C++ keyword: asm

 

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Inline assembly	History of C++

 

Keywords

alignas (C++11) alignof (C++11) and and_eq asm auto (*) bitand bitor bool break case catch char char8_t (C++20) char16_t (C++11) char32_t (C++11) class compl concept (C++20) const consteval (C++20) constexpr (C++11) constinit (C++20) const_cast continue contract_assert (C++26)   co_await (C++20) co_return (C++20) co_yield (C++20) decltype (C++11) default	delete do double dynamic_cast else enum explicit export extern false float for friend goto if inline int long mutable namespace new noexcept (C++11)   not not_eq nullptr (C++11) operator or or_eq private protected public	register reinterpret_cast requires (C++20) return short signed sizeof (*) static static_assert (C++11) static_cast struct switch template this thread_local (C++11) throw true try typedef typeid typename union unsigned using virtual void volatile wchar_t while xor xor_eq
Identifiers with special meaning
final (C++11) import (C++20) module (C++20)	override (C++11) post (C++26) pre (C++26)	replaceable_if_eligible (C++26) trivially_relocatable_if_eligible (C++26)

 

Usage

• Declaration of an inline assembly block

Example

See More

Note that while this example works well with GCC/Clang on x86_64 platform under Linux, it's not guaranteed elsewhere as asm declaration is conditionally-supported and(since C++11) implementation defined.

Run this code

#include <cstring>

int main() noexcept
{
    const char* const c_string = "Hello, world!\n";
    asm
    (R"(
        movq $1, %%rax                 # syscall number for sys_write
        movq $1, %%rdi                 # file descriptor 1 (stdout)
        movq %0, %%rsi                 # pointer to the c‐string
        movq %1, %%rdx                 # length of the c‐string
        syscall                        # invokes an OS system-call handler
    )"
    :                                  // no output operands
    :   "r"(c_string),                 // input: pointer to the c‐string
        "r"(std::strlen(c_string))     // input: size of the c‐string
    :   "%rax", "%rdi", "%rsi", "%rdx" // clobbered registers
    );
}

Output:

Hello, world!

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