text: /* SPDX-License-Identifier: GPL-2.0 */
  /*
   * See Documentation/core-api/circular-buffers.rst for more information.
   */
  
  #ifndef _LINUX_CIRC_BUF_H
  #define _LINUX_CIRC_BUF_H 1
  
  /* NOTE
   * Since we're in userspace we don't have these functions.
   * Since our caller is a single thread our memory order is well defined.
   * The idea of this userspace example is to provide useful code examples
   * that will also work in kernel space.
   */
  #define READ_ONCE(X) X
  #define WRITE_ONCE(X, Y) X=Y
  
  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  */
  