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
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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

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{constchar*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!