circ_buf.h

#ifndef _LINUX_CIRC_BUF_H
#define _LINUX_CIRC_BUF_H

/*
 * See Documentation/circular-buffers.txt for more information.
 */
struct circ_buf {

    unsigned char *buf;
   
    volatile int head;//write head
    volatile int tail;//read tail
    int size;
}__packed;
/* Return count in buffer.  */
#define CIRC_CNT(head,tail,size) (((head) - (tail)) & ((size)-1))

/* Return space available, 0..size-1.  We always leave one free char
   as a completely full buffer has head == tail, which is the same as
   empty.  */
#define CIRC_SPACE(head,tail,size) CIRC_CNT((tail),((head)+1),(size))

/* Return count up to the end of the buffer.  Carefully avoid
   accessing head and tail more than once, so they can change
   underneath us without returning inconsistent results.  */
#define CIRC_CNT_TO_END(head,tail,size) \
	({int end = (size) - (tail); \
	  int n = ((head) + end) & ((size)-1); \
	  n < end ? n : end;})

/* Return space available up to the end of the buffer.  */
#define CIRC_SPACE_TO_END(head,tail,size) \
	({int end = (size) - 1 - (head); \
	  int n = (end + (tail)) & ((size)-1); \
	  n <= end ? n : end+1;})
#define circ_empty(circ)		((circ)->head == (circ)->tail)
#define circ_clear(circ)		((circ)->head = (circ)->tail = 0)

#define circ_full(circ, size)		(circ_chars_free(circ) == (0) )//
//(((circ)->head + 1 == (circ)->tail) || (((circ)->tail == 0) && ((circ)->head == (size - 1))))

#define circ_chars_pending(circ)	\
        (CIRC_CNT((circ)->head, (circ)->tail, (circ)->size))
    
#define circ_chars_free(circ)	\
        (CIRC_SPACE((circ)->head, (circ)->tail, (circ)->size))
#endif /* _LINUX_CIRC_BUF_H  */