synchronous I/O multiplexing
*exceptfds, const struct
timespec *timeout, const
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. Exceptional conditions include the presence of
out-of-band data 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
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
initializes a descriptor set fdset to the null set.
&fdset) includes a particular descriptor
fd in fdset.
&fdset) removes fd from
&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
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
and may be reused on subsequent calls; however, it is good style to
re-initialize it before each invocation of
Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are of interest.
function is similar to
select() except that it
specifies the timeout using a timespec structure. Also, if
mask is a non-null pointer,
pselect() atomically sets the calling thread's
signal mask to the signal set pointed to by mask for
the duration of the function call. In this case, the original signal mask
will be restored before
pselect() return the number of ready descriptors
that are contained in the descriptor sets. If a descriptor is included in
multiple descriptor sets, each inclusion is counted separately. If the time
limit expires before any descriptors become ready, they return 0.
pselect() return with an error, including one due to
an interrupted call, they return -1, and the descriptor sets will be
An error return from
- One or more of readfds, writefds, or exceptfds points outside the process's allocated address space.
- One of the descriptor sets specified an invalid descriptor.
- A signal was delivered before the time limit expired and before any of the selected descriptors became ready.
- The specified time limit is invalid. One of its components is negative or too large.
- nfds was less than 0.
accept(2), clock_gettime(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2), send(2), write(2), getdtablesize(3)
pselect() functions conform to IEEE
Std 1003.1-2008 (“POSIX.1”).
select() system call first appeared in
system call has been available since OpenBSD
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. If descriptor values greater than FD_SETSIZE are possible in a program, use poll(2) instead.
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