perlfaq5 - Files and Formats ($Revision: 1.8 $, $Date:
2004/08/09 18:10:15 $)
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?
Perl does not support truly unbuffered output (except
insofar as you can "syswrite(OUT, $char, 1)"), although it
does support is "command buffering", in which a physical
write is performed after every output command.
The C standard I/O library (stdio) normally buffers characters
sent to devices so that there isn't a system call
for each byte. In most stdio implementations, the type of
output buffering and the size of the buffer varies according
to the type of device. Perl's print() and write()
functions normally buffer output, while syswrite()
bypasses buffering all together.
If you want your output to be sent immediately when you
execute print() or write() (for instance, for some network
protocols), you must set the handle's autoflush flag. This
flag is the Perl variable $| and when it is set to a true
value, Perl will flush the handle's buffer after each
print() or write(). Setting $| affects buffering only for
the currently selected default file handle. You choose
this handle with the one argument select() call (see "$|"
in perlvar and "select" in perlfunc).
Use select() to choose the desired handle, then set its
per-filehandle variables.
$old_fh = select(OUTPUT_HANDLE);
$| = 1;
select($old_fh);
Some idioms can handle this in a single statement:
select((select(OUTPUT_HANDLE), $| = 1)[0]);
$| = 1, select $_ for select OUTPUT_HANDLE;
Some modules offer object-oriented access to handles and
their variables, although they may be overkill if this is
the only thing you do with them. You can use IO::Handle:
use IO::Handle;
open(DEV, ">/dev/printer"); # but is this?
DEV->autoflush(1);
or IO::Socket:
use IO::Socket; # this one is kinda a
pipe?
my $sock = IO::Socket::INET->new( 'www.example.com:80' ) ;
$sock->autoflush();
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?
Use the Tie::File module, which is included in the standard
distribution since Perl 5.8.0.
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, $filename) or die "Can't open `$filename':
$!";
while (sysread FILE, $buffer, 4096) {
$lines += ($buffer =~ tr/0/);
}
close FILE;
This assumes po funny games with newline translations.
)
How can I useePerl's "-i" option from within a program?
a
"-i" sets thervalue of Perl's $^I variable, which in turn
affects the blhavior of "<>"; see perlrun for more
details. By iodifying the appropriate variables directly,
you can get t;e same behavior within a larger program.
For example: #
C
# ... o
{ r
localr$^I, @ARGV) = ('.orig', glob("*.c"));
whilee(<>) {
ifc($. == 1) {
t print "This line should appear at the top
of each file0; t
} y
s/p
print;
close ARGV if eof; # Reset $.
} ,
} p
# $^I and @ARGV return to their old values here
e
This block modifies all the ".c" files in the current
e
r
v
i
n
g
c
a
s
e
directory, leaving a backup of the original data from each
file in a new ".c.orig" file.
How do I make a temporary file name?
Use the File::Temp module, see File::Temp for more information.
use File::Temp qw/ tempfile tempdir /;
$dir = tempdir( CLEANUP => 1 );
($fh, $filename) = tempfile( DIR => $dir );
# or if you don't need to know the filename
$fh = tempfile( DIR => $dir );
The File::Temp has been a standard module since Perl
5.6.1. If you don't have a modern enough Perl installed,
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 name:
use IO::File;
$fh = IO::File->new_tmpfile()
or die "Unable to make new temporary file: $!";
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;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMPDIR}
|| $ENV{TEMP};
my $base_name = sprintf("%s/%d-%d-0000",
$temp_dir, $$, time());
sub temp_file {
local *FH;
my $count = 0;
until (defined(fileno(FH)) || $count++ > 100)
{
$base_name =~ s/-(+)$/"-" . (1 + $1)/e;
# O_EXCL is required for security reasons.
sysopen(FH, $base_name, O_WRONLY|O_EXCL|O_CREAT);
}
if (defined(fileno(FH))
return (*FH, $base_name);
} else {
return ();
}
}
}
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
from the output of a normal, Berkeley-style ps:
# sample input line:
# 15158 p5 T 0:00 perl
/home/tchrist/scripts/now-what
my $PS_T = 'A6 A4 A7 A5 A*';
open my $ps, '-|', 'ps';
print scalar <$ps>;
my @fields = qw( pid tt stat time command );
while (<$ps>) {
my %process;
@process{@fields} = unpack($PS_T, $_);
for my $field ( @fields ) {
print "$field: <$process{$field}>0;
}
print 'line=', pack($PS_T, @process{@fields} ),
"0;
}
We've used a hash slice in order to easily handle the
fields of each row. Storing the keys in an array means
it's easy to operate on them as a group or loop over them
with for. It also avoids polluting the program with global
variables and using symbolic references.
