memset的函数原型是

void * memset ( void * ptr, int value, size_t num );

　　这个函数的功能是将ptr所指向的某一块内存中的每个字节的内容全部设置为value指定的ASCII值, 块的大小由第三个参数指定,这个函数通常为新申请的内存做初始化工作。
英文解释：Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char).
函数的实现：

#ifndef __HAVE_ARCH_MEMSET
/**
 * memset - Fill a region of memory with the given value
 * @ptr: Pointer to the start of the area.
 * @value: The byte to fill the area with
 * @num: The size of the area.
 *
 * Do not use memset() to access IO space, use memset_io() instead.
 */
void *memset(void *ptr, int value, size_t num)
{
        char *xs = ptr;

        while (num --)
                *xs++ = value;
        return ptr;
}
EXPORT_SYMBOL(memset);

#endif