写二进制数字系统用C code [英] Writing binary number system in C code

问题描述

由于我们使用 0X preFIX为十六进制数字，而 0 八进制的人，有什么可以为二进制数做些什么呢？

As we use 0x prefix for hex numbers, and o for octal ones, is there anything that can be done for binary numbers?

我试过 B 后缀，但 GCC 没'T允许它。

I tried the b suffix, but the GCC didn't allow it.

错误：整型常量无效标b

Error: invalid suffix "b" on integer constant

这可能吗？

推荐答案

标准C没有定义二进制常量。有一个GNU（我相信）扩展，但（流行的编译器中，铛适应它）：在 0B preFIX：

Standard C doesn't define binary constants. There's a GNU (I believe) extension though (among popular compilers, clang adapts it as well): the 0b prefix:

int foo = 0b1010;

如果你想坚持使用标准的C，然后有一个选项：你可以结合宏和一个函数来创建的几乎的可读二进制常数功能：

If you want to stick with standard C, then there's an option: you can combine a macro and a function to create an almost readable "binary constant" feature:

#define B(x) S_to_binary_(#x)

static inline unsigned long long S_to_binary_(const char *s)
{
        unsigned long long i = 0;
        while (*s) {
                i <<= 1;
                i += *s++ - '0';
        }
        return i;
}

然后你可以使用它是这样的：

And then you can use it like this:

int foo = B(1010);

如果您打开沉重的编译器优化，编译器将最有可能完全（常量折叠）消除函数调用，或至少会内联，所以这不会甚至性能问题。

If you turn on heavy compiler optimizations, the compiler will most likely eliminate the function call completely (constant folding) or will at least inline it, so this won't even be a performance issue.

证明：

以下code：

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <string.h>


#define B(x) S_to_binary_(#x)

static inline unsigned long long S_to_binary_(const char *s)
{
    unsigned long long i = 0;
    while (*s) {
        i <<= 1;
        i += *s++ - '0';
    }
    return i;
}

int main()
{
    int foo = B(001100101);

    printf("%d\n", foo);

    return 0;
}

使用铛-o baz.S baz.c -Wall -O3 -S 已编制，并产生了以下组件：

has been compiled using clang -o baz.S baz.c -Wall -O3 -S, and it produced the following assembly:

    .section    __TEXT,__text,regular,pure_instructions
    .globl  _main
    .align  4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:
    pushq   %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    leaq    L_.str1(%rip), %rdi
    movl    $101, %esi               ## <= This line!
    xorb    %al, %al
    callq   _printf
    xorl    %eax, %eax
    popq    %rbp
    ret
    .cfi_endproc

    .section    __TEXT,__cstring,cstring_literals
L_.str1:                                ## @.str1
    .asciz   "%d\n"


.subsections_via_symbols

所以铛完全消除了通话的功能，并与取代它的返回值101 。整洁的，是吧？

So clang completely eliminated the call to the function, and replaced its return value with 101. Neat, huh?

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