Provided by: libparanoid-perl_2.05-2_all 
NAME
Paranoid::IO - Paranoid IO support
VERSION
$Id: lib/Paranoid/IO.pm, 2.05 2017/02/06 01:48:57 acorliss Exp $
SYNOPSIS
use Fcntl qw(:DEFAULT :flock :mode :seek);
use Paranoid::IO;
# Implicit open
$chars = pread("./foo.log", $in, 2048);
# Implcit write/append
$chars = pwrite("./bar.log", $out);
$chars = pappend("./bar.log", $out);
# Adjust block read size
PIOBLKSIZE = 8192;
# Adjust max file size for file scans
PIOMAXFSIZE = 65536;
# Explicit open
$fh = popen($filename, O_RDWR | O_CREAT | O_TRUNC, 0600);
$rv = pseek($filename, 0, SEEK_END);
if ($rv > 0) {
pseek($filename, 0, SEEK_SET) && ptruncate($filename);
}
$rv = pwrite($fileanme, $text) && pclose($filename);
$rv = pclose($filename);
DESCRIPTION
Paranoid::IO is intended to make basic file I/O access easier while keeping with the
tenets of paranoid programming. Most of these calls are essentially wrappers for the
basic system calls (exposed as sysopen, syswrite, etc.) with some additional logic to
reduce the need to explicitly code every step of normal safe access rules, such as file
locking. In the most basic of usage patterns, even explicitly opening files isn't
necessary.
For the most part the system calls that are wrapped here act identically as the underlying
calls, both in the arguments they take and the values they return. The one notable
difference, however, is the popen function itself. A glob variable isn't passed for
assignation since this module stores those references internally along with some meta
data, so popen returns file handles directly.
That semantic, however, is what gives the rest of the functions the flexibility of
accepting either a file name or a file handle to work on. In the case of file names some
of these functions can open files automatically, and the rest of the features are granted
automatically.
In the case of passing file handles the full feature set of this module is only available
if the file handle was originally opened with popen. The calls will still work even if it
wasn't, but some of the safety features, like being fork-safe, won't have the meta data to
work properly.
The features provided by this module are:
• Opportunistic file access
• File handle caching
• Fork-safe file access
• Inherent file locking
• O_APPEND access patterns where needed even for files not opened with O_APPEND
• Intelligent file tracking
The following sections will explain each feature in more depth:
Opportunistic file access
Opportunistic file access is defined here as not needing the explicit I/O handling for
what can be implied. For instance, to read content from a file one can simply use the
pread function without having to open and apply a shared file lock. In a similar manner
one should be able to write or append to a file. Files are automatically opened (with the
file mode being intuited by the type of call) as needed. Only where more complicated
access patterns (such as read/write file handles) should an explicit popen call be needed.
Opportunism is limited to where it makes sense, however. Files are not opportunistically
opened if the first I/O call is to pseek, ptell, or pflock. The intent of the file I/O
(in regards to read/write file modes) is impossible to tell within those calls.
File handle caching
This module provides a replacement for Perl's internal sysopen, which should be used even
where read/write file access is necessary. One key benefit for doing so is that it
provides internal file handle caching based on the file name. All the additional
functions provided by this module use it internally to retrieve that cached file handle to
avoid the overhead of repetitive opening and closing of files.
Fork-safe file access
A greater benefit of popen, however, is in it's fork-safe behavior. Every call checks to
see if the file handle it has was inherited from its parent, and if so, transparently
closed and reopened so I/O can continue without both processes conflicting over cursor
positions and buffers. After files are reopened read cursors are placed at the same
location they were prior to the first I/O access in the child.
File modes are preserved without the obvious conflicts of intent as well. Files opened in
the parent with O_TRUNC are reopened without that flag to prevent content from being
clobbered.
Inherent file locking
Except where explicitly ignored (like for pnlread) all read, write, and append operations
use locking internally, alleviating the need for the developer to do so explicitly. Locks
are applied and removed as quickly as possible to facilitate concurrent access.
O_APPEND access patterns
pappend allows you to mimic O_APPEND access patterns even for files that weren't
explicitly opened with O_APPEND. If you open a file with O_RDWR you can still call
pappend and the content will be appended to the end of the file, without moving the file's
cursor position for regular reads and writes.
Intelligent file tracking
popen caches file handles by file name. If files are opened with relative paths this has
the potential to cause some confusion if the process or children are changing their
working directories. In anticipation of this popen also tracks the real path (as resolved
by the realpath system call) and file name. This way you can still access the same file
regardless of the process or its children's movements on the file system.
This could be, however, a double-edged sword if your program intends to open identically
named files in multiple locations. If that is your intent you would be cautioned to avoid
using relative paths with popen.
SUBROUTINES/METHODS
PIOBLKSIZE
PIOBLKSIZE = 65536;
This lvalue function is not exported by default. It is used to determine the default
block size to read when a size is not explicitly passed. Depending on hardware and file
system parameters there might be performance gains to be had when doing default-sized
reads. The default is 4096, which is generally a safe size for most applications.
PIOMAXFSIZE
PIOMAXFSIZE = 65536;
This lvalue function is not exported by default. It is used to determine the maximum file
size that will be read. This is not used in this module, but provided for use in
dependent modules that may want to impose file size limits, such as Paranoid::IO::Line and
others.
popen
$fh = popen($filename);
$fh = popen($filename, $mode);
$fh = popen($filename, $mode, $perms);
$fh = popen($fh);
Returns a file handle if the file could be opened. If the mode is omitted the default is
O_CREAT | O_RDWR. File permissions (for newly created files) default to 0666 ^ UMASK.
