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Showing content from https://en.cppreference.com/w/cpp/language/../memory/shared_ptr/../../language/asm.html below:

asm declaration - 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.

[edit] 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 [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 <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:

[edit] 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 [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

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