PERLFAQ5
NAME DESCRIPTION AUTHOR AND COPYRIGHT
perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
This section deals with I/O and the ``f'' issues: filehandles, flushing, formats, and footers.
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 system call for each byte. Any time you use print() or write() in Perl, you go though this buffering. syswrite() circumvents stdio and buffering.
In most stdio implementations, the type of output buffering and the size of the buffer varies according to the type of device. Disk files are block buffered, often with a buffer size of more than 2k. Pipes and sockets are often buffered with a buffer size between 1/2 and 2k. Serial devices (e.g. modems, terminals) are normally line-buffered, and stdio sends the entire line when it gets the newline.
Perl does not support truly unbuffered output (except insofar as you can syswrite(OUT, $char, 1)). What it does instead support is ``command buffering'', in which a physical write is performed after every output command. This isn't as hard on your system as unbuffering, but does get the output where you want it when you want it.
If you expect characters to get to your device when you print them there, you'll want to autoflush its handle. Use select() and the $ variable to control autoflushing (see perlvar/$ and ``select'' in
$ = 1; select($old_fh);
open(DEV,
open(DEV,
$sock = IO::Socket::INET-
$sock-
Note the bizarrely hardcoded carriage return and newline in their octal equivalents. This is the ONLY way (currently) to assure a proper flush on all platforms, including Macintosh. That's the way things work in network programming: you really should specify the exact bit pattern on the network line terminator. In practice, often works, but this is not portable.
See perlfaq9 for other examples of fetching URLs over the web.
How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?
Those are operations of a text editor. Perl is not a text editor. Perl is a programming language. You have to decompose the problem into 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. 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 data at the very end of the file. A sequence of bytes can be replaced with another sequence of the same length. The $DB_RECNO array bindings as documented in 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 no locking.
$old = $file;
$new =
open(OLD,
- Correct typos, preserving case
while (
close(OLD) or die rename($old, $bak) or die
Perl can do this sort of thing for you automatically with the -i command-line switch or the closely-related $^I variable (see perlrun for more details). Note that -i may require a suffix on some non-Unix systems; see the platform-specific documentation that came with your port.
perl -pi -e 's/(^s+tests+)d+/ $1 . ++$count /e' t/op/taint.t
local($^I, @ARGV) = ('.orig', glob( If you need to seek to an arbitrary line of a file that changes infrequently, you could build up an index of byte positions of where the line ends are in the file. If the file is large, an index of every tenth or hundredth line end would allow you to seek and read fairly efficiently. If the file is sorted, try the look.pl library (part of the standard perl distribution).
In the unique case of deleting lines at the end of a file, you can use tell() and truncate(). The following code snippet deletes the last line of a file without making a copy or reading the whole file into
open (FH,
Error checking is left as an exercise for the reader.
How do I count the number of lines in a file?
One fairly efficient way is to count newlines in the file. The following program uses a feature of tr///, as documented in perlop. If your text file doesn't end with a newline, then it's not really a proper text file, so this may report one fewer line than you expect.
$lines = 0;
open(FILE, $filename) or die This assumes no funny games with newline translations.
How do I make a temporary file name?
Use the new_tmpfile class method from the IO::File module to get a filehandle opened for reading and writing. Use it if you don't need to know the file's
$fh = IO::File-
use POSIX qw(tmpnam);
do { $name = tmpnam() } until sysopen(FH, $name, O_RDWRO_CREATO_EXCL);
END { unlink($name) or die
use Fcntl; my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP} $ENV{TEMP}; my $base_name = sprintf(
How can I manipulate fixed-record-length files?
The most efficient way is using pack() and unpack(). This is faster than using substr() when taking many, many strings. It is slower for just a few.
Here is a sample chunk of code to break up and put back together again some fixed-format input lines, in this case
$PS_T = 'A6 A4 A7 A5 A*'; open(PS, We've used $$var in a way that forbidden by use strict 'refs'. That is, we've promoted a string to a scalar variable reference using symbolic references. This is ok in small programs, but doesn't scale well. It also only works on global variables, not lexicals.
How can I make a filehandle local to a subroutine? How do I pass filehandles between subroutines? How do I make an array of filehandles?
