Read from socket

Question

I need to read from an AF_UNIX socket to a buffer using the function read from C, but I don\'t know the buffer size.

I think the best way is to read

Answer 1

I think the best way is to read N bytes until the read returns 0 (no more writers in the socket). Is this correct?

0 means EOF, other side has closed the connection. If other side of communication closes the connection, then it is correct.

If connection isn't closed (multiple transfers over the same connect, chatty protocol), then the case is bit more complicated and behavior generally depends on whether you have SOCK_STREAM or SOCK_DGRAM socket.

Datagram sockets are already delimited for you by the OS.

Stream sockets do not delimit messages (all data are an opaque byte stream) and if desired one has to implement that on application level: for example by defining a size field in the message header structure or using a delimiter (e.g. '\n' for single-line text messages). In first case you would first read the header, extract length and using the length read the rest of the message. In other case, read stream into partial buffer, search for the delimiter and extract from buffer the message including the delimiter (you might need to keep the partial buffer around as depending on protocol several command can be received with single recv()/read()).

Is there a way to guess the size of the buffer being written on the socket?

For stream sockets, there is no reliable way as the other side of communication might be still in process of writing the data. Imagine the quite normal case: socket buffer is 32K and 128K is being written. Writing application would block inside send()/write(), the OS waiting for reading application to read out the data and thus free space for the next chunk of written data.

For datagram sockets, one normally knows the size of the message beforehand. Or one can try (never did that myself) recvmsg( MSG_PEEK ) and if the MSG_TRUNC is in the returned msghdr.msg_flags, try to increase the buffer size.