c - Printf in inline Assembly -

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i trying write inline assebly function exchanges 2 values.( , 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 try wirte in full assembly below 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 allowed use printf function in assembly section?

your asm code calls printf 2 integer arguments -- no format string. tries dereference first integer pointer format string , crashes.

also, calling printf clobber values in %eax , %edx, not preserved across calls in standard x86 calling conventions.


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