# `asm` declaration

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< cpp‎ | language

asm-declaration gives the ability to embed assembly language source code within a C++ program. This declaration is conditionally-supported and (since C++11)implementation defined, meaning that it may not be present and, even when provided by the implementation, (since C++11)it does not have a fixed meaning.

### Syntax

attr﻿(optional) `asm (` string-literal `)` `;` |  | (until C++26)
attr﻿(optional) `asm (` balanced-token-seq `)` `;` |  | (since C++26)

attr | - | (since C++11) any number of attributes
string-literal | - | same as in string literal, including raw string literals
balanced-token-seq | - | a sequence of tokens where parentheses, brackets and braces are balanced; any restrictions on the balanced-token-seq and its meaning are implementation-defined

### Explanation

The balanced-token-seq is typically a string literal that represents 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

### Notes

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

### Keywords

`asm`

### Example

Demonstrates two kinds of inline assembly syntax offered by the GCC/Clang compilers. This program works correctly only on the x86_64 platform under Linux.

Run this code

    #include <iostream>

    extern "C" int func(int x);
    // the definition of func is written in assembly language
    // raw string literal could be very useful
    asm(R"(
    .globl func
        .type func, @function
        func:
        .cfi_startproc
        movl %edi, %eax /* x is in RDI, see x86-64 calling convention */
        addl $1, %eax
        ret
        .cfi_endproc
    )");

    int main()
    {
        int n = func(0110);
        // formerly non-standard inline assembly, made comforming by P2361R6
        asm ("leal (%0,%0,4),%0"
             : "=r" (n)
             : "0" (n));
        std::cout << "73*5 = " << n << std::endl; // flush is intentional

        // standard inline assembly
        asm ("movq $60, %rax\n" // the exit syscall number on Linux
             "movq $2,  %rdi\n" // this program returns 2
             "syscall");
    }

Output:

    73*5 = 365

### 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

### References

- C++26 standard (ISO/IEC 14882:2026):

  - 9.10 The `asm` declaration [dcl.asm]

- C++23 standard (ISO/IEC 14882:2024):

  - 9.10 The `asm` declaration [dcl.asm]

- C++20 standard (ISO/IEC 14882:2020):

  - 9.10 The `asm` declaration [dcl.asm]

- C++17 standard (ISO/IEC 14882:2017):

  - 10.4 The `asm` declaration [dcl.asm]

- C++14 standard (ISO/IEC 14882:2014):

  - 7.4 The `asm` declaration [dcl.asm]

- C++11 standard (ISO/IEC 14882:2011):

  - 7.4 The `asm` declaration [dcl.asm]

- C++03 standard (ISO/IEC 14882:2003):

  - 7.4 The `asm` declaration [dcl.asm]

- C++98 standard (ISO/IEC 14882:1998):

  - 7.4 The `asm` declaration [dcl.asm]

### See also

- C++ ABIs

C documentation for Inline assembly

### External links

1. | GCC Inline Assembly HOWTO
2. | GCC Inline ASM — Locklessinc.com
3. | IBM XL C/C++ Inline Assembly
4. | Intel C++ Inline Assembly
5. | Visual Studio Inline Assembler
6. | Sun Studio 12 Asm Statements
7. | Inline assembly for Itanium-based HP-UX
8. | X86 calling conventions — Wikipedia

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