asm-declaration gives the ability to embed assembly language source code within a C++ program. This declaration isconditionally-supported and(since C++11)implementation defined, meaning thatit may not be present and, even when provided by the implementation,(since C++11)it does not have a fixed meaning.

[edit]Syntax

[edit]Explanation

Thebalanced-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 otherblock 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

#include <iostream> extern"C"int func(int x);// the definition of func is written in assembly language// raw string literal could be very usefulasm(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

[edit]Defect reports

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

[edit]References

[edit]See also

• C++ ABIs

[edit]External links

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