redis2.6.9源码学习---Big_Endian&Little_Endian

在阅读redis源码/src/endianconv.c时遇到了一些不理解的语句

#include <stdint.h>

/* Toggle the 16 bit unsigned integer pointed by *p from little endian to
 * big endian */
void memrev16(void *p) {
    unsigned char *x = p, t;

    t = x[0];
    x[0] = x[1];
    x[1] = t;
}

/* Toggle the 32 bit unsigned integer pointed by *p from little endian to
 * big endian */
void memrev32(void *p) {
    unsigned char *x = p, t;

    t = x[0];
    x[0] = x[3];
    x[3] = t;
    t = x[1];
    x[1] = x[2];
    x[2] = t;
}

/* Toggle the 64 bit unsigned integer pointed by *p from little endian to
 * big endian */
void memrev64(void *p) {
    unsigned char *x = p, t;

    t = x[0];
    x[0] = x[7];
    x[7] = t;
    t = x[1];
    x[1] = x[6];
    x[6] = t;
    t = x[2];
    x[2] = x[5];
    x[5] = t;
    t = x[3];
    x[3] = x[4];
    x[4] = t;
}

uint16_t intrev16(uint16_t v) {
    memrev16(&v);
    return v;
}

uint32_t intrev32(uint32_t v) {
    memrev32(&v);
    return v;
}

uint64_t intrev64(uint64_t v) {
    memrev64(&v);
    return v;
}

#ifdef TESTMAIN
#include <stdio.h>

int main(void) {
    char buf[32];

    sprintf(buf,"ciaoroma");
    memrev16(buf);
    printf("%s\n", buf);

    sprintf(buf,"ciaoroma");
    memrev32(buf);
    printf("%s\n", buf);

    sprintf(buf,"ciaoroma");
    memrev64(buf);
    printf("%s\n", buf);

    return 0;
}
#endif

 从代码可知方法是用来低位与高位交换，但为什么要交换？

这里就要了解相关的Big_Endian&Little_Endian的知识。

Big_Endian是指低地址存放最高有效字节（MSB），而Little_Endian则是低地址存放最低有效字节（LSB），不同的CPU采用不同的方式存储数据。因此考虑到兼容性的问题，必须先判断主机CPU的存储方式，再进行字节序的转换工作。

redis中/src/config.h

/* Byte ordering detection */
#include <sys/types.h> /* This will likely define BYTE_ORDER */

#ifndef BYTE_ORDER
#if (BSD >= 199103)
# include <machine/endian.h>
#else
#if defined(linux) || defined(__linux__)
# include <endian.h>
#else
#define	LITTLE_ENDIAN	1234	/* least-significant byte first (vax, pc) */
#define	BIG_ENDIAN	4321	/* most-significant byte first (IBM, net) */
#define	PDP_ENDIAN	3412	/* LSB first in word, MSW first in long (pdp)*/

#if defined(__i386__) || defined(__x86_64__) || defined(__amd64__) || \
   defined(vax) || defined(ns32000) || defined(sun386) || \
   defined(MIPSEL) || defined(_MIPSEL) || defined(BIT_ZERO_ON_RIGHT) || \
   defined(__alpha__) || defined(__alpha)
#define BYTE_ORDER    LITTLE_ENDIAN
#endif

#if defined(sel) || defined(pyr) || defined(mc68000) || defined(sparc) || \
    defined(is68k) || defined(tahoe) || defined(ibm032) || defined(ibm370) || \
    defined(MIPSEB) || defined(_MIPSEB) || defined(_IBMR2) || defined(DGUX) ||\
    defined(apollo) || defined(__convex__) || defined(_CRAY) || \
    defined(__hppa) || defined(__hp9000) || \
    defined(__hp9000s300) || defined(__hp9000s700) || \
    defined (BIT_ZERO_ON_LEFT) || defined(m68k) || defined(__sparc)
#define BYTE_ORDER	BIG_ENDIAN
#endif
#endif /* linux */
#endif /* BSD */
#endif /* BYTE_ORDER */

/* Sometimes after including an OS-specific header that defines the
 * endianess we end with __BYTE_ORDER but not with BYTE_ORDER that is what
 * the Redis code uses. In this case let's define everything without the
 * underscores. */
#ifndef BYTE_ORDER
#ifdef __BYTE_ORDER
#if defined(__LITTLE_ENDIAN) && defined(__BIG_ENDIAN)
#ifndef LITTLE_ENDIAN
#define LITTLE_ENDIAN __LITTLE_ENDIAN
#endif
#ifndef BIG_ENDIAN
#define BIG_ENDIAN __BIG_ENDIAN
#endif
#if (__BYTE_ORDER == __LITTLE_ENDIAN)
#define BYTE_ORDER LITTLE_ENDIAN
#else
#define BYTE_ORDER BIG_ENDIAN
#endif
#endif
#endif
#endif

#if !defined(BYTE_ORDER) || \
    (BYTE_ORDER != BIG_ENDIAN && BYTE_ORDER != LITTLE_ENDIAN)
	/* you must determine what the correct bit order is for
	 * your compiler - the next line is an intentional error
	 * which will force your compiles to bomb until you fix
	 * the above macros.
	 */
#error "Undefined or invalid BYTE_ORDER"
#endif

 做了详细的兼容

 

【用函数判断系统是Big Endian还是Little Endian】
 bool IsBig_Endian() 
//如果字节序为big-endian，返回true; //反之为   little-endian，返回false { 
    unsigned short test = 0x1122; 
    if(*( (unsigned char*) &test ) == 0x11)       
 return TRUE;
 else 
    return FALSE; }