Calling C Functions in Inline Assembly

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An __asm block can call C functions, including C library routines. The following example calls the printf library routine:

#include <stdio.h>

char format[] = "%s %s\n";
char hello[] = "Hello";
char world[] = "world";
void main( void )
{
   __asm
   {
      mov  eax, offset world
      push eax
      mov  eax, offset hello
      push eax
      mov  eax, offset format
      push eax
      call printf
      //clean up the stack so that main can exit cleanly
      //use the unused register ebx to do the cleanup
      pop  ebx
      pop  ebx
      pop  ebx
   }
}

Because function arguments are passed on the stack, you simply push the needed arguments—string pointers, in the previous example—before calling the function. The arguments are pushed in reverse order, so they come off the stack in the desired order. To emulate the C statement

printf( format, hello, world );

the example pushes pointers to world, hello, and format, in that order, and then calls printf.