select - synchronous I/O multiplexing
nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
Select 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. The first nfds descrip-
tors 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 con-
sisting of those descriptors that are ready for the requested operation. The total number
of ready descriptors in all the sets is returned in nfound.
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 descrip-
tor 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 nonzero 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 nor-
mally at least equal to the maximum number of descriptors supported by the system.
If timeout is a non-zero pointer, it specifies a maximum interval to wait for the selec-
tion to complete. If timeout is a zero pointer, the select blocks indefinitely. To
affect a poll, the timeout argument should be non-zero, pointing to a zero-valued timeval
Any of readfds, writefds, and exceptfds may be given as zero 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 then select returns 0. If select
returns with an error, including one due to an interrupted call, the descriptor sets will
An error return from select indicates:
[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
accept(2), connect(2), read(2), write(2), recv(2), send(2), getdtablesize(2)
Although the provision of getdtablesize(2) was intended to allow user programs to be writ-
ten independent of the kernel limit on the number of open files, the dimension of a suffi-
ciently large bit field for select remains a problem. The default size FD_SETSIZE (cur-
rently 256) is somewhat 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 sys-
tem. Thus, it is unwise to assume that the timeout value will be unmodified by the select
4.2 Berkeley Distribution May 15, 1986 SELECT(2)