CODEWITHSUNDEEP
linuxassemblyx86segmentation-faultswap
Riker
Riker

Reputation: 27

How to write a swap function in assembly?

I've been trying to figure out how to write a x86 GAS swap function for my program. I know its easier to do xchg or just write it C, but I want to be able to write it out anyways.

On my 1st midterm we were given this as as swap function:

movl 8(%ebp), %edx
movl 12(%ebp), %ecx
movl (%edx), %ebx
movl (%ecx), %eax
movl %eax, (%edx)
movl %ebx, (%ecx)

but I receive a segmentation fault when running this. Haven't been able to succeed in finding my answer anywhere else on the web, so much help would be appreciated.

EDIT:

C CODE: 

void program2()
{
int numA[2] = {5,10};
int *num1 = &numA[0];
int *num2 = &numA[1];
int loop=0;
printf("stop3\n");
for(loop=0;loop<=10;loop++)
{
    *num1 *=2;
    *num2 *=3;
    printf("%d\n%d\n",*num1,*num2);
    _asSwap(*num1,*num2);
    printf("stop5\n");
    printf("P2num1= %d\n P2num2= %d\n",*num1,*num2);

}

Assembly:

_asSwap:
push    %ebp
movl    %esp, %ebp

movl 8(%ebp), %edx
movl 12(%ebp), %ecx
movl (%edx), %ebx
movl (%ecx), %eax
movl %eax, (%edx)
movl %ebx, (%ecx)

pop %ebp
ret

Upvotes: 1

Views: 4596

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 365842

You passed the values, not the pointers. If you'd included the prototype in your C, the compiler would have caught that (instead of warning about an un-declared function, and assuming it took int parameters).

extern int _asSwap(int *a, int *b);

A debugger would also have caught this, if you checked the address that segfaulted.

Also, it's not normal to prefix your C function names with _. OS X prefixes _ onto C symbol names, and so did Linux a.out (now replaced by ELF). So you in some cases need a leading _ in the asm, but don't use it in C.

Upvotes: 1

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