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?
As of perl5.6, open() autovivifies file and directory handles
as references if you pass it an uninitialized scalar
variable. You can then pass these references just like
any other scalar, and use them in the place of named handles.
open my $fh, $file_name;
open local $fh, $file_name;
print $fh "Hello World!0;
process_file( $fh );
Before perl5.6, you had to deal with various typeglob
idioms which you may see in older code.
open FILE, "> $filename";
process_typeglob( *FILE );
process_reference( ILE );
sub process_typeglob { local *FH = shift; print
FH "Typeglob!" }
sub process_reference { local $fh = shift; print
$fh "Reference!" }
If you want to create many anonymous handles, you should
check out the Symbol or IO::Handle modules.
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
ways to get indirect filehandles:
$fh = SOME_FH; # bareword is strict-subs hostile
$fh = "SOME_FH"; # strict-refs hostile; same
package only
$fh = *SOME_FH; # typeglob
$fh = OME_FH; # ref to typeglob (bless-able)
$fh = *SOME_FH{IO}; # blessed IO::Handle from
*SOME_FH typeglob
Or, you can use the "new" method from one of the IO::*
modules to create an anonymous filehandle, store that in a
scalar variable, and use it as though it were a normal
filehandle.
use IO::Handle; # 5.004 or higher
$fh = IO::Handle->new();
Then use any of those as you would a normal filehandle.
Anywhere that Perl is expecting a filehandle, an indirect
filehandle may be used instead. An indirect filehandle is
just a scalar variable that contains a filehandle. Functions
like "print", "open", "seek", or the "<FH>" diamond
operator will accept either a named filehandle or a scalar
variable containing one:
($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh "Type it: ";
$got = <$ifh>
print $efh "What was that: $got";
If you're passing a filehandle to a function, you can
write the function in two ways:
sub accept_fh {
my $fh = shift;
print $fh "Sending to indirect filehandle0;
}
Or it can localize a typeglob and use the filehandle
directly:
sub accept_fh {
local *FH = shift;
print FH "Sending to localized filehandle0;
}
Both styles work with either objects or typeglobs of real
filehandles. (They might also work with strings under
some circumstances, but this is risky.)
accept_fh(*STDOUT);
accept_fh($handle);
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 of
hashes or arrays, can be used with built-ins like "print",
"printf", or the diamond operator. Using something other
than a simple scalar variable as a filehandle is illegal
and won't even compile:
@fd = (*STDIN, *STDOUT, *STDERR);
print $fd[1] "Type it: "; #
WRONG
$got = <$fd[0]> #
WRONG
print $fd[2] "What was that: $got"; #
WRONG
With "print" and "printf", you get around this by using a
block and an expression where you would place the filehandle:
print { $fd[1] } "funny stuff0;
printf { $fd[1] } "Pity the poor %x.0, 3_735_928_559;
# Pity the poor deadbeef.
That block is a proper block like any other, so you can
put more complicated code there. This sends the message
out to one of two places:
$ok = -x "/bin/cat";
print { $ok ? $fd[1] : $fd[2] } "cat stat $ok0;
print { $fd[ 1+ ($ok || 0) ] } "cat stat $ok0;
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 read a record
just as "<>" does. Given the initialization shown above
for @fd, this would work, but only because readline()
requires 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?
This subroutine will add commas to your number:
sub commify {
local $_ = shift;
1 while s/^([-+]?+)({3})/$1,$2/;
return $_;
}
This regex from Benjamin Goldberg will add commas to numbers:
s/(^[-+]?+?(?=(?>(?:{3})+)(?!))|G{3}(?=))/$1,/g;
It is easier to see with comments:
s/(
^[-+]? # beginning of number.