The fastest, simplest, and most direct way is to localize
Typeglobs are fast (especially compared with the alternatives) and reasonably easy to use, but they also have one subtle drawback. If you had, for example, a function named !TmpHandle?(), or a variable named %!TmpHandle?, you just hid it from yourself.
sub findme {
local *!HostFile?; open(!HostFile?, 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.
@names = qw(motd termcap passwd hosts);
my $i = 0; foreach $filename (@names) { local *FH; open(FH,
foreach $name (sort { $file{$a}[0? For passing filehandles to functions, the easiest way is to preface them with a star, as in func(*STDIN). See ``Passing Filehandles'' in perlfaq7 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 code with Symbol::gensym,
use Symbol; my $fh = gensym(); open($fh,
my $fh = !FileHandle?- Please understand that whether the filehandle happens to be a (probably 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.
How can I use a filehandle indirectly?
An indirect filehandle is using something other than a symbol in a place that a filehandle is expected. Here are
$fh = Or, you can use the 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.
use !FileHandle?;
$fh = !FileHandle?-
use IO::Handle; # 5.004 or higher
$fh = IO::Handle-
print $ofh
my $fh = shift; print $fh
local *FH = shift; print FH Both styles work with either objects or typeglobs of real filehandles. (They might also work with strings under some circumstances, but this is risky.)
accept_fh(*STDOUT);
accept_fh($handle);
print $fd[1?
This approach of treating print and printf like object methods calls doesn't work for the diamond operator. That's because it's a real operator, not just a function with a comma-less argument. Assuming you've been storing typeglobs in your structure as we did above, you can use the built-in function named readline to reads a record just as does. Given the initialization shown above for @fd, this would work, but only because readline() require a typeglob. It doesn't work with objects or strings, which might be a bug we haven't fixed yet.
$got = readline($fd[0?);
Let it be noted that the flakiness of indirect filehandles is not related to whether they're strings, typeglobs, objects, or anything else. It's the syntax of the fundamental operators. Playing the object game doesn't help you at all here.
How can I set up a footer format to be used with write()?
There's no builtin way to do this, but perlform has a couple of techniques to make it possible for the intrepid hacker.
How can I write() into a string?
See ``Accessing Formatting Internals in perlform for an swrite()'' function.
How can I output my numbers with commas added?
local $_ = shift; 1 while s/^([-+??d+)(d{3})/$1,$2/; return $_; }
$n = 23659019423.2331;
GOT: 23,659,019,423.2331
because you have to put the comma in and then recalculate your position.
Alternatively, this code commifies all numbers in a line regardless of whether they have decimal portions, are
How can I translate tildes () in a filename?
Use the glob()'') operator, documented in 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.
$filename = s{
^ # find a leading tilde ( # save this in $1 [^/? # a non-slash character
) }{ $1 ? (getpwnam($1))[7? : ( $ENV{HOME} $ENV{LOGDIR} ) }ex;
How come when I open a file read-write it wipes it out?
Because you're using something like this, which truncates the file and then gives you read-write
Whoops. You should instead use this, which will fail if the file doesn't exist.
open(FH,
Using ``
Here are examples of many kinds of file opens. Those using sysopen() all assume
use Fcntl;
sysopen(FH, $path, O_WRONLYO_EXCLO_CREAT, 0666) die $!;
sysopen(FH, $path, O_RDWRO_CREAT, 0666) die $!;
sysopen(FH, $path, O_RDWRO_EXCLO_CREAT, 0666) die $!;
Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS . That is, two processes might both successfully create or unlink the same file! Therefore O_EXCL isn't as exclusive as you might wish.
See also the new perlopentut if you have it (new for 5.6).
Why do I sometimes get an ``Argument list too long'' when I use
The operator performs a globbing operation (see above). In Perl versions earlier than v5.6.0, the internal glob() operator forks csh(1) to do the actual glob expansion, but csh can't handle more than 127 items and so gives the error message Argument list too long. People who installed tcsh as csh won't have this problem, but their users may be surprised by it.
To get around this, either upgrade to Perl v5.6.0 or later, do the glob yourself with readdir() and patterns, or use a module like Glob::KGlob, one that doesn't use the shell to do globbing.
Is there a leak/bug in glob()?
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 memory leak and/or unpredictable behavior. It's best therefore to use glob() only in list context.
