SELECT(S) UNIX System V SELECT(S)
Name
select - synchronous I/O multiplexing
Syntax
#include <sys/types.h>
#include <sys/times.h>
#include <sys/select.h>
nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
FD_SET(fd, &fdset)
FD_CLR(fd, &fdset)
FD_ISSET(fd, &fdset)
FD_ZERO(&fdset)
int fd;
fd_set 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. 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
effect 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
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 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. Or,
the device driver being polled has not
implemented select support.
See Also
read(S), write(S)
Notes
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.
Credit
This utility was developed at the University of California
at Berkeley and is used with permission.
(printed 6/20/89)