SELECT(2) BSD System Calls Manual SELECT(2)
select -- synchronous I/O multiplexing
- or -
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *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. 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. 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-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 exceptfds 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-
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), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(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 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 25, 1994 4.2 Berkeley Distribution