COMMENT(!MOD!zsh/zselect
Block and return when file descriptors are ready.
!MOD!)
The tt(zsh/zselect) module makes available one builtin command:

startitem()
findex(zselect)
cindex(select, system call)
cindex(file descriptors, waiting for)
item(tt(zselect) [ tt(-rwe) tt(-t) var(timeout) tt(-a) var(array) ] [ var(fd) ... ])(
The tt(zselect) builtin is a front-end to the `select' system call, which
blocks until a file descriptor is ready for reading or writing, or has an
error condition, with an optional timeout.  If this is not available on
your system, the command prints an error message and returns status 2
(normal errors return status 1).  For more information, see your systems
documentation for manref(select)(3).  Note there is no connection with the
shell builtin of the same name.

Arguments and options may be intermingled in any order.  Non-option
arguments are file descriptors, which must be decimal integers.  By
default, file descriptors are to be tested for reading, i.e. tt(zselect)
will return when data is available to be read from the file descriptor, or
more precisely, when a read operation from the file descriptor will not
block.  After a tt(-r), tt(-w) and tt(-e), the given file descriptors are
to be tested for reading, writing, or error conditions.  These options and
an arbitrary list of file descriptors may be given in any order.

(The presence of an `error condition' is not well defined in the
documentation for many implementations of the select system call.
According to recent versions of the POSIX specification, it is really an
em(exception) condition, of which the only standard example is out-of-band
data received on a socket.  So zsh users are unlikely to find the tt(-e)
option useful.)

The option `tt(-t) var(timeout)' specifies a timeout in hundredths of a
second.  This may be zero, in which case the file descriptors will simply
be polled and tt(zselect) will return immediately.  It is possible to call
zselect with no file descriptors and a non-zero timeout for use as a
finer-grained replacement for `sleep'; not, however, the return status is
always 1 for a timeout.

The option `tt(-a) var(array)' indicates that tt(array) should be set to
indicate the file descriptor(s) which are ready.  If the option is not
given, the array tt(reply) will be used for this purpose.  The array will
contain a string similar to the arguments for tt(zselect).  For example,

example(zselect -t 0 -r 0 -w 1)

might return immediately with status 0 and tt($reply) containing `tt(-r 0 -w
1)' to show that both file descriptors are ready for the requested
operations.

The option `tt(-A) var(assoc)' indicates that the associative array
tt(assoc) should be set to indicate the file descriptor(s) which are
ready.  This option overrides the option tt(-a), nor will tt(reply) be
modified.  The keys of tt(assoc) are the file descriptors, and the
corresponding values are any of the characters `tt(rwe)' to indicate the
condition.

The command returns 0 if some file descriptors are ready for reading.  If
the operation timed out, or a timeout of 0 was given and no file
descriptors were ready, or there was an error, it returns status 1 and
the array will not be set (nor modified in any way).  If there was an error
in the select operation the appropriate error message is printed.
)
enditem()
