如何使用C程序基于不同的长度值设置遮罩

Question

In a function I have to pass the bitlength and based on the bit length the mask will be set. For example here is my part of program

if(length == 8)
    mask = 0xff;
if(length == 7)
    mask = 0x7f;
if(length == 12)
    mask = 0x3ff;
if(length == 16)
    mask = 0xffff;

How can I use some loop statements to set the mask as the length varies from 1 to 16? It would be great if someone helps, thanks in advance.

Answer 1

How to set the mask based on the different values of lengths using C program?

Shift 1u by n , then subtract 1. No loop needed. Best to use unsigned types and guard against a large length with a mask to insure no undefined behavior (UB).

#define UINT_WIDTH 32
unsigned length = foo();
unsigned mask = (1u << (length & (UINT_WIDTH - 1)) - 1u;

How can I use some loop statements to set the mask as the length varies from 1 to 16?

This works well for [1 ... UINT_WIDTH] .

---

If using fixed width types like uint16_t , then set the ..._WIDTH mask to 16.

For portable code, UINT_WIDTH needs to be consistent with unsigned .

#include <limits.h>
#if UINT_MAX == 0xFFFF
  #define UINT_WIDTH 16
#elif UINT_MAX == 0xFFFFFFFF
  #define UINT_WIDTH 32
#elif UINT_MAX == 0xFFFFFFFFFFFFFFFF 
  #define UINT_WIDTH 64
#else
  // Very rare
  #error TBD code
#endif

Answer 2

Start with a value of zero. Then for every bit in the mask, shift left by 1 then OR a 1 bit at the end.

uint16_t mask = 0;
for (int i = 0; i < length; i++) {
    mask <<= 1;
    mask |= 1;
}

Answer 3

You can do it like the following:

uint16_t mask = 0;
for (size_t i = 0; i < length; i++)
    mask |= (1 << i);

For each iteration of the loop, you or | your mask with a bit shifted by i position to the left.