{1,3}? # first digits before first comma
(?= # followed by, (but not included
in the match) :
(?>(?:{3})+) # some positive multiple of three
digits.
(?!) # an *exact* multiple, not x * 3 +
1 or whatever.
)
| # or:
G{3} # after the last group, get three
digits
(?=) # but they have to have more digits
after them.
)/$1,/xg;
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 File::KGlob module (available
from CPAN) gives more portable glob functionality.
Within Perl, you may use this directly:
$filename =~ s{
^ ~ # find a leading tilde
( # save this in $1
[^/] # a non-slash character
* # repeated 0 or more times (0
means me)
)
}{
$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 access:
open(FH, "+> /path/name"); # WRONG (almost
always)
Whoops. You should instead use this, which will fail if
the file doesn't exist.
open(FH, "+< /path/name"); # open for update
Using ">" always clobbers or creates. Using "<" never
does either. The "+" doesn't change this.
Here are examples of many kinds of file opens. Those
using sysopen() all assume
use Fcntl;
To open file for reading:
open(FH, "< $path") ||
die $!;
sysopen(FH, $path, O_RDONLY) ||
die $!;
To open file for writing, create new file if needed or
else truncate old file:
open(FH, "> $path") || die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT) ||
die $!;
sysopen(FH, $path, O_WRONLY|O_TRUNC|O_CREAT, 0666) ||
die $!;
To open file for writing, create new file, file must not
exist:
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT) ||
die $!;
sysopen(FH, $path, O_WRONLY|O_EXCL|O_CREAT, 0666) ||
die $!;
To open file for appending, create if necessary:
open(FH, ">> $path") || die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT) ||
die $!;
sysopen(FH, $path, O_WRONLY|O_APPEND|O_CREAT, 0666) ||
die $!;
To open file for appending, file must exist:
sysopen(FH, $path, O_WRONLY|O_APPEND) ||
die $!;
To open file for update, file must exist:
open(FH, "+< $path") ||
die $!;
sysopen(FH, $path, O_RDWR) ||
die $!;
To open file for update, create file if necessary:
sysopen(FH, $path, O_RDWR|O_CREAT) ||
die $!;
sysopen(FH, $path, O_RDWR|O_CREAT, 0666) ||
die $!;
To open file for update, file must not exist:
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT) ||
die $!;
sysopen(FH, $path, O_RDWR|O_EXCL|O_CREAT, 0666) ||
die $!;
To open a file without blocking, creating if necessary:
sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT)
or die "can't open /foo/somefile: $!":
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 File::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 anglebracket
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 ">" or trailing
blanks?
Normally perl ignores trailing blanks in filenames, and
interprets certain leading characters (or a trailing "|")
to mean something special.
The three argument form of open() lets you specify the
mode separately from the filename. The open() function
treats special mode characters and whitespace in the filename
as literals
open FILE, "<", " file "; # filename is "
file "
open FILE, ">", ">file"; # filename is ">file"
It may be a lot clearer to use sysopen(), though:
use Fcntl;
$badpath = "<<<something really wicked ";
sysopen (FH, $badpath, O_WRONLY | O_CREAT | O_TRUNC)
or die "can't open $badpath: $!";
How can I reliably rename a file?
If your operating system supports a proper mv(1) utility
or its functional equivalent, this works:
rename($old, $new) or system("mv", $old, $new);
It may be more portable 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 rename(), which
preserves meta-information like permissions, timestamps,
inode info, etc.
Newer versions of File::Copy export 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 advi-
sory. 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, ">file.lock")?
A common bit of code NOT TO USE is this:
sleep(3) while -e "file.lock"; # PLEASE DO NOT
USE
open(LCK, "> file.lock"); # THIS BROKEN CODE
This is a classic race condition: you take two steps to do
something which must be done in one. That's why computer
hardware provides an atomic test-and-set instruction. In
theory, this "ought" to work:
sysopen(FH, "file.lock", O_WRONLY|O_EXCL|O_CREAT)
or die "can't open file.lock: $!";
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, "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, "0) or die
"can't write numfile: $!";
close FH or die
"can't close numfile: $!";
Here's a much better web-page hit counter:
$hits = int( (time() - 850_000_000) / rand(1_000) );
If the count doesn't impress your friends, then the code
might. :-)
All I want to do is append a small amount of text to the [Toc] [Back]
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 guaranteed
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.
