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From Paul W. Nelson (firstname.lastname@example.org):
In the traditional BSD socket implementation, sockets that are atomic such as UDP keep received data in lists of mbufs. An mbuf is a fixed size buffer that is shared by various protocol stacks. When you set your receive buffer size, the protocol stack keeps track of how many bytes of mbuf space are on the receive buffer, not the number of actual bytes. This approach is used because the resource you are controlling is really how many mbufs are used, not how many bytes are being held in the socket buffer. (A socket buffer isn't really a buffer in the traditional sense, but a list of mbufs).
For example: Lets assume your UNIX has a small mbuf size of 256 bytes. If your receive socket buffer is set to 4096, you can fit 16 mbufs on the socket buffer. If you receive 16 UDP packets that are 10 bytes each, your socket buffer is full, and you have 160 bytes of data. If you receive 16 UDP packets that are 200 bytes each, your socket buffer is also full, but contains 3200 bytes of data. FIONREAD returns the total number of bytes, not the number of messages or bytes of mbufs. Because of this, it is not a good indicator of how full your receive buffer is.
Additionaly, if you receive UDP messages that are 260 bytes, you use up two mbufs, and can only recieve 8 packets before your socket buffer is full. In this case, only 2080 bytes of the 4096 are held in the socket buffer.
This example is greatly simplified, and the real socket buffer algorithm also takes into account some other parameters. Note that some older socket implementations use a 128 byte mbuf.