Mixing C and Assembly

Defining in assembly and using in C
The assembly code which defines functions and variables:

# define a global variable asm_var and export it so that C code can use it.
.data
.global asm_var
asm_var:
.long

# define a function asm_fun and export it so that C code can use it.
.text
.global asm_fun
asm_fun:
    ret;

C code:

void asm_fun();
extern int asm_var;

int x;
void main()
{
	asm_fun();
	asm_var++;
}

Points to understand:

• Assembly code can define variables and functions which can be used in C program.
• The assembly code must export variables and functions which will be used from C code. In this example .global is used to export asm_var and asm_fun.
• The C code should declare variables and function before using. These declarations are similar to what is done in the header file. The declaration can be done in c source files or c header files.

Defining in C and using in assembly
C code

void fun()
{
}

Assembly code:

.text
.global main
main:
    call fun
    ret;

Points to understand:

• Functions and variables defined in C are always exported unless you use the static keyword. You do not have to export each symbol as you do in assembly.
• The assembly can call function or use variables as if they are defined in assembly code itself.

Mixing assembly inside C code

There are few things that you can not do in pure C code at all. For those things you have to use assembly. For example if you want to raise a hardware exception, then you need to write assembly instructions. C does not provide any syntax to raise hardware exceptions. To avoid writing code in separate assembly files and call from C code, C provides a way to insert assembly code inside a C code. This is done by asm keyword.

Here is an example:

int main()
{
	asm("int $3");
	return 0;
}

Assembly which is generated form this C code will be:

    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    int $3
    movl    $0, %eax
    popl    %ebp
    ret