通信用リングバッファのサンプル。ライトポインタはリードポインタを追い越せない。リードポインタもライトポインタを追い越せない。

mystd.h

/* mystd.h */
#ifndef __MYSTD_H__
#define __MYSTD_H__

/* types and values */
typedef unsigned char  u1;
typedef unsigned short u2;
typedef unsigned long  u4;

typedef signed char  s1;
typedef signed short s2;
typedef signed long  s4;

typedef unsigned char bool;
#define FALSE 0
#define TRUE  1

#define LO    0
#define HI    1

#define OFF   0
#define ON    1

#define EXIT_SUCCESS    0
#define EXIT_FAILURE    1


/* pseudo functions */
#define __ei()    sei()
#define __di()    cli()



#endif  /* __MYSTD_H__ */

result.txt

$gcc -std=c99 ring_buffer.c && ./a.out
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
 w                             
FF  FF  FF  FF  FF  FF  FF  FF
 r                             
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
 w                             
FF  FF  FF  FF  FF  FF  FF  FF
 r                             
write: 3
write: 4
write: 5
write: 6
write: 7
write: 8
write: 9
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
                             w 
FF   3   4   5   6   7   8   9
 r                             
read: 3
read: 4
read: 5
read: 6
read: 7
read: 8
read: 9
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
                             w 
FF   3   4   5   6   7   8   9
                             r 
write: 17
write: 18
write: 19
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
         w                     
17  18  19   5   6   7   8   9
                             r 
read: 17
read: 18
read: 19
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
         w                     
17  18  19   5   6   7   8   9
         r                     
write: 1A
write: 1B
write: 1C
write: 1D
write: 1E
write: 1F
write: 20
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
     w                         
1F  20  19  1A  1B  1C  1D  1E
         r                     
read: 1A
read: 1B
read: 1C
read: 1D
read: 1E
read: 1F
read: 20
-------------------------------------------------
[0] [1] [2] [3] [4] [5] [6] [7]
     w                         
1F  20  19  1A  1B  1C  1D  1E
     r                         
$

ring_buffer.c

#include <stdio.h>
#include <stdlib.h>

#include "mystd.h"

#define SZ 8
u1 buf[SZ];
u1 pw, pr;

void dump(void)
{
    u1 dpw[SZ], dpr[SZ];

    for(int i=0; i<SZ; i++) {
        dpw[i] = dpr[i] = ' ';
    }

    dpw[pw] = 'w';
    dpr[pr] = 'r';

    printf("-------------------------------------------------\n");
    printf("[%1d] [%1d] [%1d] [%1d] [%1d] [%1d] [%1d] [%1d]\n", 0, 1, 2, 3, 4, 5, 6, 7);
    printf(" %1c   %1c   %1c   %1c   %1c   %1c   %1c   %1c \n", dpw[0], dpw[1], dpw[2], dpw[3], dpw[4], dpw[5], dpw[6], dpw[7]);
    printf("%2X  %2X  %2X  %2X  %2X  %2X  %2X  %2X\n", buf[0], buf[1], buf[2], buf[3], buf[4], buf[5], buf[6], buf[7]);
    printf(" %1c   %1c   %1c   %1c   %1c   %1c   %1c   %1c \n", dpr[0], dpr[1], dpr[2], dpr[3], dpr[4], dpr[5], dpr[6], dpr[7]);
}

void init_buf(void)
{
    for(int i=0; i<SZ; i++)
        buf[i] = 0xFF;

    pw = 0;
    pr = 0;
}

void write(u1 data)
{
    u1 nextw = (pw+1) % SZ;
    
    if(nextw == pr) return;

    pw = nextw;
    buf[pw] = data;
    printf("write: %X\n", buf[pw]);
}

void read()
{
    u1 nextw = (pw+1) % SZ;
    u1 nextr = (pr+1) % SZ;

    if(nextw == nextr) return;

    pr = nextr;
    printf("read: %X\n", buf[pr]);
}

int main(void)
{
    u1 v = 3;

    init_buf();
    dump();

    for(int i=0; i<20; i++) read();
    dump();

    for(int i=0; i<20; i++) write(v++);
    dump();

    for(int i=0; i<20; i++) read();
    dump();

    for(int i=0; i<3; i++) write(v++);
    dump();
    
    for(int i=0; i<20; i++) read();
    dump();

    for(int i=0; i<20; i++) write(v++);
    dump();
    
    for(int i=0; i<20; i++) read();
    dump();

    return EXIT_SUCCESS;
}