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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

Calling C Functions in Inline Assembly

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

// InlineAssembler_Calling_C_Functions_in_Inline_Assembly.cpp
// processor: x86
#include <stdio.h>

char format[] = "%s %s\n";
char hello[] = "Hello";
char world[] = "world";
int main( void )
      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.

END Microsoft Specific

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