Question

With the following pseudo-Python script for sending data to a local socket:

s = socket.socket(AF_UNIX, SOCK_STREAM)
s.connect("./sock.sock")
s.send("test\n")
s.send("aaa\0")
s.close()

My C program will randomly end up recving the following buffers:

• test\n
• test\n<random chars>
• test\naaa (as expected)

The socket is being recv()‘d after select() points that the socket is readable. Question is, how to avoid the first two cases?

And side question: Is it possible to send the following two messages from that script:

• asd\0
• dsa\0

And have select() to show the socket as readable on each of those sends, or will it only do that if I run the script again (restarting the socket client connection) and sending a message for each connect?

Answer 1

At a guess, the len argument to recv specifies a maximum amount of data to read, not the precise amount to be returned. recv is free to return any amount of data up to len bytes instead.

If you want to read a specific number of bytes, call recv in a loop.

 int bytes = 0;
 while (bytes < len) {
    int remaining = len - bytes;
    int read = recv(sockfd, buf+bytes, remaining, 0);
    if (read < 0) {
        // error
        break;
    }
    bytes += read;
 }

As noted by junix, if you’ll need to send unpredictable amounts of data, consider defining a simple protocol that either starts each message with a note of its length or ends with a particular byte or sequence of bytes.