How do I randomly update a binary file?
If you're just trying to patch a binary, in many cases
something as simple as this works:
perl -i -pe 's{window manager}{window mangler}g'
/usr/bin/emacs
However, if you have fixed sized records, then you might
do something more like this:
$RECSIZE = 220; # size of record, in bytes
$recno = 37; # which record to update
open(FH, "+<somewhere") || die "can't update somewhere: $!";
seek(FH, $recno * $RECSIZE, 0);
read(FH, $record, $RECSIZE) == $RECSIZE || die "can't
read record $recno: $!";
# munge the record
seek(FH, -$RECSIZE, 1);
print FH $record;
close 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 file test 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. Some platforms may not
have all of these times. See perlport for details. 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 humanreadable
form.
Here's an example:
$write_secs = (stat($file))[9];
printf "file %s updated at %s0, $file,
scalar localtime($write_secs);
If you prefer something more legible, use the File::stat
module (part of the standard distribution in version 5.004
and later):
# error checking left as an exercise for reader.
use File::stat;
use Time::localtime;
$date_string = ctime(stat($file)->mtime);
print "file $file updated at $date_string0;
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 < 2) {
die "usage: cptimes timestamp_file other_files
...0;
}
$timestamp = shift;
($atime, $mtime) = (stat($timestamp))[8,9];
utime $atime, $mtime, @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?
To connect one filehandle to several output filehandles,
you can use the IO::Tee or Tie::FileHandle::Multiplex modules.
If you only have to do this once, you can print individually
to each filehandle.
for $fh (FH1, FH2, FH3) { print $fh "whatever0 }
How can I read in an entire file all at once?
You can use the File::Slurp module to do it in one step.
use File::Slurp;
$all_of_it = read_file($filename); # entire file
in scalar
@all_lines = read_file($filename); # one line perl element
The customary Perl approach for processing all the lines
in a file is to do so one line at a time:
open (INPUT, $file) || die "can't open $file:
$!";
while (<INPUT>) {
chomp;
# do something with $_
}
close(INPUT) || die "can't close $file:
$!";
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 you see someone
do this:
@lines = <INPUT>;
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
Tie::File module, or the 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.
You can read the entire filehandle contents into a scalar.
{
local(*INPUT, $/);
open (INPUT, $file) || die "can't open $file:
$!";
$var = <INPUT>;
}
That temporarily undefs your record separator, and will
automatically close the file at block exit. If the file
is already open, just use this:
$var = do { local $/; <INPUT> };
For ordinary files you can also use the read function.
read( INPUT, $var, -s INPUT );
The third argument tests the byte size of the data on the
INPUT filehandle and reads that many bytes into the buffer
$var.
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
("abc0ef", for instance, gets treated as two paragraphs
and not three), or "0 to accept empty paragraphs.
Note that a blank line must have no blanks in it. Thus
"fred0tuff0 is one paragraph, but
"fred0tuff0 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.
#!/usr/bin/perl -w
use strict;
$| = 1;
for (1..4) {
my $got;
print "gimme: ";
$got = getone();
print "--> $got0;
}
exit;
BEGIN {
use POSIX qw(:termios_h);
my ($term, $oterm, $echo, $noecho, $fd_stdin);
$fd_stdin = fileno(STDIN);
$term = POSIX::Termios->new();
$term->getattr($fd_stdin);
$oterm = $term->getlflag();
$echo = ECHO | ECHOK | ICANON;
$noecho = $oterm & ~$echo;
sub cbreak {
$term->setlflag($noecho);
$term->setcc(VTIME, 1);
$term->setattr($fd_stdin, TCSANOW);
}
sub cooked {
$term->setlflag($oterm);
$term->setcc(VTIME, 0);
$term->setattr($fd_stdin, TCSANOW);
}
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, "</dev/tty");
print "Gimme a char: ";
ReadMode "raw";
$key = ReadKey 0, *TTY;
ReadMode "normal";
printf "0ou said %s, char number %03d0,
$key, ord $key;
How can I tell whether there's a character waiting on a
filehandle?
The very first thing you should do is look into getting
the Term::ReadKey extension from CPAN. As we mentioned
earlier, it now even has limited support for non-portable
(read: not open systems, closed, proprietary, not POSIX,
not Unix, etc) systems.
