asm declaration - cppreference.com

From cppreference.com

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.

Notes

Feature-test macroValueStdFeature

__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"intfunc(intx);// the definition of func is written in assembly language// raw string literal could be very usefulasm(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)");intmain(){intn=func(0110);// formerly non-standard inline assembly, made comforming by P2361R6asm("leal (%0,%0,4),%0":"=r"(n):"0"(n));std::cout<<"73*5 = "<<n<<std::endl;// flush is intentional// standard inline assemblyasm("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]