Inline assembly (typically introduced by theasm keyword) gives the ability to embed assembly language source code within a C program.

Unlike in C++, inline assembly is treated as an extension in C. It 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.

[edit]Syntax

[edit]Explanation

This kind of inline assembly syntax is accepted by the C++ standard and calledasm-declaration in C++. Thestring_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.

asm-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

MSVC does not support inline assembly on the ARM and x64 processors, and only support the form introduced by__asm on x86 processors.

When compiling in ISO C mode by GCC or Clang (e.g. with option-std=c11),__asm__ must be used instead ofasm.

[edit]Examples

#include <stdio.h> externint func(void);// the definition of func is written in assembly language__asm__(".globl func\n\t"".type func, @function\n\t""func:\n\t"".cfi_startproc\n\t""movl $7, %eax\n\t""ret\n\t"".cfi_endproc"); int main(void){int n= func();// gcc's extended inline assembly    __asm__("leal (%0,%0,4),%0":"=r"(n):"0"(n));printf("7*5 = %d\n", n);fflush(stdout);// flush is intentional // standard inline assembly in C++    __asm__("movq $60, %rax\n\t"// the exit syscall number on Linux"movq $2,  %rdi\n\t"// this program returns 2"syscall");}

7*5 = 35

[edit]References

[edit]See also

[edit]External links

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