sockatmark - determine whether a socket is at the out-of-band mark
#include <sys/socket.h> int sockatmark(int s);
The sockatmark() function determines whether the socket specified by the descriptor s is at the out-of-band data mark. If the protocol for the socket supports out-of-band data by marking the stream with an out-of-band data mark, the sockatmark() function returns 1 when all data preceding the mark has been read and the out-of-band data mark is the first element in the receive queue. The sockatmark() function does not remove the mark from the stream.
Upon successful completion, the sockatmark() function returns a value indicating whether the socket is at an out-of-band data mark. If the protocol has marked the data stream and all data preceding the mark has been read, the return value is 1. If there is no mark, or if data precedes the mark in the receive queue, the sockatmark() function returns 0. Otherwise, it returns −1 and sets errno to indicate the error.
The sockatmark() function will fail if:
The s argument is not a valid file descriptor.
The s argument does not specify a descriptor for a socket.
The use of this function between receive operations allows an application to determine which received data precedes the out-of-band data and which follows the out-of-band data.
There is an inherent race condition in the use of this function. On an empty receive queue, the current read of the location might well be at the "mark", but the system has no way of knowing that the next data segment that will arrive from the network will carry the mark, and sockatmark() will return false, and the next read operation will silently consume the mark.
Hence, this function can only be used reliably when the application already knows that the out-of-band data has been seen by the system or that it is known that there is data waiting to be read at the socket, either by SIGURG or select(3C).
See attributes(7) for descriptions of the following attributes: