print "Text\n" if -T _;
print "Binary\n" if -B _;
+As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
+test operators, in a way that C<-f -w -x $file> is equivalent to
+C<-x $file && -w _ && -f _>. (This is only syntax fancy : if you use
+the return value of C<-f $file> as an argument to another filetest
+operator, no special magic will happen.)
+
=item abs VALUE
=item abs
Changes the permissions of a list of files. The first element of the
list must be the numerical mode, which should probably be an octal
-number, and which definitely should I<not> a string of octal digits:
+number, and which definitely should I<not> be a string of octal digits:
C<0644> is okay, C<'0644'> is not. Returns the number of files
successfully changed. See also L</oct>, if all you have is a string.
If you chomp a list, each element is chomped, and the total number of
characters removed is returned.
+If the C<encoding> pragma is in scope then the lengths returned are
+calculated from the length of C<$/> in Unicode characters, which is not
+always the same as the length of C<$/> in the native encoding.
+
Note that parentheses are necessary when you're chomping anything
that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
C<open>.) However, an explicit C<close> on an input file resets the line
counter (C<$.>), while the implicit close done by C<open> does not.
-If the file handle came from a piped open C<close> will additionally
-return false if one of the other system calls involved fails or if the
+If the file handle came from a piped open, C<close> will additionally
+return false if one of the other system calls involved fails, or if the
program exits with non-zero status. (If the only problem was that the
-program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
+program exited non-zero, C<$!> will be set to C<0>.) Closing a pipe
also waits for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards, and
implicitly puts the exit status value of that command into C<$?>.
=item getsockopt SOCKET,LEVEL,OPTNAME
-Returns the socket option requested, or undef if there is an error.
+Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
+Options may exist at multiple protocol levels depending on the socket
+type, but at least the uppermost socket level SOL_SOCKET (defined in the
+C<Socket> module) will exist. To query options at another level the
+protocol number of the appropriate protocol controlling the option
+should be supplied. For example, to indicate that an option is to be
+interpreted by the TCP protocol, LEVEL should be set to the protocol
+number of TCP, which you can get using getprotobyname.
+
+The call returns a packed string representing the requested socket option,
+or C<undef> if there is an error (the error reason will be in $!). What
+exactly is in the packed string depends in the LEVEL and OPTNAME, consult
+your system documentation for details. A very common case however is that
+the option is an integer, in which case the result will be an packed
+integer which you can decode using unpack with the C<i> (or C<I>) format.
+
+An example testing if Nagle's algorithm is turned on on a socket:
+
+ use Socket;
+
+ defined(my $tcp = getprotobyname("tcp"))
+ or die "Could not determine the protocol number for tcp";
+ # my $tcp = Socket::IPPROTO_TCP; # Alternative
+ my $packed = getsockopt($socket, $tcp, Socket::TCP_NODELAY)
+ or die "Could not query TCP_NODELAY SOCKEt option: $!";
+ my $nodelay = unpack("I", $packed);
+ print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
+
=item glob EXPR
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
-Also see the C<timegm> function provided by the C<Time::Local> module,
-and the strftime(3) function available via the POSIX module.
-
-This scalar value is B<not> locale dependent (see L<perllocale>), but
-is instead a Perl builtin. Also see the C<Time::Local> module, and the
-strftime(3) and mktime(3) functions available via the POSIX module. To
-get somewhat similar but locale dependent date strings, set up your
-locale environment variables appropriately (please see L<perllocale>)
-and try for example:
+If you need local time instead of GMT use the L</localtime> builtin.
+See also the C<timegm> function provided by the C<Time::Local> module,
+and the strftime(3) and mktime(3) functions available via the L<POSIX> module.
- use POSIX qw(strftime);
- $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
-
-Note that the C<%a> and C<%b> escapes, which represent the short forms
-of the day of the week and the month of the year, may not necessarily
-be three characters wide in all locales.
+This scalar value is B<not> locale dependent (see L<perllocale>), but is
+instead a Perl builtin. To get somewhat similar but locale dependent date
+strings, see the example in L</localtime>.
=item goto LABEL
or another C<grep>) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.
