|SELECT(2)||System Calls Manual||SELECT(2)|
synchronous I/O multiplexing
fd_set *restrict readfds, fd_set
*restrict writefds, fd_set *restrict errorfds,
struct timeval *restrict timeout);
examines the I/O descriptor sets whose addresses are passed in
readfds, writefds, and
errorfds 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. (Example: If you have set two file descriptors "4"
and "17", nfds should not be "2",
but rather "17 + 1" or "18".) 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.
&fdset_copy) replaces an already allocated
&fdset_copy file descriptor set with a copy of
&fdset_orig. The behavior of these macros is
undefined if a descriptor value is less than zero or greater than or equal
FD_SETSIZE, which is normally at least equal to
the maximum number of descriptors supported by the system.
If timeout is not a 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 not be a null pointer, but it should point 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 errorfds may be given as null pointers if no descriptors are of interest.
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,
returns 0. If
select() returns with an error,
including one due to an interrupted call, the descriptor sets will be
unmodified and the global variable errno will be set
to indicate the error.
An error return from
select() now returns with
errno set to EINVAL when nfds is
greater than FD_SETSIZE. Use a smaller value for nfds
or compile with -D_DARWIN_UNLIMITED_SELECT.
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 1024) is somewhat smaller 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
the inclusion of ⟨sys/types.h⟩.
select() should probably have been
designed to return the time remaining from the original timeout, if any, by
modifying the time value in place. However, it is unlikely this semantic
will ever be implemented, as the change would cause source code
compatibility problems. In general 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.
select() function call appeared in
|March 18, 2015||BSD 4.2|