=head2 How do I flush/unbuffer an output filehandle? Why must I do this?
The C standard I/O library (stdio) normally buffers characters sent to
-devices. This is done for efficiency reasons, so that there isn't a
+devices. This is done for efficiency reasons so that there isn't a
system call for each byte. Any time you use print() or write() in
Perl, you go though this buffering. syswrite() circumvents stdio and
buffering.
low-level calls to read, write, open, close, and seek.
Although humans have an easy time thinking of a text file as being a
-sequence of lines that operates much like a stack of playing cards -- or
-punch cards -- computers usually see the text file as a sequence of bytes.
+sequence of lines that operates much like a stack of playing cards--or
+punch cards--computers usually see the text file as a sequence of bytes.
In general, there's no direct way for Perl to seek to a particular line
of a file, insert text into a file, or remove text from a file.
-(There are exceptions in special circumstances. You can add or remove at
-the very end of the file. Another is replacing a sequence of bytes with
-another sequence of the same length. Another is using the C<$DB_RECNO>
-array bindings as documented in L<DB_File>. Yet another is manipulating
-files with all lines the same length.)
+(There are exceptions in special circumstances. You can add or remove
+data at the very end of the file. A sequence of bytes can be replaced
+with another sequence of the same length. The C<$DB_RECNO> array
+bindings as documented in L<DB_File> also provide a direct way of
+modifying a file. Files where all lines are the same length are also
+easy to alter.)
The general solution is to create a temporary copy of the text file with
the changes you want, then copy that over the original. This assumes
=head2 How do I make a temporary file name?
Use the C<new_tmpfile> class method from the IO::File module to get a
-filehandle opened for reading and writing. Use this if you don't
-need to know the file's name.
+filehandle opened for reading and writing. Use it if you don't
+need to know the file's name:
use IO::File;
$fh = IO::File->new_tmpfile()
or die "Unable to make new temporary file: $!";
-Or you can use the C<tmpnam> function from the POSIX module to get a
-filename that you then open yourself. Use this if you do need to know
-the file's name.
+If you do need to know the file's name, you can use the C<tmpnam>
+function from the POSIX module to get a filename that you then open
+yourself:
+
use Fcntl;
use POSIX qw(tmpnam);
# now go on to use the file ...
-If you're committed to doing this by hand, use the process ID and/or
-the current time-value. If you need to have many temporary files in
-one process, use a counter:
+If you're committed to creating a temporary file by hand, use the
+process ID and/or the current time-value. If you need to have many
+temporary files in one process, use a counter:
BEGIN {
use Fcntl;
# *HostFile automatically closes/disappears here
}
-Here's how to use this in a loop to open and store a bunch of
+Here's how to use typeglobs in a loop to open and store a bunch of
filehandles. We'll use as values of the hash an ordered
pair to make it easy to sort the hash in insertion order.
}
For passing filehandles to functions, the easiest way is to
-preface them with a star, as in func(*STDIN). See L<perlfaq7/"Passing
-Filehandles"> for details.
+preface them with a star, as in func(*STDIN).
+See L<perlfaq7/"Passing Filehandles"> for details.
If you want to create many anonymous handles, you should check out the
Symbol, FileHandle, or IO::Handle (etc.) modules. Here's the equivalent
$file{$filename} = [ $i++, $fh ];
}
-Or here using the semi-object-oriented FileHandle module, which certainly
+Here's using the semi-object-oriented FileHandle module, which certainly
isn't light-weight:
use FileHandle;
}
Please understand that whether the filehandle happens to be a (probably
-localized) typeglob or an anonymous handle from one of the modules,
+localized) typeglob or an anonymous handle from one of the modules
in no way affects the bizarre rules for managing indirect handles.
See the next question.
An indirect filehandle is using something other than a symbol
in a place that a filehandle is expected. Here are ways
-to get those:
+to get indirect filehandles:
$fh = SOME_FH; # bareword is strict-subs hostile
$fh = "SOME_FH"; # strict-refs hostile; same package only
$fh = \*SOME_FH; # ref to typeglob (bless-able)
$fh = *SOME_FH{IO}; # blessed IO::Handle from *SOME_FH typeglob
-Or to use the C<new> method from the FileHandle or IO modules to
+Or, you can use the C<new> method from the FileHandle or IO modules to
create an anonymous filehandle, store that in a scalar variable,
and use it as though it were a normal filehandle.
accept_fh($handle);
In the examples above, we assigned the filehandle to a scalar variable
-before using it. That is because only simple scalar variables,
-not expressions or subscripts into hashes or arrays, can be used with
-built-ins like C<print>, C<printf>, or the diamond operator. These are
+before using it. That is because only simple scalar variables, not
+expressions or subscripts of hashes or arrays, can be used with
+built-ins like C<print>, C<printf>, or the diamond operator. Using
+something other than a simple scalar varaible as a filehandle is
illegal and won't even compile:
@fd = (*STDIN, *STDOUT, *STDERR);
because you have to put the comma in and then recalculate your
position.
