;
printf("--- name=%s\n" ,name ) ;
printf("--- a=%i\n" ,a ) ;
printf("--- b=%li\n" ,b ) ;
printf("--- c=%e\n" ,*c ) ;
printf("--- d=%i\n" ,d ) ;
}
---------------------------------------------------------
Inside the function_A, I try to guess the stack size. I forgot to tell
you: function_A is a variadic
function, so you can use va_start, va_arg and va_end to parse the
unknown arguments. I assume that
all the arguments are 4 bytes wide (float are 4 bytes wide, but are
expanded to 8 bytes on the stack.
This is the problem ?).
Function_A is
---------------------------------------------------------
void function_A(char *name,void *function,...)
{
va_list ap = NULL ;
void *dummy = NULL ;
int cpt = 1 ;
size_t size = 0 ;
void *arguments = NULL ;
/* Guessing stack size */
/* I am using the va_arg macro */
printf("Function A : stack size\n") ;
va_start(ap,function) ;
dummy = va_arg(ap,void *) ;
printf("--- arg #%i : %p\n",cpt,dummy) ;
while(dummy != NULL)
{
cpt++ ;
dummy = va_arg(ap,void *) ;
printf("--- arg #%i : %p\n",cpt,dummy) ;
}
va_end(ap) ;
/* Now calculating the stack size. Every argument shoud be 4 bytes wide */
size = cpt * 4 ;
printf("--- %i args, stack size=%i\n",cpt,size) ;
/* Storing arguments in the stack */
printf("Function A : applying arguments\n") ;
arguments = __builtin_apply_args(size) ;
printf("Function A : calling function\n") ;
/* The call */
__builtin_apply(function,arguments,size) ;
}
---------------------------------------------------------
The main program is
---------------------------------------------------------
int main(int argc,char *argv[])
{
float flottant = 0.0 ;
flottant = 7.999 ;
printf("--------------------------Test A calling B\n\n") ;
function_A("le test",function_B,5,67L,&flottant,10,NULL) ;
return 0 ;
}
---------------------------------------------------------
And the result :
---------------------------------------------------------
--------------------------Test A calling B
Function A : stack size
--- arg #1 : 0x5
--- arg #2 : 0x43
--- arg #3 : 0xbfffed84
--- arg #4 : 0xa
--- arg #5 : (nil)
--- 5 args, stack size=20
Function A : applying arguments
Function A : calling function
Function B : printing
--- name=le test
--- a=5
--- b=67
--- c=7.999000e+00
--- d=5
---------------------------------------------------------
Ok, it works. So, where is the problem ? Well, I used a pointer to give
the float argument. But I want to give a float,
not a pointer to a float. So, lets examine function_C and the main program:
---------------------------------------------------------
void function_C(char *name,void *function,int a,long b,float *c,float
d,int e)
{
printf("Function B : printing\n") ;
printf("--- name=%s\n" ,name ) ;
printf("--- a=%i\n" ,a ) ;
printf("--- b=%li\n" ,b ) ;
printf("--- c=%e\n" ,*c ) ;
printf("--- d=%e\n" ,d ) ;
printf("--- e=%i\n" ,e ) ;
}
/***/
int main(int argc,char *argv[])
{
float flottant = 0.0 ;
flottant = 7.999 ;
printf("--------------------------Test A calling C\n\n") ;
function_A("le test",function_C,5,67L,&flottant,flottant,10,NULL) ;
return 0 ;
}
---------------------------------------------------------
And now the trace of the execution:
---------------------------------------------------------
--------------------------Test A calling C
Function A : stack size
--- arg #1 : 0x5
--- arg #2 : 0x43
--- arg #3 : 0xbfffed84
--- arg #4 : 0xe0000000
--- arg #5 : 0x401ffef9
--- arg #6 : 0xa
--- arg #7 : (nil)
--- 7 args, stack size=28
Function A : applying arguments
Function A : calling function
Function B : printing
--- name=le test
--- a=5
--- b=67
--- c=7.999000e+00
--- d=-3.689349e+19
--- e=1075838713
---------------------------------------------------------
Arrgh !!! Big problem on the float... If I give the correct size for the
stack (remember, a float
is 8 bytes wide in the stack) the result is the same.
So, finally, my question (sorry for the long explanation):
Is there a solution ? Shall I use only 32 bits arguments, ie pointers
instead of float ? Am I wrong or
is there a bug somewhere ?
I don't see why the __builtin_apply function has a different behaviour
for floats and for other types...
(By the way, I am working on Linux RedHat, AMD K6II.)
Any help is (really) appreciated,
Sincerely yours,
Pierre Nguyen Tuong.
next reply other threads:[~2001-10-10 11:26 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-10-10 11:26 Pierre NGUYEN-TUONG [this message]
2001-10-10 12:14 ` John Love-Jensen
2005-07-21 22:52 Constructing function calls Jean-Sebastien Legare
2005-07-21 23:03 Meissner, Michael
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