SELECT(2) System Calls Manual SELECT(2)
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 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. 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 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 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 normally 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 selection 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 structure.
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 be
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 too large.
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 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 somewhat larger 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 defini-
tion 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.
4.2 Berkeley Distribution May 15, 1986 SELECT(2)