>> Hope this makes things more clear. > > Yep, that helps considerably. I've extensively reworked your text, > adding quite a bit more detail. Could you check the text below please? Yup, this is exactly what I was trying to say! Thanks! > Cheers, > > Michael > > SO_PEEK_OFF (since Linux 3.4) > This option, which is currently supported only for unix(7) > sockets, sets the value of the "peek offset" for the > recv(2) system call when used with MSG_PEEK flag. > > When this option is set to a negative value (it is set to > -1 for all new sockets), traditional behavior is provided: > recv(2) with the MSG_PEEK flag will peek data from the > front of the queue. > > When the option is set to a value greater than or equal to > zero, then the next peek at data queued in the socket will > occur at the byte offset specified by the option value. > At the same time, the "peek offset" will be incremented by > the number of bytes that were peeked from the queue, so > that a subsequent peek will return the next data in the > queue.i > > If data is removed from the front of the queue via a call > to recv(2) (or similar) without the MSG_PEEK flag, the > "peek offset" will be decreased by the number of bytes > removed. In other words, receiving data without the > MSG_PEEK flag will cause the "peek offset" to be adjusted > to maintain the correct relative position in the queued > data, so that a subsequent peek will retrieve the data > that would have been retrieved had the data not been > removed. > > For datagram sockets, if the "peek offset" points to the > middle of a packet, the data returned will be marked with > the MSG_TRUNC flag. > > The following example serves to illustrate the use of > SO_PEEK_OFF. Suppose a stream socket has the following > queued input data: > > aabbccddeeff > > The following sequence of recv(2) calls would have the > effect noted in the comments: > > int ov = 4; // Set peek offset to 4 > setsockopt(fd, SOL_SOCKET, SO_PEEK_OFF, &ov, sizeof(ov)); > > recv(fd, buf, 2, MSG_PEEK); // Peeks "cc"; offset set to 6 > recv(fd, buf, 2, MSG_PEEK); // Peeks "dd"; offset set to 8 > recv(fd, buf, 2, 0); // Reads "aa"; offset set to 6 > recv(fd, buf, 2, MSG_PEEK); // Peeks "ee"; offset set to 8 > . > -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
