asm declaration
It can be used to pass information to the back-end of the compiler.
This declaration is conditionally-supported and implementation defined, meaning that it may not be present and, even when provided by the implementation, it does not have a fixed meaning
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[edit] Syntax
asm ( string_literal ) ;
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[edit] Explanation
The string_literal is typically 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 only appear inside a block (a function body or another compound statement).
| This section is incomplete Reason: write a note on GCC extended assembly syntax, since it is now supported by Intel, IBM, Sun (as of v12), etc |
[edit] Examples
Demonstrates two kinds of inline assembly syntax offered by the GCC compiler. This program will only work correctly on x86_64 platform under Linux.
#include <iostream> int main() { int n = 7; // extended inline assembly asm ("leal (%0,%0,4),%0" : "=r" (n) : "0" (n)); std::cout << "7*5 = " << n << '\n'; // standard inline assembly asm ("movq $60, %rax\n\t" // the exit syscall number on Linux "movq $2, %rdi\n\t" // this program returns 2 "syscall"); }
Output:
7*5 = 35