> Woytech Karl-Heinz wrote:
> When the application code below gets started our OS finds the 'imex'
> structure and correctly fills out the required function
> pointers so that
> calls to the fn's result in the code provided by the os
> getting called.
>
> Thus the explicit call
> jimbob(815);
> translates to
> push $0x32f
> call *0x804912c
That's because it's not an explicit call to a function, but through a
function pointer which you declared.
> BUT the structure copy x=y becomes
> push $0x16
> push %eax
> push %edx
> call 8049128 <- no '*'
>
> which is a direct jump into memory instead of a jump to the
> dereference
That's because the compiler does not read your memcpy declaration.
It assumes that there is a memcpy function and generates a call to it.
That generation is not influenced by your redeclaration of memcpy.
You must provide a real memcpy function.
> ie can i get gcc to produce the same derefecenced call as per the
> explicit calls?
Forget about it.
> gcc testjmp.c -c -o testjmp.o -ffreestanding -march=i386 -g -Os
> -fno-builtin
> ld testjmp.o -o testjmp -static -nostdlib
> objdump testjmp -DSx > testjmp.dis
>
> testjmp.c:
>
> // structure def for our import/export table
> typedef struct imex_s {
> char name[16];
> void *ptr;
> } imex_t;
>
> // fixed typedefs for our funcions
> typedef int (*jimbob_t)(int jb);
> typedef void (*memcpy_t)(char *d, char *s, int l);
ANSI C memcpy is void * (void *, const void *, size_t).
> // memory space for the to be linked fn's will be in .bss
> jimbob_t jimbob;
> memcpy_t memcpy;
Call these jimbob_ptr, memcpy_ptr.
> // our import table will be in .const
> const imex_t imex[] =
> {
> { "jimbob", (void*)&jimbob},
&jimbob_ptr
> { "memcpy", (void*)&memcpy}
&memcpy_ptr
> };
Now write this function and link it to your program:
/* wrapper for indirection */
void *memcpy(void *dest, void *src, size_t len)
{
memcpy_ptr(dest, src, len);
return dest;
}
Note that this wrapper not only ensures that control is routed
to your function, but also provides ANSI C compatibility.