-Alternatively, this commifies all numbers in a line regardless of
+Alternatively, this code commifies all numbers in a line regardless of
whether they have decimal portions, are preceded by + or -, or
whatever:
=head2 How can I translate tildes (~) in a filename?
-Use the <> (glob()) operator, documented in L<perlfunc>. This
-requires that you have a shell installed that groks tildes, meaning
-csh or tcsh or (some versions of) ksh, and thus may have portability
-problems. The Glob::KGlob module (available from CPAN) gives more
-portable glob functionality.
+Use the <> (glob()) operator, documented in L<perlfunc>. Older
+versions of Perl require that you have a shell installed that groks
+tildes. Recent perl versions have this feature built in. The
+Glob::KGlob module (available from CPAN) gives more portable glob
+functionality.
Within Perl, you may use this directly:
Be warned that neither creation nor deletion of files is guaranteed to
be an atomic operation over NFS. That is, two processes might both
-successful create or unlink the same file! Therefore O_EXCL
-isn't so exclusive as you might wish.
+successfully create or unlink the same file! Therefore O_EXCL
+isn't as exclusive as you might wish.
See also the new L<perlopentut> if you have it (new for 5.6).
Due to the current implementation on some operating systems, when you
use the glob() function or its angle-bracket alias in a scalar
-context, you may cause a leak and/or unpredictable behavior. It's
+context, you may cause a memory leak and/or unpredictable behavior. It's
best therefore to use glob() only in list context.
=head2 How can I open a file with a leading ">" or trailing blanks?
Normally perl ignores trailing blanks in filenames, and interprets
certain leading characters (or a trailing "|") to mean something
-special. To avoid this, you might want to use a routine like this.
-It makes incomplete pathnames into explicit relative ones, and tacks a
+special. To avoid this, you might want to use a routine like the one below.
+It turns incomplete pathnames into explicit relative ones, and tacks a
trailing null byte on the name to make perl leave it alone:
sub safe_filename {
use Fcntl;
$badpath = "<<<something really wicked ";
- open (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
+ sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
or die "can't open $badpath: $!";
For more information, see also the new L<perlopentut> if you have it
=head2 How can I reliably rename a file?
-Well, usually you just use Perl's rename() function. But that may not
-work everywhere, in particular, renaming files across file systems.
+Well, usually you just use Perl's rename() function. That may not
+work everywhere, though, particularly when renaming files across file systems.
Some sub-Unix systems have broken ports that corrupt the semantics of
-rename() -- for example, WinNT does this right, but Win95 and Win98
+rename()--for example, WinNT does this right, but Win95 and Win98
are broken. (The last two parts are not surprising, but the first is. :-)
If your operating system supports a proper mv(1) program or its moral
It may be more compelling to use the File::Copy module instead. You
just copy to the new file to the new name (checking return values),
-then delete the old one. This isn't really the same semantics as a
+then delete the old one. This isn't really the same semantically as a
real rename(), though, which preserves metainformation like
permissions, timestamps, inode info, etc.
-The newer version of File::Copy exports a move() function.
+Newer versions of File::Copy exports a move() function.
=head2 How can I lock a file?
Some versions of flock() can't lock files over a network (e.g. on NFS file
systems), so you'd need to force the use of fcntl(2) when you build Perl.
-But even this is dubious at best. See the flock entry of L<perlfunc>,
+But even this is dubious at best. See the flock entry of L<perlfunc>
and the F<INSTALL> file in the source distribution for information on
building Perl to do this.
Two potentially non-obvious but traditional flock semantics are that
-it waits indefinitely until the lock is granted, and that its locks
+it waits indefinitely until the lock is granted, and that its locks are
I<merely advisory>. Such discretionary locks are more flexible, but
offer fewer guarantees. This means that files locked with flock() may
be modified by programs that do not also use flock(). Cars that stop
stop for red lights. See the perlport manpage, your port's specific
documentation, or your system-specific local manpages for details. It's
best to assume traditional behavior if you're writing portable programs.
