SELECT(2) BSD System Calls Manual SELECT(2)NAME
select — synchronous I/O multiplexing
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
struct timeval *timeout);
DESCRIPTIONSelect() 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 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. 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 descrip‐
tors 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(&fdsetx) 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 max‐
imum 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 affect a poll, the timeout argument should be
non-nil, pointing to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds 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‐
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 com‐
ponents is negative or too large.
SEE ALSOaccept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2),
recv(2), send(2), write(2)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 256) is some‐
what larger 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 return the time remaining from the original
timeout, if any, by modifying the time value in place. This may be
implemented in future versions of the system. Thus, it is unwise to
assume that the timeout value will be unmodified by the select() call.
The select function call appeared in 4.2BSD.
4.2 Berkeley Distribution March 25, 1994 4.2 Berkeley Distribution