asm 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

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

Keywords

asm

Example

#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");
}

73*5 = 365

Defect reports

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

References

See also

• C++ ABIs

External links