NAME
select - synchronous I/O multiplexing

SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>

nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;

FD_SET(fd, &fdset)
FD_CLR(fd, &fdset)
FD_ISSET(fd, &fdset)
FD_ZERO(&fdset)
int fd;
fd_set 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-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 total number of ready descriptors in all the sets is returned
in nfound.

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 nonzero 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-zero pointer, it specifies a maximum interval
to wait for the selection to complete. If timeout is a zero
pointer, the select blocks indefinitely. To affect a poll, the
timeout argument should be non-zero, pointing to a zero-valued
timeval structure.

Any of readfds, writefds, and exceptfds may be given as zero
pointers if no descriptors are of interest.

RETURN VALUE
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 then 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:

[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), read(2), write(2), recv(2), send(2),
getdtablesize(2)

BUGS
Although the provision of getdtablesize(2) 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 size FD_SETSIZE
(currently 256) is somewhat larger 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>.

Select 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 will be unmodified by the select
call.