circ_buf.h

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

#ifndef _LINUX_CIRC_BUF_H
#define _LINUX_CIRC_BUF_H 1

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

/* 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;})

#endif /* _LINUX_CIRC_BUF_H  */