Confused about the 'cpu_set_t' structure definition in sched.h-CodePudding

From what I have read online and seen in struct.h , cpu_set_t is a struct that contains a bit mask, where each bit represents a CPU that can be used by the thread/process.

This is how it is defined:

typedef struct
{ __CPU_BITTYPE  __bits[ CPU_SETSIZE / __CPU_BITS ];
} cpu_set_t

__CPU_BITTYPE is an unsigned long int.

Doesn't the above cpu_set_t definition mean that the struct contains an array called __bits of size CPU_SETSIZE / __CPU_BITS? What am I missing here? How is it interpreted as a bit field?

CodePudding user response：

Yes, it's an array. This is to allow for the possibility that there may be more CPUs than the number of bits in an integer.

CPU_SETSIZE is the total number of CPUs in a CPU set, and __CPU_BITS is the number of bits used in each integer to hold the bit mask. To access the bit for a particular CPU, you divide the CPU number by __CPU_BITS to get the array index, and use the modulus as a left shift to get to that bit in the integer. There's almost certainly a macro that performs this operation.

This is generally how bit arrays are implemented in C when you can't limit the number of elements to the size in bits of an integer type.