.\" $NetBSD: 1.5.t,v 1.4 2013/10/06 05:45:19 dholland Exp $ .\" .\" Copyright (c) 1983, 1993, 1994 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)1.5.t 8.6 (Berkeley) 5/29/94 .\" .Sh 2 16 "Descriptors .Sh 3 16 "The reference table .PP Each process has access to resources through \fIdescriptors\fP. Each descriptor is a handle allowing processes to reference objects such as files, devices and communications links. .PP Rather than allowing processes direct access to descriptors, the system introduces a level of indirection, so that descriptors may be shared between processes. Each process has a \fIdescriptor reference table\fP, containing pointers to the actual descriptors. The descriptors themselves therefore may have multiple references, and are reference counted by the system. .PP Each process has a limited size descriptor reference table, where the current size is returned by the .Fn getdtablesize call: .DS .Fd getdtablesize 0 "get descriptor table size nds = getdtablesize(); result int nds; .DE and guaranteed to be at least 64. The maximum number of descriptors is a resource limit (see section .Xr 1.6.3 ). The entries in the descriptor reference table are referred to by small integers; for example if there are 64 slots they are numbered 0 to 63. .Sh 3 16 "Descriptor properties .PP Each descriptor has a logical set of properties maintained by the system and defined by its \fItype\fP. Each type supports a set of operations; some operations, such as reading and writing, are common to several abstractions, while others are unique. For those types that support random access, the current file offset is stored in the descriptor. The generic operations applying to many of these types are described in section .Xr 2.1 . Naming contexts, files and directories are described in section .Xr 2.2 . Section .Xr 2.3 describes communications domains and sockets. Terminals and (structured and unstructured) devices are described in section .Xr 2.4 . .Sh 3 16 "Managing descriptor references .LP A duplicate of a descriptor reference may be made by doing: .DS .Fd dup 1 "duplicate an existing file descriptor new = dup(old); result int new; int old; .DE returning a copy of descriptor reference \fIold\fP which is indistinguishable from the original. The value of \fInew\fP chosen by the system will be the smallest unused descriptor reference slot. A copy of a descriptor reference may be made in a specific slot by doing: .DS .Fd dup2 2 "duplicate an existing file descriptor dup2(old, new); int old, new; .DE The .Fn dup2 call causes the system to deallocate the descriptor reference current occupying slot \fInew\fP, if any, replacing it with a reference to the same descriptor as old. .LP Descriptors are deallocated by: .DS .Fd close 1 "delete a descriptor close(old); int old; .DE .Sh 3 16 "Multiplexing requests .PP The system provides a standard way to do synchronous and asynchronous multiplexing of operations. Synchronous multiplexing is performed by using the .Fn select call to examine the state of multiple descriptors simultaneously, and to wait for state changes on those descriptors. Sets of descriptors of interest are specified as bit masks, as follows: .DS .Fd select 5 "synchronous I/O multiplexing nds = select(nd, in, out, except, tvp); result int nds; int nd; result fd_set *in, *out, *except; struct timeval *tvp; FD_CLR(fd, &fdset); FD_COPY(&fdset, &fdset2); FD_ISSET(fd, &fdset); FD_SET(fd, &fdset); FD_ZERO(&fdset); int fd; fs_set fdset, fdset2; .DE The .Fn select call examines the descriptors specified by the sets \fIin\fP, \fIout\fP and \fIexcept\fP, replacing the specified bit masks by the subsets that select true for input, output, and exceptional conditions respectively (\fInd\fP indicates the number of file descriptors specified by the bit masks). If any descriptors meet the following criteria, then the number of such descriptors is returned in \fInds\fP and the bit masks are updated. .if n .ds bu * .if t .ds bu \(bu .IP \*(bu A descriptor selects for input if an input oriented operation such as .Fn read or .Fn receive is possible, or if a connection request may be accepted (see sections .Xr 2.1.3 and .Xr 2.3.1.4 ). .IP \*(bu A descriptor selects for output if an output oriented operation such as .Fn write or .Fn send is possible, or if an operation that was ``in progress'', such as connection establishment, has completed (see sections .Xr 2.1.3 and .Xr 2.3.1.5 ). .IP \*(bu A descriptor selects for an exceptional condition if a condition that would cause a SIGURG signal to be generated exists (see section .Xr 1.3.2 ), or other device-specific events have occurred. .LP For these tests, an operation is considered to be possible if a call to the operation would return without blocking (even if the O_NONBLOCK flag were not set). For example, a descriptor would test as ready for reading if a read call would return immediately with data, an end-of-file indication, or an error other than EWOULDBLOCK. .LP If none of the specified conditions is true, the operation waits for one of the conditions to arise, blocking at most the amount of time specified by \fItvp\fP. If \fItvp\fP is given as NULL, the .Fn select waits indefinitely. .LP Options affecting I/O on a descriptor may be read and set by the call: .DS .Fd fcntl 3 "file control dopt = fcntl(d, cmd, arg); result int dopt; int d, cmd, arg; .DE .DS .TS l s l l. /* command values */ F_DUPFD /* return a new descriptor */ F_GETFD /* get file descriptor flags */ F_SETFD /* set file descriptor flags */ F_GETFL /* get file status flags */ F_SETFL /* set file status flags */ F_GETOWN /* get SIGIO/SIGURG proc/pgrp */ F_SETOWN /* set SIGIO/SIGURG proc/pgrp */ F_GETLK /* get blocking lock */ F_SETLK /* set or clear lock */ F_SETLKW /* set lock with wait */ .TE .DE The F_DUPFD \fIcmd\fP provides identical functionality to .Fn dup2 ; it is provided solely for POSIX compatibility. The F_SETFD \fIcmd\fP can be used to set the close-on-exec flag for a file descriptor. The F_SETFL \fIcmd\fP may be used to set a descriptor in non-blocking I/O mode and/or enable signaling when I/O is possible. F_SETOWN may be used to specify a process or process group to be signaled when using the latter mode of operation or when urgent indications arise. The .Fn fcntl system call also provides POSIX-compliant byte-range locking on files. However the semantics of unlocking on every .Fn close rather than last close makes them useless. Much better semantics and faster locking are provided by the .Fn flock system call (see section .Xr 2.2.7 ). The .Fn fcntl and .Fn flock locks can be used concurrently; they will serialize against each other properly. .PP Operations on non-blocking descriptors will either complete immediately, return the error EWOULDBLOCK, partially complete an input or output operation returning a partial count, or return an error EINPROGRESS noting that the requested operation is in progress. A descriptor which has signalling enabled will cause the specified process and/or process group be signaled, with a SIGIO for input, output, or in-progress operation complete, or a SIGURG for exceptional conditions. .PP For example, when writing to a terminal using non-blocking output, the system will accept only as much data as there is buffer space, then return. When making a connection on a \fIsocket\fP, the operation may return indicating that the connection establishment is ``in progress''. The .Fn select facility can be used to determine when further output is possible on the terminal, or when the connection establishment attempt is complete.