служебный знак (of-band mark)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
sockatmark — determine whether a socket is at the out-of-band
mark
Синопсис (Synopsis)
#include <sys/socket.h>
int sockatmark(int s);
Описание (Description)
The sockatmark() function shall determine whether the socket
specified by the descriptor s is at the out-of-band data mark
(see Section 2.10.12, Socket Out-of-Band Data 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() function
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.
Возвращаемое значение (Return value)
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
indicate the error.
Ошибки (Error)
The sockatmark() function shall fail if:
EBADF The s argument is not a valid file descriptor.
ENOTTY The file associated with the s argument is not a socket.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
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 (via SIGURG or select()). See Section
2.10.11, Socket Receive Queue, Section 2.10.12, Socket Out-of-
Band Data State, Section 2.10.14, Signals, and pselect() for
details.
Обоснование (Rationale)
The sockatmark() function replaces the historical SIOCATMARK
command to ioctl() which implemented the same functionality on
many implementations. Using a wrapper function follows 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 implemented as follows:
#include <sys/ioctl.h>
int sockatmark(int s)
{
int val;
if (ioctl(s,SIOCATMARK,&val)==-1)
return(-1);
return(val);
}
The use of [ENOTTY] to indicate an incorrect descriptor type
matches the historical behavior of SIOCATMARK.
Будущие направления (Future directions)
None.
Смотри также (See also)
Section 2.10.12, Socket Out-of-Band Data State, pselect(3p),
recv(3p), recvmsg(3p)
The Base Definitions volume of POSIX.1‐2017, sys_socket.h(0p)