You should also check out the Frequently Asked Questions
list in comp.unix.* for things like this: the answer is
essentially the same. It's very system dependent. Here's
one solution that works on BSD systems:
sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
If you want to find out how many characters are waiting,
there's also the FIONREAD ioctl call to be looked at. The
h2ph tool that comes with Perl tries to convert C include
files to Perl code, which can be "require"d. FIONREAD
ends up defined as a function in the sys/ioctl.ph file:
require 'sys/ioctl.ph';
$size = pack("L", 0);
ioctl(FH, FIONREAD(), $size) or die "Couldn't call
ioctl: $!0;
$size = unpack("L", $size);
If h2ph wasn't installed or doesn't work for you, you can
grep the include files by hand:
% grep FIONREAD /usr/include/*/*
/usr/include/asm/ioctls.h:#define FIONREAD 0x541B
Or write a small C program using the editor of champions:
% cat > fionread.c
#include <sys/ioctl.h>
main() {
printf("%#08x0, FIONREAD);
}
^D
% cc -o fionread fionread.c
% ./fionread
0x4004667f
And then hard code it, leaving porting as an exercise to
your successor.
$FIONREAD = 0x4004667f; # XXX: opsys dependent
$size = pack("L", 0);
ioctl(FH, $FIONREAD, $size) or die "Couldn't call
ioctl: $!0;
$size = unpack("L", $size);
FIONREAD requires a filehandle connected to a stream,
meaning that sockets, pipes, and tty devices work, but not
files.
How do I do a "tail -f" in perl?
First try
seek(GWFILE, 0, 1);
The statement "seek(GWFILE, 0, 1)" doesn't change the current
position, but it does clear the end-of-file condition
on the handle, so that the next <GWFILE> makes Perl try
again to read something.
If that doesn't work (it relies on features of your stdio
implementation), then you need something more like this:
for (;;) {
for ($curpos = tell(GWFILE); <GWFILE>; $curpos =
tell(GWFILE)) {
# search for some stuff and put it into files
}
# sleep for a while
seek(GWFILE, $curpos, 0); # seek to where we
had been
}
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 example:
open(LOG, ">>/foo/logfile");
open(STDERR, ">&LOG");
Or even with a literal numeric descriptor:
$fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT, "<&=$fd"); # like fdopen(3S)
Note that "<&STDIN" makes a copy, but "<&=STDIN" make an
alias. That means if you close an aliased handle, all
aliases become inaccessible. This is not true with a
copied one.
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, 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 $rc == -1;
Or, just use the fdopen(3S) feature of open():
{
local *F;
open F, "<&=$fd" or die "Cannot reopen fd=$fd:
$!";
close F;
}
Why can't I use "C:empoo" in DOS paths? Why doesn't
`C:empoo.exe` work?
Whoops! You just put a tab and a formfeed into that filename!
Remember that within double quoted strings
("likehis"), 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++, awk, Tcl, Java, or Python, just to 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
"-i" clobber protected files? Isn't this a bug in Perl?
This is elaborately and painstakingly described in the
file-dir-perms article in the "Far More Than You Ever
Wanted To Know" collection in
http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .
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?
Here's an algorithm from the Camel Book:
srand;
rand($.) < 1 && ($line = $_) while <>;
This has a significant advantage in space over reading the
whole file in. You can find a proof of this method in The
Art of Computer Programming, Volume 2, Section 3.4.2, by
Donald E. Knuth.
You can use the File::Random module which provides a function
for that algorithm:
use File::Random qw/random_line/;
my $line = random_line($filename);
Another way is to use the Tie::File module, which treats
the entire file as an array. Simply access a random array
element.
Why do I get weird spaces when I print an array of lines?
Saying
print "@lines0;
joins together the elements of @lines with a space between
them. If @lines were "("little", "fluffy", "clouds")"
then the above statement would print
little fluffy clouds
but if each element of @lines was a line of text, ending a
newline character "("little0, "fluffy0, "clouds0)"
then it would print:
little
fluffy
clouds
If your array contains lines, just print them:
print @lines;
Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
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.
perl v5.8.5 2002-11-06 22 [ Back ] |
