available


Name:
available - check if data is available from an istream
Synopsis:
istream available -> istream {true|false}  

Examples:
myfifo available { getline } if % read it data is available.  

Description:
"available" gives the answer to one question:  
--Is there at least one character waiting to be read
from the istream?--
If "available" returns true, it can be safely assumed that
reading one character from the given istream is safe,
i.e. it will NEITHER BLOCK nor yield EOF or an error.

Alternative: Functions available_is (undocumented)
-> behavior and synopsis are the same.

Parameters:
In:  istream: The istream to check.  
Out: true or false, indicating if data is waiting on the stream.

Bugs:
-  

Diagnostics:
If a system-error occurs, a code is stored in "sys_errno"  
(contained in errordict) to identify the error, and "sys_errname"
is set to the error message. Then a "SystemError" is raised.
The following system errors may be issued, according to the
POSIX standard (errors in parentheses are not
expected to occur in this routines' context):

(EACCES) Search permission is denied for a
directory in a files path prefix.
(EAGAIN) The ON_NONBLOCK flag is set for a file
descriptor and the process would be
delayed in the I/O operation.
EBADF Invalid file descriptor. (With the current
implementation, this indicates trouble
getting a fildescriptor from a stream. If
it occurs, ask the author for a proper
solution!)
(EDEADLK) A fcntl with function F_SETLKW would
cause a deadlock.
EINTR Function was interrupted by a signal.
(EINVAL) Invalid argument.
(EMFILE Too many file descriptors are in use by
this process.
(ENOLCK) No locks available.

Remarks:
"available" will be typically used with pipes or fifos.  

There are two possible reasons why "available" may return false:
1. There are processes writing to the pipe/fifo, but none
of the is currently writing data to it.
A subsequent read attempt will block until data becomes
available.
2. There are no processes writing to the pipe (any longer).
A subsequent read attempt will yield EOF.
It is NOT possible to tell these possibilities apart! This is
not a fault of the implementation of this function. It is generally
impossible to do this. The only way to know is to start a read
attempt. If it blocks, you know the answer - but you could wait
forever. Anyway, there normally is no need to distinguish between
these alternatives: Just NEVER try a read attempt, if "available"
returned false. Even if temporarily no process was connected to
the stream, it will return true as soon as the connection is re-
established and data is waiting.

"available" just tells you if -one- character may be read safely.
It is left to the programmer to assure that a given amount of
data (e.g. upto the next linefeed) may be read.

Author:
R Kupper  

FirstVersion:
May 10 1999  

SeeAlso: Source:
/home/nest/work/nest-2.14.0/sli/processes.h