asm declaration - cppreference.com (original) (raw)

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.

Contents

• 1 Syntax
• 2 Explanation
• 3 Notes
• 4 Keywords
• 5 Example
• 6 Defect reports
• 7 References
• 8 See also
• 9 External links

[edit] Syntax

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

[edit] 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.

[edit] Notes

[edit] Keywords

asm

[edit] 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.

#include   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 << "735 = " << 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:

[edit] Defect reports

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

[edit] 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]

[edit] See also

• C++ ABIs

[edit] External links