asm declaration
asm-declaration gives the ability to embed assembly language source code within a C++ program. This declaration is conditionally-supported and implementation defined, meaning that it may not be present and, even when provided by the implementation, it does not have a fixed meaning.
Syntax
asm ( string_literal ) ;
|
|||||||||
Explanation
The string_literal is typically a short program written in assembly language, which is executed whenever this declaration is executed. Different C++ compilers have wildly varying rules for asm-declarations, and different conventions for the interaction with the surrounding C++ code.
As other block declarations, this declaration can appear inside a block (a function body or another compound statement), and, as all other declarations, this declaration can also appear outside a block.
| This section is incomplete Reason: write a note on GCC extended assembly syntax, since it is now supported by Intel, IBM, Sun (as of v12), etc |
Examples
Demonstrates two kinds of inline assembly syntax offered by the GCC compiler. This program will only work correctly on x86_64 platform under Linux.
#include <iostream> extern "C" int func(); // the definition of func is written in assembly language asm(".globl func\n\t" ".type func, @function\n\t" "func:\n\t" ".cfi_startproc\n\t" "movl $7, %eax\n\t" "ret\n\t" ".cfi_endproc"); int main() { int n = func(); // extended inline assembly asm ("leal (%0,%0,4),%0" : "=r" (n) : "0" (n)); std::cout << "7*5 = " << n << std::endl; // flush is intentional // standard inline assembly asm ("movq $60, %rax\n\t" // the exit syscall number on Linux "movq $2, %rdi\n\t" // this program returns 2 "syscall"); }
Output:
7*5 = 35