select.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" @(#)select.2 1.37 90/02/01 SMI; from UCB 4.2
.\" Copyright (c) 1983 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
.TH select 2
.SH Name
select \- synchronous I/O multiplexing
.SH Syntax
.nf
#include <sys/types.h>
#include <sys/time.h>

int select (\fInfds\fP, \fIreadfds\fP, \fIwritefds\fP, \fIexceptfds\fP, \fItimeout\fP)
int \fInfsd\fP;
fd_set \fI*readfds\fP, \fI*writefds\fP, \fI*exceptfds\fP;
struct timeval *\fItimeout\fP;

FD_SET (\fIfd, fdset\fP)
FD_CLR (\fIfd, fdset\fP)
FD_ISSET (\fIfd, fdset\fP)
FD_ZERO (\fIfdset\fP)
int \fIfd\fP;
fd_set \fI*fdset\fP;
.fi
.SH Description
.NXR "select system call"
.NXR "I/O multiplexing"
The
.PN select 
system call examines the I/O descriptor sets whose addresses
are passed in \fIreadfds\fP, \fIwritefds\fP, 
and \fIexceptfds\fP to see if
some of their descriptors are ready for reading, ready for
writing, or have an exception condition pending. The \fInfds\fP
parameter is
the number of bits to be checked in each bit mask that
represent a file descriptor; the descriptors from 0 through
\fInfds\fP-1 in the descriptor sets are examined. Typically
\fInfds\fP has the value returned by 
.MS getdtablesize 2
for the maximum
number of file descriptors. On return, 
.PN select
replaces
the given descriptor sets with subsets consisting of those
descriptors that are ready for the requested operation. The
total number of ready descriptors in all the sets is
returned.
.PP
The descriptor sets are stored as bit fields in arrays of
integers. The following macros are provided for manipulating such descriptor 
sets: 
.RS 5
.TP 20
FD_ZERO(\fIfdset\fP)
Initializes  a
descriptor set \fIfdset\fP to the null set. 
.TP
FD_SET(\fIfd\fP, \fIfdset\fP)
Includes a particular descriptor \fIfd\fP in \fIfdset\fP. 
.TP
FD_CLR(\fIfd\fP, \fIfdset\fP) 
Removes \fIfd\fP from \fIfdset\fP. 
.TP
FD_ISSET(\fIfd\fP, \fIfdset\fP) 
Is
nonzero if \fIfd\fP is a member of \fIfdset\fP; otherwise the value is
set to zero. 
.RE
.PP
The
behavior of these macros is undefined if a descriptor value
is less than zero or greater than or equal to FD_SETSIZE,
which is equal to the maximum number of
descriptors that can be supported by the system.
.PP
If \fItimeout\fP is not a NULL pointer, it specifies a maximum
interval to wait for the selection to complete. If \fItimeout\fP
is a NULL pointer, the 
.PN select 
call blocks indefinitely. To
effect a poll, the \fItimeout\fP argument should be a non-NULL
pointer, pointing to a zero-valued timeval structure.
.PP
Any of \fIreadfds\fP, \fIwritefds\fP, and \fIexceptfds\fP may be given as NULL
pointers if no descriptors are of interest.
.PP
Selecting true for reading on a socket descriptor upon which
a 
.PN listen 
call has been performed indicates that a subsequent 
.PN accept
call on that descriptor will not block.
.SH Restrictions
The
.PN select
call may indicate that a
descriptor is ready for writing when in fact an attempt to
write would block. This can happen if system resources
necessary for a write are exhausted or otherwise unavailable. 
If an application deems it critical that writes to a
file descriptor not block, it should set the descriptor for
non-blocking I/O using the F_SETFL request to the
.PN fcntl
call.
.SH Return Values
The 
.PN select
call returns a non-negative value on success. 
A positive value indicates the number of ready descriptors in the
descriptor sets. A 0 indicates that the time limit referred
to by \fItimeout\fP expired. 
On failure, 
.PN select
returns -1,
sets errno to indicate the error, and does not change the descriptor sets.
.SH Diagnostics
.TP 12
[EBADF]
One of the descriptor sets specified an invalid descriptor.
.TP
[EFAULT]
One of the pointers given in the call referred to a 
non-existent portion of the process's address space.
.TP
[EINTR]
A signal was delivered before any of the selected events occurred, 
or before the time limit expired.
.TP
[EINVAL]
A component of the pointed-to time limit is outside the acceptable range. 
The t_sec range must be between 0 and 10^8, inclusive; 
t_usec must be greater than or equal to 0, and less than 10^6.
.SH See Also
accept(2), connect(2), fcntl(2), gettimeofday(2), listen(2), 
read(2), recv(2), send(2), write(2), getdtablesize(2)