CODEWITHSUNDEEP
cassemblyinline-assembly
Pegah.A
Pegah.A

Reputation: 85

Printf in inline Assembly

I am trying to write an inline assebly function which exchanges two values.( and i'm using extended ASM format)

This code works:

#include <stdio.h>
void Exchange(int *x, int *y)
{
printf("In Exchange function: Before exchange x is: %d\n",*x);
printf("In Exchange function: Before exchange y is: %d\n",*y);

asm("xchg %%eax,%%edx\n\t" \
:"+a"(*x),"+d"(*y));

printf("In Exchange function: After exchange x is: %d\n",*x);
printf("In Exchange function: After exchange y is: %d\n",*y);


}


int main()
{
int x=20;
int y=30;
printf("In main: Before exchange x is: %d\n",x);
printf("In main: Before exchange y is: %d\n",y);
Exchange(&x,&y);
printf("In main: After exchange x is: %d\n",x);
printf("In main: After exchange y is: %d\n",y);


return 0;
}

but when i try to wirte it in full assembly like below i get segmentation fault (core dumped) error.

void Exchange(int *x, int *y)
{
asm("subl $8,%%esp\n\t" \
    "movl %%eax,4(%%esp)\n\t" \
    "movl %%edx,(%%esp)\n\t" \
    "call printf\n\t" \
    "addl $8,%%esp\n\t" \   
    "xchg %%eax,%%edx\n\t" \
    "subl $8,%%esp\n\t" \
    "movl %%eax,4(%%esp)\n\t" \
    "movl %%edx,(%%esp)\n\t" \
    "call printf\n\t" \
    "addl $8,%%esp\n\t" \
:"+a"(*x),"+d"(*y));

}
 int main() 
{   
int x=20;
int y=30; 
printf("In main: Before exchange x is: %d\n",x);
printf("In main: Before exchange y is: %d\n",y);
Exchange(&x,&y);
printf("In main: After exchange x is: %d\n",x);
printf("In main: After exchange y is: %d\n",y);


return 0;
}

Aren't we allowed to use printf function in the assembly section?

Upvotes: 0

Views: 1034

Answers (1)

Chris Dodd
Chris Dodd

Reputation: 126418

Your asm code calls printf with two integer arguments -- no format string. So it tries to dereference the first integer as a pointer to a format string and crashes.

Also, calling printf will clobber the values in %eax and %edx, as they are not preserved across calls in the standard x86 calling conventions.

Upvotes: 3

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