The
select ();
system call
examines the I/O descriptor sets whose addresses are passed in
Fa readfds ,
Fa writefds ,
and
Fa exceptfds
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
The only exceptional condition detectable is out-of-band
data received on a socket.
The first
Fa nfds
descriptors are checked in each set;
i.e., the descriptors from 0 through
Fa nfds Ns No -1
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.
The
select ();
system call
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
Fa fdset
to the null set.
FD_SET (fd &fdset);
includes a particular descriptor
Fa fd
in
Fa fdset .
FD_CLR (fd &fdset);
removes
Fa fd
from
Fa fdset .
FD_ISSET (fd &fdset);
is non-zero if
Fa fd
is a member of
Fa 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
Fa timeout
is not a null pointer, it specifies the maximum interval to wait for the
selection to complete.
System activity can lengthen the interval by
an indeterminate amount.
If
Fa timeout
is a null pointer, the select blocks indefinitely.
To effect a poll, the
Fa timeout
argument should not be a null pointer,
but it should point to a zero-valued timeval structure.
Any of
Fa readfds ,
Fa writefds ,
and
Fa exceptfds
may be given as null pointers if no descriptors are of interest.
RETURN VALUES
The
select ();
system call
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 system call,
the descriptor sets will be unmodified.
ERRORS
An error return from
select ();
indicates:
Bq Er EBADF
One of the descriptor sets specified an invalid descriptor.
Bq Er EFAULT
One of the arguments
Fa readfds , writefds , exceptfds ,
or
Fa timeout
points to an invalid address.
Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
Bq Er EINVAL
The specified time limit is invalid.
One of its components is
negative or too large.
The default size of
FD_SETSIZE
is currently 1024.
In order to accommodate programs which might potentially
use a larger number of open files with
select (,);
it is possible
to increase this size by having the program define
FD_SETSIZE
before the inclusion of any header which includes
In sys/types.h .
If
Fa nfds
is greater than the number of open files,
select ();
is not guaranteed to examine the unused file descriptors.
For historical
reasons,
select ();
will always examine the first 256 descriptors.
STANDARDS
The
select ();
system call and
FD_CLR (,);
FD_ISSET (,);
FD_SET (,);
and
FD_ZERO ();
macros conform with
St -p1003.1-2001 .
HISTORY
The
select ();
system call appeared in
BSD 4.2
BUGS
St -susv2
allows systems to modify the original timeout in place.
Thus, it is unwise to assume that the timeout value will be unmodified
by the
select ();
system call.
