asm declaration

 

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Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (C++11) consteval (C++20) constinit (C++20) Storage class specifiers Translation-unit-local (C++20) class/struct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile Pack indexing specifier (C++26)     Elaborated type specifier Attributes (C++11)

Declarators Reference Pointer Array Block declarations Simple-declaration →Structured binding declaration (C++17) Alias declaration (C++11) Namespace alias definition using declaration using directive static_assert declaration (C++11) asm declaration Opaque enum declaration (C++11) Other declarations Namespace definition Function declaration Class template declaration Function template declaration Explicit template instantiation (C++11) Explicit template specialization Linkage specification Attribute declaration (C++11) Empty declaration

 

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 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