| SELECT(2) |
AerieBSD 1.0 Refernce Manual |
SELECT(2) |
NAME
select —
synchronous I/O multiplexing
SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
#include <string.h>
#include <unistd.h>
int
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
FD_SET(fd &fdset);
FD_CLR(fd &fdset);
FD_ISSET(fd &fdset);
FD_ZERO(&fdset);
DESCRIPTION
select();
examines the I/O descriptor sets whose addresses are passed in
readfds,
writefds,
and
exceptfds
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
The first
nfds
descriptors are checked in each set;
i.e., the descriptors from 0 through
nfds
in the descriptor sets are examined.
On return,
select();
replaces the given descriptor sets
with subsets consisting of those descriptors that are ready
for the requested operation.
select();
returns the total number of ready descriptors in all the sets.
The descriptor sets are stored as bit fields in arrays of integers.
The following macros are provided for manipulating such descriptor sets:
FD_ZERO(&fdset);
initializes a descriptor set
fdset
to the null set.
FD_SET(fd &fdset);
includes a particular descriptor
fd
in
fdset.
FD_CLR(fd &fdset);
removes
fd
from
fdset.
FD_ISSET(fd &fdset);
is non-zero if
fd
is a member of
fdset,
zero otherwise.
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 normally at least equal
to the maximum number of descriptors supported by the system.
If
timeout
is a non-null pointer, it specifies a maximum interval to wait for the
selection to complete.
If
timeout
is a null pointer, the select blocks indefinitely.
To effect a poll, the
timeout
argument should be non-null, pointing to a zero-valued timeval structure.
timeout
is not changed by
select();,
and may be reused on subsequent calls; however, it is good style to
re-initialize it before each invocation of
select();.
Any of
readfds,
writefds,
and
exceptfds
may be given as null pointers if no descriptors are of interest.
RETURN VALUES
select();
returns the number of ready descriptors that are contained in
the descriptor sets, or \-1 if an error occurred.
If the time limit expires,
select();
returns 0.
If
select();
returns with an error, including one due to an interrupted call,
the descriptor sets will be unmodified.
ERRORS
An error return from
select();
indicates:
- [EFAULT]
-
One or more of
readfds,
writefds,
or
exceptfds
points outside the process's allocated address space.
- [EBADF]
-
One of the descriptor sets specified an invalid descriptor.
- [EINTR]
-
A signal was delivered before the time limit expired and
before any of the selected events occurred.
- [EINVAL]
-
The specified time limit is invalid.
One of its components is negative or too large.
SEE ALSO
accept(2),
connect(2),
gettimeofday(2),
poll(2),
read(2),
recv(2),
send(2),
write(2),
getdtablesize(3)
HISTORY
The
select();
function call appeared in
4.2BSD.
BUGS
Although the provision of
getdtablesize(3)
was intended to allow user programs to be written independent
of the kernel limit on the number of open files, the dimension
of a sufficiently large bit field for select remains a problem.
The default bit size of
fd_set
is based on the symbol
FD_SETSIZE
(currently 1024),
but that is somewhat smaller than the current kernel limit
to the number of open files.
However, in order to accommodate programs which might potentially
use a larger number of open files with select, it is possible
to increase this size within a program by providing
a larger definition of
FD_SETSIZE
before the inclusion of
‹sys/types.h›.
The kernel will cope, and the userland libraries provided with the
system are also ready for large numbers of file descriptors.
Alternatively, to be really safe, it is possible to allocate
fd_set
bit-arrays dynamically.
The idea is to permit a program to work properly even if it is
execve(2/Ns)
with 4000 file descriptors pre-allocated.
The following illustrates the technique which is used by
userland libraries:
fd_set *fdsr;
int max = fd;
fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
sizeof(fd_mask));
if (fdsr == NULL) {
...
return (-1);
}
FD_SET(fd, fdsr);
n = select(max+1, fdsr, NULL, NULL, &tv);
\&...
free(fdsr);
Alternatively, it is possible to use the
poll(2)
interface.
poll(2)
is more efficient when the size of
select(Ns 's);
fd_set
bit-arrays are very large, and for fixed numbers of
file descriptors one need not size and dynamically allocate a
memory object.
select();
should probably have been designed to return the time remaining from the
original timeout, if any, by modifying the time value in place.
Even though some systems stupidly act in this different way, it is
unlikely this semantic will ever be commonly implemented, as the
change causes massive source code compatibility problems.
Furthermore, recent new standards have dictated the current behaviour.
In general, due to the existence of those brain-damaged
non-conforming systems, it is unwise to assume that the timeout
value will be unmodified by the
select();
call, and the caller should reinitialize it on each invocation.
Calculating the delta is easily done by calling
gettimeofday(2)
before and after the call to
select();,
and using
timersub();
(as described in
getitimer(2/)).
Internally to the kernel,
select();
works poorly if multiple processes wait on the same file descriptor.
Given that, it is rather surprising to see that many daemons are
written that way (i.e.,
httpd(8/)).
| AerieBSD 1.0 Reference Manual |
August 26 2008 |
SELECT(2) |