linux内核的队列实现移植

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在c编程中有时会使用一些常用的数据结构，如队列。每次都手写一遍有些麻烦，写一个通用的比较好，而内核有实现队列，直接移植出来就好了。

内核的队列实现

　　内核的队列实现在linux-2.6.32.68/kernel/kfifo.c和对应的kfifo.h中，主要接口罗列如下：

struct kfifo {    unsigned char *buffer;  /* the buffer holding the data */    unsigned int size;  /* the size of the allocated buffer */    unsigned int in;    /* data is added at offset (in % size) */    unsigned int out;   /* data is extracted from off. (out % size) */    spinlock_t *lock;   /* protects concurrent modifications */};extern struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask,                 spinlock_t *lock);extern void kfifo_free(struct kfifo *fifo);static inline void kfifo_reset(struct kfifo *fifo);static inline unsigned int kfifo_put(struct kfifo *fifo,                const unsigned char *buffer, unsigned int len);static inline unsigned int kfifo_get(struct kfifo *fifo,                     unsigned char *buffer, unsigned int len);static inline unsigned int kfifo_len(struct kfifo *fifo);                     

函数名字已非常清晰的表明了用途，无需多言。说一下，它的实现里面我觉得比较好的几个点。

• 内核实现的亮点
  较我自己以前实现的队列而言，在队列发生循环时，会通过取余的方式去更新队尾和队头下标。这样队尾下标有可能比队头下标小。计算队列长度时，相减又需要考虑此种情况。
  而内核in和out是一直递增的（因为是无符号，溢出也没关系），put和get时通过fifo->in & (fifo->size - 1)方式取余计算，分开拷贝。内核的put核心代码如下：
  

unsigned int __kfifo_put(struct kfifo *fifo,            const unsigned char *buffer, unsigned int len){    unsigned int l;    len = min(len, fifo->size - fifo->in + fifo->out);    /* first put the data starting from fifo->in to buffer end */    l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));    memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);    /* then put the rest (if any) at the beginning of the buffer */    memcpy(fifo->buffer, buffer + l, len - l);    fifo->in += len;    return len;}

移植

　　移植到普通应用，可以视情况去掉还是保留锁机制，在这儿就去掉了，再替换掉一些kmalloc这样的内核函数,实现is_power_of_2和roundup_pow_of_two两个函数就可以了。为了通用性，fifo.c里的代码都是对char型指针结合长度参数进行写入读取，而在自己使用时，可以在fifo.h里封装一下，方便自己的数据存放和读取。我在测试时封装成了存取int整数。

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fifo.c

#include <stdio.h>#include <string.h>#include <stdlib.h>#include "fifo.h"static inline int is_power_of_2(unsigned int n){    return !((n - 1) & n);}unsigned int roundup_pow_of_two(unsigned int n){    if(is_power_of_2(n))        return n;    int i = 32;    while(i--)    {        if((n >> i) & 1)            break;    }    n = 1 << (i + 1);    return n;} struct fifo *fifo_init(unsigned char *buffer, unsigned int size){    struct fifo *fifo;    fifo = malloc(sizeof(struct fifo));    if (!fifo)    {        fprintf(stderr, "malloc %ubyte failed\n", sizeof(struct fifo));        return NULL;    }    fifo->buffer = buffer;    fifo->size = size;    fifo->in = fifo->out = 0;    return fifo;}struct fifo *fifo_alloc(unsigned int size){    unsigned char *buffer;    struct fifo *ret;     /* round up to the next power of 2 */    if(size > 0x80000000)    {        fprintf(stderr, "The size is too lager\n");        return NULL;    }    size = roundup_pow_of_two(size);    buffer = malloc(size);    if (!buffer)    {        fprintf(stderr, "malloc %ubyte failed\n", size);        return NULL;    }    ret = fifo_init(buffer, size);    if (!ret)        free(buffer);    return ret;}void fifo_free(struct fifo *fifo){    free(fifo->buffer);    free(fifo);}unsigned int _fifo_put(struct fifo *fifo, const unsigned char *buffer, unsigned int len){    unsigned int l;    len = min(len, fifo->size - fifo->in + fifo->out);    /* first put the data starting from fifo->in to buffer end */    l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));    memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);    /* then put the rest (if any) at the beginning of the buffer */    memcpy(fifo->buffer, buffer + l, len - l);    fifo->in += len;    return len;}unsigned int _fifo_get(struct fifo *fifo, unsigned char *buffer, unsigned int len){    unsigned int l;    len = min(len, fifo->in - fifo->out);    /* first get the data from fifo->out until the end of the buffer */    l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));    memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);    /* then get the rest (if any) from the beginning of the buffer */    memcpy(buffer + l, fifo->buffer, len - l);    fifo->out += len;    return len;}

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fifo.h

#ifndef _FIFO_H_#define _FIFO_H_struct fifo {    unsigned char *buffer;  /* the buffer holding the data */    unsigned int size;  /* the size of the allocated buffer */    unsigned int in;    /* data is added at offset (in % size) */    unsigned int out;   /* data is extracted from off. (out % size) */};#define min(a, b) ((a) > (b) ? (b) : (a))extern struct fifo *fifo_alloc(unsigned int size);extern void fifo_free(struct fifo *fifo);extern unsigned int _fifo_put(struct fifo *fifo,                 const unsigned char *buffer, unsigned int len);extern unsigned int _fifo_get(struct fifo *fifo,                unsigned char *buffer, unsigned int len);static inline void _fifo_reset(struct fifo *fifo){    fifo->in = fifo->out = 0;}static inline unsigned int _fifo_len(struct fifo *fifo){    return fifo->in - fifo->out;}/****************************************************//* 根据需要进行封装，这儿我封装为保存int数/****************************************************/static inline void fifo_reset(struct fifo *fifo){    _fifo_reset(fifo);}static inline unsigned int fifo_len(struct fifo *fifo){    return _fifo_len(fifo)/4;}static inline unsigned int fifo_put(struct fifo *fifo, int n){    return _fifo_put(fifo, (unsigned char *)&n, sizeof(int));}static inline unsigned int fifo_get(struct fifo *fifo, int *n){    return _fifo_get(fifo, (unsigned char *)n, sizeof(int));}#endif