How can I open a file with a leading ``
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 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
local $_ = shift; s#^([^./?)#./$1#; $_ .=
$badpath =
This assumes that you are using POSIX (portable operating systems interface) paths. If you are on a closed, non-portable, proprietary system, you may have to adjust the above.
It would be a lot clearer to use sysopen(),
$badpath = For more information, see also the new perlopentut if you have it (new for 5.6).
How can I reliably rename a file?
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 are broken. (The last two parts are not surprising, but the first is. :-)
If your operating system supports a proper mv(1)
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 semantically as a real rename(), though, which preserves metainformation like permissions, timestamps, inode info, etc.
Newer versions of File::Copy exports a move() function.
How can I lock a file?
Perl's builtin flock() function (see perlfunc for details) will call flock(2) if that exists, fcntl(2) if it doesn't (on perl version 5.004 and later), and lockf(3) if neither of the two previous system calls exists. On some systems, it may even use a different form of native locking. Here are some gotchas with Perl's flock():
1
Produces a fatal error if none of the three system calls (or their close equivalent) exists.
2
lockf(3) does not provide shared locking, and requires that the filehandle be open for writing (or appending, or read/writing).
3
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 perlfunc and the 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 are 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 for red lights get on well with each other, but not with cars that don't 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. (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 ``File Locking'' in perlopentut if you have it (new for 5.6).
__Why can't I just open( FH , ``
A common bit of code NOT TO USE is
except that lamentably, file creation (and deletion) is not atomic over NFS , so this won't work (at least, not every time) over the net. Various schemes involving link() have been suggested, but these tend to involve busy-wait, which is also subdesirable.
I still don't get locking. I just want to increment the number in the file. How can I do this?
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. 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 qw(:DEFAULT :flock);
sysopen(FH,
If the count doesn't impress your friends, then the code might. :-)
How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases
$recno = 37; # which record to update open(FH, Locking and error checking are left as an exercise for the reader. Don't forget them or you'll be quite sorry.
How do I get a file's timestamp in perl?
If you want to retrieve the time at which the file was last read, written, or had its meta-data (owner, etc) changed, you use the -M, -A, or -C filetest operations as documented in perlfunc. These retrieve the age of the file (measured against the start-time of your program) in days as a floating point number. To retrieve the ``raw time in seconds since the epoch, you would call the stat function, then use localtime(), gmtime(), or POSIX::strftime()'' to convert this into human-readable form.
printf
use File::stat; use Time::localtime; $date_string = ctime(stat($file)- The POSIX::strftime() approach has the benefit of being, in theory, independent of the current locale. See perllocale for details.
How do I set a file's timestamp in perl?
You use the utime() function documented in ``utime'' in perlfunc. By way of example, here's a little program that copies the read and write times from its first argument to all the rest of them.
if (@ARGV
Error checking is, as usual, left as an exercise for the reader.
Note that utime() currently doesn't work correctly with Win95/NT ports. A bug has been reported. Check it carefully before using utime() on those platforms.
How do I print to more than one file at once?
If you only have to do this once, you can do
open (STDOUT,
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 written in Perl and offers much greater functionality than the stock version.
How can I read in an entire file all at once?
The customary Perl approach for processing all the lines in
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 the array actually accesses the corresponding line in the file.
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
$var = `cat $file`;
This tiny but expedient solution is neat, clean, and portable to all systems on which decent tools have been installed. For those who prefer not to use the toolbox, you can of course read the file manually, although this makes for more complicated code.
{
local(*INPUT, $/); open (INPUT, $file) die
How can I read in a file by paragraphs?
Use the $/ variable (see perlvar for details). You can either set it to to eliminate empty paragraphs (, for instance, gets treated as two paragraphs and not three), or
to accept empty
paragraphs.
Note that a blank line must have no blanks in it. Thus
is one paragraph, but is two.
How can I read a single character from a file? From the keyboard?
You can use the builtin 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 ``getc'' in perlfunc.
If your system supports the portable operating system programming interface ( POSIX ), you can use the following code, which you'll note turns off echo processing as well.
use strict; $ = 1; for (1..4) { my $got; print
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin); $fd_stdin = fileno(STDIN); $term = POSIX::Termios- $echo = ECHO ECHOK ICANON;
$noecho = $oterm
sub cbreak {
$term-
sub cooked {
$term-
sub getone {
my $key = ''; cbreak(); sysread(STDIN, $key, 1); cooked(); return $key; }
} END { cooked() }
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?;
open(TTY, For legacy DOS systems, Dan Carson
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
$old_ioctl
This is all trial and error I did a long time ago; I hope I'm reading the file that worked...