Failures to open a file will result in an undef return value, with a text description of
the fault stored in Paranoid::ERROR.
If a file handle is passed to popen it will attempt to match it to a tracked file handle
and, if identified, take the appropriate action. If it doesn't match any tracked file
handles it will just return that file handle back to the caller.
pclose
$rv = pclose($filename);
$rv = pclose($fh);
Returns the value from close. Attempts to close a file that's already closed is
considered a success, and true value is returned. Handing it a stale file handle,
however, will be handed to the internal close, with all the expected results.
preopen
$rv = preopen();
$rv = preopen(@filenames);
$rv = preopen(@filehandles);
This checks each tracked file handle (i.e., file handles that were opened by popen) and
reopens them if necessary. This is typically only useful after a fork. It is also not
striclty necessary since every call to a function in this module does that with every
invocation, but if you have several file handles that you may not access immediately you
run the risk of the parent moving the current file position before the child gets back to
those files. You may or may not care. If you do, use this function immediately after a
fork.
Called with a list of file names means that only those files are examined and reopened.
Any failure to reopen any single file handle will result in a false return value. That
said, any failures will not interrupt the function from trying every file in the list.
pcloseAll
$rv = pcloseAll();
$rv = pcloseAll(@filenames);
$rv = pcloseAll(@filehandles);
This function returns a boolean value denoting any errors while trying to close every
tracked file handle. This function is also not strictly necessary for all the normal Perl
I/O reasons, but it's here for those that want to be explicit.
ptell
$pos = ptell($filename);
$pos = ptell($fh);
Returns the current position of the file cursor. Returns the results of sysseek, which
means that any successful seek is true, even if the cursor is at the beginning of the
file. In that instance it returns "0 but true" which is boolean true while converting to
an integer appropriately.
Any failures are returned as false or undef.
pseek
$rv = pseek($filename, $pos, $whence);
$rv = pseek($fh, $pos, $whence);
This returns the return value from sysseek. The appropriate whence values sould be one of
the SEEK_* constants as exported by Fcntl.
pflock
$rv = pflock($filename, $locktype);
$rv = pflock($fh, $locktype);
This returns the return value from flock. The appropriate lock type values should be one
of the LOCK_* constants as exported by Fcntl.
NOTE: If you wish to control file locking yourself you can do so with this function. Any
existing locks explicitly applied are tracked and will cause read/write functions to
refrain from attempting to apply (and remove) locks automatically.
pread
$bytes = pread($filename, $text, $length);
$bytes = pread($filename, $text, $length, $offset);
$bytes = pread($fh, $text, $length);
$bytes = pread($fh, $text, $length, $offset);
This returns the number of bytes read, or undef on errors. If this is called prior to an
explicit popen it will default to a mode of O_RDONLY. Length defaults to PIOBLKSIZE.
pnlread
$bytes = pnlread($filename, $text, $length);
$bytes = pnlread($filename, $text, $length, $offset);
$bytes = pnlread($fh, $text, $length);
$bytes = pnlread($fh, $text, $length, $offset);
File locks are inherent on all reads and writes. There are plenty of legitimate scenarios
where a read needs to be done ignoring any file locks. It is for those situations that
this function exists. It acts identically in every way to pread with the lone exception
that it does not perform file locking.
pwrite
$bytes = pwrite($filename, $text);
$bytes = pwrite($filename, $text, $length);
$bytes = pwrite($filename, $text, $length, $offset);
$bytes = pwrite($fh, $text);
$bytes = pwrite($fj, $text, $length);
$bytes = pwrite($fh, $text, $length, $offset);
This returns the number of bytes written, or undef for any critical failures. If this is
called prior to an explicit popen it uses a default mode of O_WRONLY | O_CREAT | O_TRUNC.
pappend
$bytes = pappend($filename, $text);
$bytes = pappend($filename, $text, $length);
$bytes = pappend($filename, $text, $length, $offset);
$bytes = pappend($fh, $text);
$bytes = pappend($fh, $text, $length);
$bytes = pappend($fh, $text, $length, $offset);
This behaves identically to pwrite with the sole exception that this preserves the file
position after explicitly seeking and writing to the end of the file. The default mode
here, however, would be O_WRONLY | O_CREAT | O_APPEND for those files not explicitly
opened.
ptruncate
$rv = ptruncate($filename);
$rv = ptruncate($filename, $pos);
$rv = ptruncate($fh);
$rv = ptruncate($fh, $pos);
This returns the result of the internal truncate call. If called without an explicit
popen it will open the named file with the default mode of O_RDWR | O_CREAT. Omitting the
position to truncate from will result in the file being truncated at the beginning of the
file.
DEPENDENCIES
o Cwd
o Fcntl
o IO::Handle
o Paranoid
o Paranoid::Debug
o Paranoid::Input
BUGS AND LIMITATIONS
It may not always be benficial to cache file handles. You must explicitly pclose file
handles to avoid that. That said, with straight Perl you'd have to either explicitly
close the file handles or use lexical scoping, anyway. From that perspective I don't find
it onerous to do so, especially with all of the other code-saving features this module
provides.
AUTHOR
Arthur Corliss (corliss@digitalmages.com)
LICENSE AND COPYRIGHT
This software is licensed under the same terms as Perl, itself. Please see
http://dev.perl.org/licenses/ for more information.
(c) 2005 - 2017, Arthur Corliss (corliss@digitalmages.com)