On Mon, Mar 17, 2008 at 4:47 PM, Dermot <[EMAIL PROTECTED]> wrote:
snip
> print $fh $status."\n";
> print STDERR "$0: $! $?\n";
snip
$? holds the status returned by the last external call (call to
system() function, pipes, qx// operator, call to wait() or waitpid(),
or possibly the gethost*() functions), but you are not making an
external call in your code, so it is inappropriate to be checking its
value.
$! is not gaurenteed to have a valid value unless an error code has
been returned by the last function called, so unless print has an
error $! could be anything (and often is something weird).
from perldoc perlvar
$? The status returned by the last pipe close, backtick ('')
command, successful call to wait() or waitpid(), or from the
system() operator. This is just the 16−bit status word
returned by the wait() system call (or else is made up to look
like it). Thus, the exit value of the subprocess is really
("$? >> 8"), and "$? & 127" gives which signal, if any, the
process died from, and "$? & 128" reports whether there was a
core dump. (Mnemonic: similar to sh and ksh.)
Additionally, if the "h_errno" variable is supported in C, its
value is returned via $? if any "gethost*()" function fails.
If you have installed a signal handler for "SIGCHLD", the value
of $? will usually be wrong outside that handler.
Inside an "END" subroutine $? contains the value that is going
to be given to "exit()". You can modify $? in an "END"
subroutine to change the exit status of your program. For
example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma "use vmsish 'status'" makes $? reflect
the actual VMS exit status, instead of the default emulation of
POSIX status; see "$?" in perlvms for details.
snip
$! If used numerically, yields the current value of the C "errno"
variable, or in other words, if a system or library call fails,
it sets this variable. This means that the value of $! is
meaningful only immediately after a failure:
if (open(FH, $filename)) {
# Here $! is meaningless.
...
} else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# here $! is meaningless.
In the above meaningless stands for anything: zero, non‐zero,
"undef". A successful system or library call does not set the
variable to zero.
If used as a string, yields the corresponding system error
string. You can assign a number to $! to set errno if, for
instance, you want "$!" to return the string for error n, or
you want to set the exit value for the die() operator.
(Mnemonic: What just went bang?)
--
Chas. Owens
wonkden.net
The most important skill a programmer can have is the ability to read.
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