SOCKATMARK(P) POSIX Programmer's Manual SOCKATMARK(P)
sockatmark - determine whether a socket is at the out-of-band mark
int sockatmark(int s);
The sockatmark() function shall determine whether the socket specified by the descriptor s
is at the out-of-band data mark (see the System Interfaces volume of IEEE Std 1003.1-2001,
Section 2.10.12, Socket Out-of-Band State). If the protocol for the socket supports out-
of-band data by marking the stream with an out-of-band data mark, the sockatmark() func-
tion shall return 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 shall not
remove the mark from the stream.
Upon successful completion, the sockatmark() function shall return 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 shall be 1; if
there is no mark, or if data precedes the mark in the receive queue, the sockatmark()
function shall return 0. Otherwise, it shall return a value of -1 and set errno to indi-
cate the error.
The sockatmark() function shall fail if:
EBADF The s argument is not a valid file descriptor.
ENOTTY The s argument does not specify a descriptor for a socket.
The following sections are informative.
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 con-
sume 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 wait-
ing to be read at the socket (via SIGURG or select()). See Socket Receive Queue , Socket
Out-of-Band Data State , Signals , and pselect() for details.
The sockatmark() function replaces the historical SIOCATMARK command to ioctl() which
implemented the same functionality on many implementations. Using a wrapper function fol-
lows the adopted conventions to avoid specifying commands to the ioctl() function, other
than those now included to support XSI STREAMS. The sockatmark() function could be imple-
mented as follows:
int sockatmark(int s)
The use of [ENOTTY] to indicate an incorrect descriptor type matches the historical behav-
ior of SIOCATMARK.
pselect() , recv() , recvmsg() , the Base Definitions volume of IEEE Std 1003.1-2001,
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SOCKATMARK(P)