pread 和 lseek 无法处理套接字文件描述符 [英] pread and lseek not working on socket file descriptor

问题描述

这个问题是关于系统调用 pread 和 lseek 的.我有一个套接字类型的文件描述符.每当从网络层读取数据包时，就会向其中添加数据.我想知道定期出现在文件描述符中的数据量是多少.

This question is on system calls pread and lseek . I have a file descriptor of socket type . Data is added to it whenever packet is read from the network layer . I would like to know whats the amount of data present in the file descriptor at regular intervals .

我尝试使用系统调用 pread 和 lseek ，因此我只知道数据量而不是读取数据本身.但是，这两个调用都失败了，给出了非法搜索错误.套接字类型文件描述符上是否有任何其他系统调用或套接字 fd 不支持 pread 和 lseek?.

I tried using the systems calls pread and lseek , so that I know only the amount of data rather than reading data itself . But , both the calls fails giving Illegal seek error. Are there any other systems calls on the socket type file descriptor or pread and lseek are not supported on socket fd? .

最好的亚什

推荐答案

Linux 联机帮助页说sockets接口不支持seek操作:

Linux manpages say that sockets interface doesn't support seek operations:

寻找或调用具有非零位置的 pread(2) 或 pwrite(2) 是套接字不支持.

Seeking, or calling pread(2) or pwrite(2) with a nonzero position is not supported on sockets.

您可以尝试使用以下标志设置 (MSG_PEEK|MSG_DONTWAIT) 指定足够大小的缓冲区来发出 recv.

You can try issuing recv with with the following flags set (MSG_PEEK|MSG_DONTWAIT) specifying the buffer of a sufficient size.

此操作将从套接字的接收缓冲区中复制数据，但不会将其弹出，即后续的 recv/read 调用将读取所有相同的数据 + 一些更多的数据可能到达套接字，因此返回的字节数可能更大.

This operation will copy the data from socket's receive buffer, but won't pop it out i.e. a subsequent recv/read call will read all the same data + some more data might arrive on socket so the number of bytes returned might be even greater.

缓冲区大小是一个非常棘手的问题 - 它应该大于套接字接收缓冲区，否则您的 recv 调用可能会返回您的缓冲区中的字节数提供，但插座实际上还有更多.

The buffer size is a very sticky point there - it should be greater than the socket receive buffer, otherwise there is a chance your recv call will return the number of bytes there are in the buffer you provided, but the socket actually has some more.

套接字接收缓冲区的大小可以通过带有选项名称SO_RCVBUF的getsockopt函数获得.

The size of the socket receive buffer can be obtained by means of getsockopt function with the option name SO_RCVBUF.

顺便说一句，我认为更好的选择是实际读取数据并将其保存在缓冲区中的某处.当您阅读了足够多的数据后，对其进行一些操作.与不提取数据直接查看套接字接收缓冲区相比，这似乎是一种更好、更常见的方法.

Btw, I think a better option would be to actually read the data and keep it in the buffer somewhere. When you've read enough data, do some actions with it. This seems to be a better and more common approach than peeking into the socket receive buffer without extracting data.

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