-(But if you're not, you should as always feel perfectly free to write
+(If you're not, you should as always feel perfectly free to write
for your own system's idiosyncrasies (sometimes called "features").
Slavish adherence to portability concerns shouldn't get in the way of
your getting your job done.)
-For more information on file locking, see also L<perlopentut/"File
-Locking"> if you have it (new for 5.6).
+For more information on file locking, see also
+L<perlopentut/"File Locking"> if you have it (new for 5.6).
=back
Didn't anyone ever tell you web-page hit counters were useless?
They don't count number of hits, they're a waste of time, and they serve
-only to stroke the writer's vanity. Better to pick a random number.
-It's more realistic.
+only to stroke the writer's vanity. It's better to pick a random number;
+they're more realistic.
Anyway, this is what you can do if you can't help yourself.
- use Fcntl ':flock';
+ use Fcntl qw(:DEFAULT :flock);
sysopen(FH, "numfile", O_RDWR|O_CREAT) or die "can't open numfile: $!";
flock(FH, LOCK_EX) or die "can't flock numfile: $!";
$num = <FH> || 0;
seek(FH, 0, 0) or die "can't rewind numfile: $!";
truncate(FH, 0) or die "can't truncate numfile: $!";
(print FH $num+1, "\n") or die "can't write numfile: $!";
- # Perl as of 5.004 automatically flushes before unlocking
- flock(FH, LOCK_UN) or die "can't flock numfile: $!";
close FH or die "can't close numfile: $!";
Here's a much better web-page hit counter:
If the count doesn't impress your friends, then the code might. :-)
+=head2 All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?
+
+If you are on a system that correctly implements flock() and you use the
+example appending code from "perldoc -f flock" everything will be OK
+even if the OS you are on doesn't implement append mode correctly (if
+such a system exists.) So if you are happy to restrict yourself to OSs
+that implement flock() (and that's not really much of a restriction)
+then that is what you should do.
+
+If you know you are only going to use a system that does correctly
+implement appending (i.e. not Win32) then you can omit the seek() from
+the above code.
+
+If you know you are only writing code to run on an OS and filesystem that
+does implement append mode correctly (a local filesystem on a modern
+Unix for example), and you keep the file in block-buffered mode and you
+write less than one buffer-full of output between each manual flushing
+of the buffer then each bufferload is almost garanteed to be written to
+the end of the file in one chunk without getting intermingled with
+anyone else's output. You can also use the syswrite() function which is
+simply a wrapper around your systems write(2) system call.
+
+There is still a small theoretical chance that a signal will interrupt
+the system level write() operation before completion. There is also a
+possibility that some STDIO implementations may call multiple system
+level write()s even if the buffer was empty to start. There may be some
+systems where this probability is reduced to zero.
+
=head2 How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases something as
close FH;
Locking and error checking are left as an exercise for the reader.
-Don't forget them, or you'll be quite sorry.
+Don't forget them or you'll be quite sorry.
=head2 How do I get a file's timestamp in perl?
Note that utime() currently doesn't work correctly with Win95/NT
ports. A bug has been reported. Check it carefully before using
-it on those platforms.
+utime() on those platforms.
=head2 How do I print to more than one file at once?
close(STDOUT) or die "Closing: $!\n";
Otherwise you'll have to write your own multiplexing print
-function -- or your own tee program -- or use Tom Christiansen's,
-at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz, which is
+function--or your own tee program--or use Tom Christiansen's,
+at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz , which is
written in Perl and offers much greater functionality
than the stock version.
This is tremendously more efficient than reading the entire file into
memory as an array of lines and then processing it one element at a time,
-which is often -- if not almost always -- the wrong approach. Whenever
+which is often--if not almost always--the wrong approach. Whenever
you see someone do this:
@lines = <INPUT>;
-You should think long and hard about why you need everything loaded
+you should think long and hard about why you need everything loaded
at once. It's just not a scalable solution. You might also find it
more fun to use the standard DB_File module's $DB_RECNO bindings,
which allow you to tie an array to a file so that accessing an element
On very rare occasion, you may have an algorithm that demands that
the entire file be in memory at once as one scalar. The simplest solution
-to that is:
+to that is
$var = `cat $file`;
You can use the builtin C<getc()> function for most filehandles, but
it won't (easily) work on a terminal device. For STDIN, either use
-the Term::ReadKey module from CPAN, or use the sample code in
+the Term::ReadKey module from CPAN or use the sample code in
L<perlfunc/getc>.
