Dxferp: a pointer to user memory of at least dxfer_len bytes in length for data transfer.
dxferp：指向数据传输时长度至少为dxfer_len字节的用户内存的指针。
Larger data types are made by simply treating multiple bytes as a single unit (using two memory locations together for a 16-bit number, for instance).
较大的数据类型可以通过简单地将多个字节当作一个单位实现(例如，将两个内存位置放到一起作为一个16位的数字)。
This request asks for sizeof(data) bytes of data copied from the address in main memory denoted by argp to be copied into the object data.
这个请求会要求将sizeof(data)字节的数据从 argp所指向的主存地址处拷贝到 data 对象中。
It always USES four bytes in computer memory, but this does not include the storage for the data representing the value of the variable.
它总是在计算机内存中使用四个字节，但这不包括表示变量值的数据的存储。
Overhead in memory is very small: You basically have a 4- byte overhead per interface (the vtable pointers in the object's instance data) plus 4 bytes for the reference counter.
内存开销非常小：基本上每个接口需要额外4个字节（对象实例数据里虚函数表指针），加上引用计数的4个字节。
System counters within this category track memory data such as the number of bytes available and the number of bytes cached.
此类别内的系统计数器跟踪内存数据，如可用字节数和缓存的字节数。
System counters within this category track memory data such as the number of bytes available and the number of bytes cached.
此类别内的系统计数器跟踪内存数据，如可用字节数和缓存的字节数。
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