DragonFly On-Line Manual Pages
SELECT(2) DragonFly System Calls Manual SELECT(2)
select -- synchronous I/O multiplexing
Standard C Library (libc, -lc)
select(int nfds, fd_set * restrict readfds, fd_set * restrict writefds,
fd_set * restrict exceptfds, struct timeval * restrict timeout);
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 only exceptional condition
detectable is out-of-band data received on a socket. 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. 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 non-NULL, it specifies the maximum interval to wait for the
selection to complete. System activity can lengthen the interval by an
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.
Any of readfds, writefds, and exceptfds may be given as null pointers if
no descriptors are of interest.
Even if no descriptors are of interest, the timeout works as described,
effectively putting the process into an interruptible sleep for the
specified timeout. If timeout is NULL, the process will block until a
signal is received.
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
An error return from select() indicates:
[EBADF] One of the descriptor sets specified an invalid
[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.
[EINVAL] nfds was invalid.
accept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2),
recv(2), send(2), write(2), clocks(7)
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
If 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.
The select() function call appeared in 4.2BSD.
Version 2 of the Single UNIX Specification (``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() call.
DragonFly 5.1 February 22, 2018 DragonFly 5.1