I've got -masm=intel on so I can use good ol' Intel syntax. I need to pop a value off the stack into my unsigned short "cur_val", how do I reference "cur_val"?
Although it's Intel syntax, it's not Fasm dialect, I'm a little confused.Code:struct bigint_t init_bigint_t(unsigned long count, ...) { struct bigint_t new_bigint; for(;count > 0;count--) { block cur_val; asm("pop word cur_val\n\t"); /* Error: junk `cur_val' after expression */ } }
Well that depends entirely on your compiler.
There is NO standard for what asm does, so you should at least start with the manual.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
I'm using GCC, so I guess that would mean I'm really using GAS in Intel mode().
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
Yeah, I can use Google to. The hard part is finding documentation with Intel syntax turned on. It really does matther.
Intel(Fasm):
See how nice and neat that is, with it's continuity and no stupid "%"s.Code:lea eax, dword ptr [esi + ebx * 4 - 0x10]
AT&T:
Just seeing it... Ewww... I just threw up a little... AT&T syntax is sickening.Code:leal -0x10(%esi, %ebx, 4), %eax
I'm getting cur_val undefined, why? Am I supposed to use some mangled form of "cur_val"?Code:struct bigint_t init_bigint_t(unsigned long count, ...) { struct bigint_t new_bigint; for(;count > 0;count--) { block cur_val asm("_cur_val"); asm volatile ( "pop word ptr [cur_val]#" ); } }
I went ahead and just did it with stdarg instead. Wasn't as hard as I was expecting either.Code:struct bigint_t init_bigint_t(unsigned long count, ...) { va_list blocks; struct bigint_t new_bigint; new_bigint.count = count; va_start(blocks, count); new_bigint.root = (struct bigint_node*)malloc(sizeof(struct bigint_node)); /* Alloc space for root */ struct bigint_node *cur_node = new_bigint.root; /* Set current node to root */ for(;count > 0;count--) { cur_node->val = va_arg(blocks, block); /* Get block */ cur_node->next = (struct bigint_node*)malloc(0); /* Allocate next node */ cur_node = cur_node->next; /* Set current node to next node(the node just allocated) */ } va_end(blocks); return new_bigint; }
malloc(0) is asking for trouble...
Why does the syntax matter?
What matters is getting the constraint strings right. The actual instruction between the first pair of quotes doesn't really matter.
This "works" (compiles without complaint)
It doesn't work because the top of the stack are not parameters, but the return address (and other things).Code:$ cat foo.c #include <stdio.h> void foo ( unsigned long count, ...) { for(;count > 0;count--) { int cur_val; asm("pop %0\n" : :"m"(cur_val) ); printf("Value=%x\n", cur_val); } } int main() { foo(3,22,44,66); return 0; } $ gcc -masm=intel foo.c $ ./a.out Value=0 Value=8048500 Value=8048500
The variadic parameters would be accessible though ebp, not esp.
The generated code around the asm looks like this
Code:.globl foo .type foo, @function foo: push ebp mov ebp, esp sub esp, 24 jmp .L2 .L3: #APP # 9 "foo.c" 1 pop DWORD PTR [ebp-4] # 0 "" 2 #NO_APP mov eax, DWORD PTR [ebp-4]
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
The malloc(0) thing was just an accident, it was supposed to be "malloc(sizeof(struct bigint_node))".
I'm not sure what I was thinking wanting to pop a param off the stack on my 64 bit box(Not that it would work in cdecl, as you've pointed out.). I guess I forgot that they use a weird register based calling convention.
