环形缓冲区circ_buf详解

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#include<stdio.h>
#define MAX_SIZE  1024
struct circ_buf {
	char *buf;
	int head;
	int tail;
};

/* Return count in buffer.  */
/*返回该缓冲区head-tail之间的差值*/
#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.  */
/*刨去head-tail之间的大小，整个缓冲区还有多少空间*/
#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.  */
/*head到整个缓冲区的结束位置被tail分成了两部分，取这两部分的最小值*/
#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.  */
/*返回head到整个缓冲区结尾有多少空间可以用*/
#define CIRC_SPACE_TO_END(head,tail,size) \
	({int end = (size) - 1 - (head); \
	  int n = (end + (tail)) & ((size)-1); \
	  n <= end ? n : end+1;})

int main()
{
    int ret = 0;
    struct circ_buf circ;
    circ.head = circ.tail = 0;

    printf("cnt          = %d \n", CIRC_CNT(circ.head, circ.tail, MAX_SIZE));
    printf("space        = %d \n", CIRC_SPACE(circ.head, circ.tail, MAX_SIZE));
    printf("cnt to end   = %d \n", CIRC_CNT_TO_END(circ.head, circ.tail, MAX_SIZE));
    printf("space to end = %d \n\r\n", CIRC_SPACE_TO_END(circ.head, circ.tail, MAX_SIZE));

    circ.head = 5;
    printf("cnt          = %d \n", CIRC_CNT(circ.head, circ.tail, MAX_SIZE));
    printf("space        = %d \n", CIRC_SPACE(circ.head, circ.tail, MAX_SIZE));
    printf("cnt to end   = %d \n", CIRC_CNT_TO_END(circ.head, circ.tail, MAX_SIZE));
    printf("space to end = %d \n\r\n", CIRC_SPACE_TO_END(circ.head, circ.tail, MAX_SIZE));

    circ.tail = 5;
    printf("cnt          = %d \n", CIRC_CNT(circ.head, circ.tail, MAX_SIZE));
    printf("space        = %d \n", CIRC_SPACE(circ.head, circ.tail, MAX_SIZE));
    printf("cnt to end   = %d \n", CIRC_CNT_TO_END(circ.head, circ.tail, MAX_SIZE));
    printf("space to end = %d \n\r\n", CIRC_SPACE_TO_END(circ.head, circ.tail, MAX_SIZE));

    circ.head = 15;
    printf("cnt          = %d \n", CIRC_CNT(circ.head, circ.tail, MAX_SIZE));
    printf("space        = %d \n", CIRC_SPACE(circ.head, circ.tail, MAX_SIZE));
    printf("cnt to end   = %d \n", CIRC_CNT_TO_END(circ.head, circ.tail, MAX_SIZE));
    printf("space to end = %d \n\r\n", CIRC_SPACE_TO_END(circ.head, circ.tail, MAX_SIZE));

    circ.tail = 10;
    printf("cnt          = %d \n", CIRC_CNT(circ.head, circ.tail, MAX_SIZE));
    printf("space        = %d \n", CIRC_SPACE(circ.head, circ.tail, MAX_SIZE));
    printf("cnt to end   = %d \n", CIRC_CNT_TO_END(circ.head, circ.tail, MAX_SIZE));
    printf("space to end = %d \n\r\n", CIRC_SPACE_TO_END(circ.head, circ.tail, MAX_SIZE));

    circ.tail = 800;
    printf("cnt          = %d \n", CIRC_CNT(circ.head, circ.tail, MAX_SIZE));
    printf("space        = %d \n", CIRC_SPACE(circ.head, circ.tail, MAX_SIZE));
    printf("cnt to end   = %d \n", CIRC_CNT_TO_END(circ.head, circ.tail, MAX_SIZE));
    printf("space to end = %d \n\r\n", CIRC_SPACE_TO_END(circ.head, circ.tail, MAX_SIZE));

    return 0;
}

程序运行结果为：