How can I tell whether there's a character waiting on a filehandle?
The very first thing you should do is look into getting the Term::!ReadKey? extension from CPAN . As we mentioned earlier, it now even has limited support for non-portable (read: not open systems, closed, proprietary, not POSIX , not Unix, etc) systems.
You should also check out the Frequently Asked Questions list in comp.unix.* for things like this: the answer is essentially the same. It's very system dependent. Here's one solution that works on BSD
my($rin, $nfd); vec($rin, fileno(STDIN), 1) = 1; return $nfd = select($rin,undef,undef,0); }
/usr/include/asm/ioctls.h:#define FIONREAD 0x541B
And then hard-code it, leaving porting as an exercise to your successor.
$FIONREAD = 0x4004667f; # XXX: opsys dependent $size = pack(
FIONREAD requires a filehandle connected to a stream, meaning that sockets, pipes, and tty devices work, but not files.
How do I do a tail -f in perl?
First try
seek(GWFILE, 0, 1);
The statement seek(GWFILE, 0, 1) doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next GWFILE
If that doesn't work (it relies on features of your stdio implementation), then you need something more like
for (;;) {
for ($curpos = tell(GWFILE); If this still doesn't work, look into the POSIX module. POSIX defines the clearerr() method, which can remove the end of file condition on a filehandle. The method: read until end of file, clearerr(), read some more. Lather, rinse, repeat.
There's also a File::Tail module from CPAN .
How do I dup() a filehandle in Perl?
If you check ``open in perlfunc, you'll see that several of the ways to call open()'' should do the trick. For
open(MHCONTEXT, Note that ``
Error checking, as always, has been left as an exercise for the reader.
How do I close a file descriptor by number?
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 to,
$rc = syscall(
local *F; open F,
Why can't I use ``C:tempfoo'' in DOS paths? What doesn't `C:tempfoo.exe` work?
Whoops! You just put a tab and a formfeed into that filename! Remember that within double quoted strings (``likethis), the backslash is an escape character. The full list of these is in ``Quote and Quote-like Operators in perlop. Unsurprisingly, you don't have a file called ``c:(tab)emp(formfeed)oo or ``c:(tab)emp(formfeed)oo.exe on your legacy DOS filesystem.
Either single-quote your strings, or (preferably) use forward slashes. Since all DOS and Windows versions since something like MS-DOS 2.0 or so have treated / and \ the same in a path, you might as well use the one that doesn't clash with Perl--or the POSIX shell, ANSI C and C
mention a few. POSIX paths are more portable, too.
Why doesn't glob(``*.*'') get all the files?
Because even on non-Unix ports, Perl's glob function follows standard Unix globbing semantics. You'll need glob( to get all (non-hidden) files. This makes glob() portable even to legacy systems. Your port may include proprietary globbing functions as well. Check its documentation for details.
Why does Perl let me delete read-only files? Why does
Perl?__
This is elaborately and painstakingly described in the ``Far
More Than You Ever Wanted To Know'' in
http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms
.
The executive summary: learn how your filesystem works. The permissions on a file say what can happen to the data in that file. The permissions on a directory say what can happen to the list of files in that directory. If you delete a file, you're removing its name from the directory (so the operation depends on the permissions of the directory, not of the file). If you try to write to the file, the permissions of the file govern whether you're allowed to.
How do I select a random line from a file?
rand($.) 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 the algorithm's correctness.
Why do I get weird spaces when I print an array of lines?
Saying
joins together the elements of @lines with a space between them. If @lines were ( then the above statement would print
little fluffy clouds
fluffy clouds
Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved.
When included as an integrated part of the Standard Distribution of Perl or of its documentation (printed or otherwise), this works is covered under Perl's Artistic License. For separate distributions of all or part of this FAQ outside of that, see perlfaq.
Irrespective of its distribution, all code examples here are in the public domain. You are permitted and encouraged to use this code and any derivatives thereof in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit to the FAQ would be courteous but is not required.
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