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.


attr(optional) asm ( string-literal ) ;
attr - (since C++11) any number of attributes
string-literal - a string literal


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.


Feature-test macro Value Std Comment
__cpp_constexpr 201907L (C++20) Trivial default initialization and asm-declaration in constexpr functions


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
// raw string literal could be very useful
.globl func
    .type func, @function
    movl $7, %eax
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


7*5 = 35

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
CWG 195 C++98 it was required to support all asm declarations made conditionally-supported
CWG 2262 C++11 attributes could not be applied to asm declarations allowed

See also

C documentation for Inline assembly
1. GCC Inline Assembly HOWTO
2. IBM XL C/C++ Inline Assembly
3. Intel C++ Inline Assembly
4. Visual Studio Inline Assembler
5. Sun Studio 12 Asm Statements
6. Inline assembly for Itanium-based HP-UX

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