include/linux/circ_buf.h

#ifndef _LINUX_CIRC_BUF_H
#define _LINUX_CIRC_BUF_H 1

struct circ_buf {
	char *buf;
	int head;
	int tail;
};

#define CIRC_CNT(head,tail,size) (((head) - (tail)) & ((size)-1))

#define CIRC_SPACE(head,tail,size) CIRC_CNT((tail),((head)+1),(size))

#define CIRC_CNT_TO_END(head,tail,size) \
	({int end = (size) - (tail); \
	  int n = ((head) + end) & ((size)-1); \
	  n < end ? n : end;})

#define CIRC_SPACE_TO_END(head,tail,size) \
	({int end = (size) - 1 - (head); \
	  int n = (end + (tail)) & ((size)-1); \
	  n <= end ? n : end+1;})

#endif