NAME
select —
synchronous I/O multiplexing
SYNOPSIS
#include
<sys/types.h>
#include <sys/time.h>
#include <unistd.h>
int
select(int
nfds, fd_set
*readfds, fd_set
*writefds, fd_set
*exceptfds, struct
timeval *timeout);
FD_SET(fd,
&fdset);
FD_CLR(fd,
&fdset);
FD_ISSET(fd,
&fdset);
FD_ZERO(&fdset);
DESCRIPTION
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(&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 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 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 VALUES
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
unmodified.
ERRORS
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.
SEE ALSO
accept(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
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 system. Thus, it is
unwise to assume that the timeout value will be unmodified by the
select() call.
HISTORY
The select function call appeared in
4.2BSD.