C语言 用C代码编写二进制数系统
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Writing binary number system in C code
提问by cipher
As we use 0xprefix for hexadecimal numbers, and 0for octal ones, is there anything that can be done for binary numbers?
正如我们0x对十六进制数和0八进制数使用前缀一样，对二进制数有什么可以做的吗？
I tried the bsuffix, but the GCCdidn't allow it.
我尝试了b后缀，但GCC不允许。
Error: invalid suffix "b" on integer constant
错误：整数常量上的后缀“b”无效
Is it possible?
是否可以？
回答by
Standard C doesn't define binary constants. There's a GNU (I believe) extension though (among popular compilers, clang adapts it as well): the 0bprefix:
标准 C 没有定义二进制常量。虽然有一个 GNU（我相信）扩展（在流行的编译器中，clang 也适应了它）：0b前缀：
int foo = 0b1010;
If you want to stick with standard C, then there's an option: you can combine a macro and a function to create an almostreadable "binary constant" feature:
如果你想坚持使用标准 C，那么有一个选择：你可以结合一个宏和一个函数来创建一个几乎可读的“二进制常量”特性：
#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;
}
And then you can use it like this:
然后你可以像这样使用它：
int foo = B(1010);
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.
如果您打开大量编译器优化，编译器很可能会完全消除函数调用（常量折叠）或至少将其内联，因此这甚至不会成为性能问题。
Proof:
证明：
The following code:
以下代码：
#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;
}
has been compiled using clang -o baz.S baz.c -Wall -O3 -S, and it produced the following assembly:
已使用 编译clang -o baz.S baz.c -Wall -O3 -S，并生成以下程序集：
.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 1, %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
So clangcompletely eliminated the call to the function, and replaced its return value with 101. Neat, huh?
因此clang完全消除了对函数的调用，并将其返回值替换为101. 整洁吧？
回答by Avidan Borisov
Use BOOST_BINARY(Yes, you can use it in C).
使用BOOST_BINARY（是的，您可以在 C 中使用它）。
#include <boost/utility/binary.hpp>
...
int bin = BOOST_BINARY(110101);
This macro is expanded to an octal literal during preprocessing.
此宏在预处理期间扩展为八进制文字。
