synchronous I/O multiplexing
cc . . . -lc
int select(nfds, readfds, writefds, exceptfds, timeout)
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
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;
that is, the descriptors from 0 through
nfds-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 return value of select is
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.
removes fd from fdset.
is nonzero if fd is a member of fdset,
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.
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
argument should be non-NULL,
pointing to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds
may be given as NULL pointers if no descriptors are of interest.
returns the number of ready descriptors that are contained in
the descriptor sets, or -1 if an error occurred.
If the time limit expires then select
If select returns with an error,
including one due to an interrupted call,
the descriptor sets are unmodified.
An error return from select indicates:
One of the descriptor sets specified an invalid descriptor.
A signal was delivered before the time limit expired and
before any of the selected events occurred.
The specified time limit is invalid. One of its components is
negative or too large. Or, the device driver being polled has not
implemented select support.
should probably return the time remaining from the original timeout,
if any, by modifying the time value in place.
This may be implemented in future versions of the system.
Thus, it is unwise to assume that the timeout value is unmodified
by the select call.
select is not part of any currently supported standard;
it was developed at
the University of California at Berkeley
and is maintained by The SCO Group.
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003