If your system supports the portable operating system programming
END { cooked() }
-The Term::ReadKey module from CPAN may be easier to use. Recent version
+The Term::ReadKey module from CPAN may be easier to use. Recent versions
include also support for non-portable systems as well.
use Term::ReadKey;
printf "\nYou said %s, char number %03d\n",
$key, ord $key;
-For legacy DOS systems, Dan Carson <dbc@tc.fluke.COM> reports the following:
-
-To put the PC in "raw" mode, use ioctl with some magic numbers gleaned
-from msdos.c (Perl source file) and Ralf Brown's interrupt list (comes
-across the net every so often):
-
- $old_ioctl = ioctl(STDIN,0,0); # Gets device info
- $old_ioctl &= 0xff;
- ioctl(STDIN,1,$old_ioctl | 32); # Writes it back, setting bit 5
-
-Then to read a single character:
-
- sysread(STDIN,$c,1); # Read a single character
-
-And to put the PC back to "cooked" mode:
-
- ioctl(STDIN,1,$old_ioctl); # Sets it back to cooked mode.
-
-So now you have $c. If C<ord($c) == 0>, you have a two byte code, which
-means you hit a special key. Read another byte with C<sysread(STDIN,$c,1)>,
-and that value tells you what combination it was according to this
-table:
-
- # PC 2-byte keycodes = ^@ + the following:
-
- # HEX KEYS
- # --- ----
- # 0F SHF TAB
- # 10-19 ALT QWERTYUIOP
- # 1E-26 ALT ASDFGHJKL
- # 2C-32 ALT ZXCVBNM
- # 3B-44 F1-F10
- # 47-49 HOME,UP,PgUp
- # 4B LEFT
- # 4D RIGHT
- # 4F-53 END,DOWN,PgDn,Ins,Del
- # 54-5D SHF F1-F10
- # 5E-67 CTR F1-F10
- # 68-71 ALT F1-F10
- # 73-77 CTR LEFT,RIGHT,END,PgDn,HOME
- # 78-83 ALT 1234567890-=
- # 84 CTR PgUp
-
-This is all trial and error I did a long time ago, I hope I'm reading the
-file that worked.
-
=head2 How can I tell whether there's a character waiting on a filehandle?
The very first thing you should do is look into getting the Term::ReadKey
ioctl(FH, $FIONREAD, $size) or die "Couldn't call ioctl: $!\n";
$size = unpack("L", $size);
-FIONREAD requires a filehandle connected to a stream, meaning sockets,
+FIONREAD requires a filehandle connected to a stream, meaning that sockets,
pipes, and tty devices work, but I<not> files.
=head2 How do I do a C<tail -f> in perl?
This should rarely be necessary, as the Perl close() function is to be
used for things that Perl opened itself, even if it was a dup of a
-numeric descriptor, as with MHCONTEXT above. But if you really have
+numeric descriptor as with MHCONTEXT above. But if you really have
to, you may be able to do this:
require 'sys/syscall.ph';
$rc = syscall(&SYS_close, $fd + 0); # must force numeric
die "can't sysclose $fd: $!" unless $rc == -1;
-Or just use the fdopen(3S) feature of open():
+Or, just use the fdopen(3S) feature of open():
{
local *F;
Either single-quote your strings, or (preferably) use forward slashes.
Since all DOS and Windows versions since something like MS-DOS 2.0 or so
have treated C</> and C<\> the same in a path, you might as well use the
-one that doesn't clash with Perl -- or the POSIX shell, ANSI C and C++,
+one that doesn't clash with Perl--or the POSIX shell, ANSI C and C++,
awk, Tcl, Java, or Python, just to mention a few. POSIX paths
are more portable, too.
This has a significant advantage in space over reading the whole
file in. A simple proof by induction is available upon
-request if you doubt its correctness.
+request if you doubt the algorithm's correctness.
=head2 Why do I get weird spaces when I print an array of lines?
joins together the elements of C<@lines> with a space between them.
If C<@lines> were C<("little", "fluffy", "clouds")> then the above
-statement would print:
+statement would print
little fluffy clouds
projects / p5sagit/p5-mst-13.2.git / blobdiff