SELECT(2) SELECT(2)
NAME
select - synchronous I/O multiplexing
SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
fdset *readfds, *writefds, *exceptfds;
struct timeval *timeout;
FDSET(fd, &fdset)
FDCLR(fd, &fdset)
FDISSET(fd, &fdset)
FDZERO(&fdset)
int fd;
fdset fdset;
DESCRIPTION
The select command 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
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SELECT(2) SELECT(2)
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.
RETURN VALUE
The select command 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 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), read(2), write(2), recv(2), send(2),
getdtablesize(2)
BUGS
Although the provision of getdtablesize(2) was intended to
allow user programs to be written independent of the kernel
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SELECT(2) SELECT(2)
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>.
The select call 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.
Page 3 May 1989