Calling C Functions in Inline Assembly

 

For the latest documentation on Visual Studio 2017, see Visual Studio 2017 Documentation.

For the latest documentation on Visual Studio 2017, see Calling C Functions in Inline Assembly on docs.microsoft.com.

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 )  
{  
   __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.

END Microsoft Specific

Inline Assembler

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