linux 5.10 CIRC_SPACE_TO_END

CIRC_SPACE_TO_END is a macro defined in the Linux kernel version 5.10 that calculates the number of bytes available from the current position in a circular buffer to the end of the buffer. The macro takes two parameters: the current position in the buffer (represented as a pointer), and the size of the buffer (represented as an integer). The macro uses the modulo operator to handle wrapping around the end of the buffer when calculating the available space.

Here is the definition of the CIRC_SPACE_TO_END macro in the Linux kernel version 5.10:

#define CIRC_SPACE_TO_END(head, tail, size) \
({ (head >= tail) ? (size - (head - tail)) : (tail - head - 1); })

The macro uses a ternary operator to check if the head of the buffer is greater than or equal to the tail. If so, it subtracts the head from the tail and subtracts the result from the size of the buffer to get the available space. If not, it subtracts the tail from the head and subtracts one (to account for the fact that there is one less byte available when the head is immediately before the tail) from the result to get the available space.

For example, suppose you have a circular buffer of size 10, with the head at index 8 and the tail at index 2. The CIRC_SPACE_TO_END macro would be called with the head pointer, tail pointer, and size of the buffer as arguments:

int head = 8;
int tail = 2;
int size = 10;
int space_to_end = CIRC_SPACE_TO_END(&head, &tail, size);

The macro would calculate that there are 4 bytes available from index 8 to the end of the buffer, and assign that value to the space_to_end variable.