Digital Mages

Digital Mages - Paranoid::IO(3)

Paranoid::IO

Section: User Contributed Perl Documentation (3)
Updated: 2017-02-06
Index  

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 addtional 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)


 

Index

NAME
VERSION
SYNOPSIS
DESCRIPTION
Opportunistic file access
File handle caching
Fork-safe file access
Inherent file locking
O_APPEND access patterns
Intelligent file tracking
SUBROUTINES/METHODS
PIOBLKSIZE
PIOMAXFSIZE
popen
pclose
preopen
pcloseAll
ptell
pseek
pflock
pread
pnlread
pwrite
pappend
ptruncate
DEPENDENCIES
BUGS AND LIMITATIONS
AUTHOR
LICENSE AND COPYRIGHT