select - synchronous I/O multiplexing
int 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 file descriptors are
checked in each set; i.e. the file descriptors from 0 through nfds - 1
will be examined (see getdtablehi(3) for largest open descriptor). 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 the return value.
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.
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.
An error return from select indicates:
[EBADF] One of the descriptor sets specified an invalid
[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.
poll(2), accept(2), connect(2), read(2), write(2), recv(2), send(2),
Some devices do not support polling via the select(2) and poll(2) system
calls. Doing a select or poll on a file descriptor associated with an
"un-pollable" device will cause the select or poll to return immediately
with a success value of 0 and the with the corresponding file descriptor
events of queried set true. For instance, if a select or poll is
performed on a read file descriptor associated with an un-pollable
device, the call would return immediately, even though there may be
nothing to read on the device. A subsequent read(2) in this situation
might return with a "bytes-read" count of 0 or might block if the device
supports read blocking. Devices which exhibit this behavior (especially
those from third-party vendors) should be suspected as not supporting
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.
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