1 #ifndef _LINUX_CIRC_BUF_H
2 #define _LINUX_CIRC_BUF_H 1
3 
4 struct circ_buf {
5 	char *buf;
6 	int head;
7 	int tail;
8 };
9 
10 /* Return count in buffer.  */
11 #define CIRC_CNT(head,tail,size) (((head) - (tail)) & ((size)-1))
12 
13 /* Return space available, 0..size-1.  We always leave one free char
14    as a completely full buffer has head == tail, which is the same as
15    empty.  */
16 #define CIRC_SPACE(head,tail,size) CIRC_CNT((tail),((head)+1),(size))
17 
18 /* Return count up to the end of the buffer.  Carefully avoid
19    accessing head and tail more than once, so they can change
20    underneath us without returning inconsistent results.  */
21 #define CIRC_CNT_TO_END(head,tail,size) \
22 	({int end = (size) - (tail); \
23 	  int n = ((head) + end) & ((size)-1); \
24 	  n < end ? n : end;})
25 
26 /* Return space available up to the end of the buffer.  */
27 #define CIRC_SPACE_TO_END(head,tail,size) \
28 	({int end = (size) - 1 - (head); \
29 	  int n = (end + (tail)) & ((size)-1); \
30 	  n <= end ? n : end+1;})
31 
32 #endif /* _LINUX_CIRC_BUF_H  */
33