SELECT(2) BSD System Calls Manual SELECT(2)
FD_CLR, FD_COPY, FD_ISSET, FD_SET, FD_ZERO, select -- synchronous I/O multiplexing
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);
select() 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 operation. select() returns the total number of ready descriptors in all
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
Any of readfds, writefds, and errorfds may be given as nil 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, select() returns 0. If select() returns with an error, including one due to an interrupted call, the descriptor sets will be unmod-
ified 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 allocate the requested number of file descriptors.
[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.
[EINVAL] ndfs is greater than FD_SETSIZE and _DARWIN_UNLIMITED_SELECT is not defined.
- or -
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_DAR-
accept(2), connect(2), connectx(2), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(2), compat(5)
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 poten-
tially 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
The select() function call appeared in 4.2BSD.
4.2 Berkeley Distribution March 18, 2015 4.2 Berkeley Distribution