1 /*
2  * See Documentation/circular-buffers.txt for more information.
3  */
4 
5 #ifndef _LINUX_CIRC_BUF_H
6 #define _LINUX_CIRC_BUF_H 1
7 
8 struct circ_buf {
9 	char *buf;
10 	int head;
11 	int tail;
12 };
13 
14 /* Return count in buffer.  */
15 #define CIRC_CNT(head,tail,size) (((head) - (tail)) & ((size)-1))
16 
17 /* Return space available, 0..size-1.  We always leave one free char
18    as a completely full buffer has head == tail, which is the same as
19    empty.  */
20 #define CIRC_SPACE(head,tail,size) CIRC_CNT((tail),((head)+1),(size))
21 
22 /* Return count up to the end of the buffer.  Carefully avoid
23    accessing head and tail more than once, so they can change
24    underneath us without returning inconsistent results.  */
25 #define CIRC_CNT_TO_END(head,tail,size) \
26 	({int end = (size) - (tail); \
27 	  int n = ((head) + end) & ((size)-1); \
28 	  n < end ? n : end;})
29 
30 /* Return space available up to the end of the buffer.  */
31 #define CIRC_SPACE_TO_END(head,tail,size) \
32 	({int end = (size) - 1 - (head); \
33 	  int n = (end + (tail)) & ((size)-1); \
34 	  n <= end ? n : end+1;})
35 
36 #endif /* _LINUX_CIRC_BUF_H  */
37