SELECT(2) BSD System Calls Manual SELECT(2)NAME
FD_CLR, FD_COPY, FD_ISSET, FD_SET, FD_ZERO, select — synchronous I/O mul‐
FD_CLR(fd, fd_set *fdset);
FD_COPY(fd_set *fdset_orig, fd_set *fdset_copy);
FD_ISSET(fd, fd_set *fdset);
FD_SET(fd, fd_set *fdset);
select(int nfds, fd_set *restrict readfds, fd_set *restrict writefds,
fd_set *restrict errorfds, struct timeval *restrict timeout);
DESCRIPTIONSelect() 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 condi‐
tion 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 consist‐
ing 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. FD_COPY(&fdset_orig,
&fdset_copy) replaces an already allocated &fdset_copy file descriptor
set with a copy of &fdset_orig. The behavior of these macros is unde‐
fined 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-nil pointer, it specifies a maximum interval to wait
for the selection to complete. If timeout is a nil pointer, the select
blocks indefinitely. To effect a poll, the timeout argument should be
non-nil, pointing to a zero-valued timeval structure. Timeout is not
changed by select(), and may be reused on subsequent calls, however it is
good style to re-initialize it before each invocation of select().
Any of readfds, writefds, and errorfds may be given as nil pointers if no
descriptors are of interest.
RETURN VALUESSelect() 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, includ‐
ing one due to an interrupted call, the descriptor sets will be unmodi‐
fied and the global variable errno will be set to indicate the error.
An error return from select() indicates:
[EAGAIN] The kernel was (perhaps temporarily) unable to allo‐
cate the requested number of file descriptors.
[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 com‐
ponents is negative or too large.
[EINVAL] ndfs is greater than FD_SETSIZE and _DARWIN_UNLIM‐
ITED_SELECT is not defined.
- or -
COMPATIBILITYselect() now returns with errno set to EINVAL when nfds is greater than
FD_SETSIZE. Use a smaller value for nfds or compile with -D_DAR‐
SEE ALSOaccept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2),
recv(2), send(2), write(2), compat(5)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 1024) is some‐
what 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 FD_SETSIZE
before 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.
The select() function call appeared in 4.2BSD.
4.2 Berkeley Distribution March 25, 1994 4.2 Berkeley Distribution