What is the purpose of “int mask = ~0;”?

Question

I saw the following line of code here in C.

 int mask = ~0;

I have printed the value of mask in C and C++. It always prints

Answer 1

You are studying a coding challenge with a number of restrictions on operators and language constructions to perform given tasks.

The first problem is return the value -1 without the use of the - operator.

On machines that represent negative numbers with two's complement, the value -1 is represented with all bits set to 1, so ~0 evaluates to -1:

/* 
 * minusOne - return a value of -1 
 *   Legal ops: ! ~ & ^ | + << >>
 *   Max ops: 2
 *   Rating: 1
 */
int minusOne(void) {
  // ~0 = 111...111 = -1
  return ~0;
}

Other problems in the file are not always implemented correctly. The second problem, returning a boolean value representing the fact the an int value would fit in a 16 bit signed short has a flaw:

/* 
 * fitsShort - return 1 if x can be represented as a 
 *   16-bit, two's complement integer.
 *   Examples: fitsShort(33000) = 0, fitsShort(-32768) = 1
 *   Legal ops: ! ~ & ^ | + << >>
 *   Max ops: 8
 *   Rating: 1
 */
int fitsShort(int x) {
  /* 
   * after left shift 16 and right shift 16, the left 16 of x is 00000..00 or 111...1111
   * so after shift, if x remains the same, then it means that x can be represent as 16-bit
  */
  return !(((x << 16) >> 16) ^ x); 
}

Left shifting a negative value or a number whose shifted value is beyond the range of int has undefined behavior, right shifting a negative value is implementation defined, so the above solution is incorrect (although it is probably the expected solution).