endianconv.h

/* variants of the function doing the actual conversion only if the target
 * host is big endian */
#if (BYTE_ORDER == LITTLE_ENDIAN)
#define memrev16ifbe(p) ((void)(0))
#define memrev32ifbe(p) ((void)(0))
#define memrev64ifbe(p) ((void)(0))
#define intrev16ifbe(v) (v)
#define intrev32ifbe(v) (v)
#define intrev64ifbe(v) (v)
#else
#define memrev16ifbe(p) memrev16(p)
#define memrev32ifbe(p) memrev32(p)
#define memrev64ifbe(p) memrev64(p)
#define intrev16ifbe(v) intrev16(v)
#define intrev32ifbe(v) intrev32(v)
#define intrev64ifbe(v) intrev64(v)
#endif

定义大端到小端转换的宏
比如主机是大端序时，intrev32ifbe(v)用于将小端转为大端，intrev32(v)定义如下：

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

/* 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;
}

memrev32ifbe§在主机是小端序时，对指针p不做处理；在主机是大端时，需要将小端转为大端