+If C<$_> is lexical in the scope where the C<grep> appears (because it has
+been declared with C<my $_>) then, in addition the be locally aliased to
+the list elements, C<$_> keeps being lexical inside the block; i.e. it
+can't be seen from the outside, avoiding any potential side-effects.
+
See also L</map> for a list composed of the results of the BLOCK or EXPR.
=item hex EXPR
=item localtime EXPR
+=item localtime
+
Converts a time as returned by the time function to a 9-element list
with the time analyzed for the local time zone. Typically used as
follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
- localtime(time);
+ localtime(time);
All list elements are numeric, and come straight out of the C `struct
-tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
-specified time. $mday is the day of the month, and $mon is the month
-itself, in the range C<0..11> with 0 indicating January and 11
-indicating December. $year is the number of years since 1900. That
-is, $year is C<123> in year 2023. $wday is the day of the week, with
-0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
-the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
-is true if the specified time occurs during daylight savings time,
-false otherwise.
+tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
+of the specified time.
-Note that the $year element is I<not> simply the last two digits of
-the year. If you assume it is, then you create non-Y2K-compliant
-programs--and you wouldn't want to do that, would you?
+C<$mday> is the day of the month, and C<$mon> is the month itself, in
+the range C<0..11> with 0 indicating January and 11 indicating December.
+This makes it easy to get a month name from a list:
-The proper way to get a complete 4-digit year is simply:
+ my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
+ print "$abbr[$mon] $mday";
+ # $mon=9, $mday=18 gives "Oct 18"
- $year += 1900;
+C<$year> is the number of years since 1900, not just the last two digits
+of the year. That is, C<$year> is C<123> in year 2023. The proper way
+to get a complete 4-digit year is simply:
-And to get the last two digits of the year (e.g., '01' in 2001) do:
+ $year += 1900;
- $year = sprintf("%02d", $year % 100);
+To get the last two digits of the year (e.g., '01' in 2001) do:
+
+ $year = sprintf("%02d", $year % 100);
+
+C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
+Wednesday. C<$yday> is the day of the year, in the range C<0..364>
+(or C<0..365> in leap years.)
+
+C<$isdst> is true if the specified time occurs during Daylight Saving
+Time, false otherwise.
If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
-This scalar value is B<not> locale dependent, see L<perllocale>, but
-instead a Perl builtin. Also see the C<Time::Local> module
-(to convert the second, minutes, hours, ... back to seconds since the
-stroke of midnight the 1st of January 1970, the value returned by
-time()), and the strftime(3) and mktime(3) functions available via the
-POSIX module. To get somewhat similar but locale dependent date
-strings, set up your locale environment variables appropriately
-(please see L<perllocale>) and try for example:
+This scalar value is B<not> locale dependent but is a Perl builtin. For GMT
+instead of local time use the L</gmtime> builtin. See also the
+C<Time::Local> module (to convert the second, minutes, hours, ... back to
+the integer value returned by time()), and the L<POSIX> module's strftime(3)
+and mktime(3) functions.
+
+To get somewhat similar but locale dependent date strings, set up your
+locale environment variables appropriately (please see L<perllocale>) and
+try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
+ # or for GMT formatted appropriately for your locale:
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
most cases. See also L</grep> for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.
+If C<$_> is lexical in the scope where the C<map> appears (because it has
+been declared with C<my $_>) then, in addition the be locally aliased to
+the list elements, C<$_> keeps being lexical inside the block; i.e. it
+can't be seen from the outside, avoiding any potential side-effects.
+
C<{> starts both hash references and blocks, so C<map { ...> could be either
the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
ahead for the closing C<}> it has to take a guess at which its dealing with
to the temporary file first. You will need to seek() to do the
reading.
-File handles can be opened to "in memory" files held in Perl scalars via:
+Since v5.8.0, perl has built using PerlIO by default. Unless you've
+changed this (ie Configure -Uuseperlio), you can open file handles to
+"in memory" files held in Perl scalars via:
open($fh, '>', \$variable) || ..
open(ARTICLE, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
- open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
+ open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# in memory files
}
}
+See L<perliol/> for detailed info on PerlIO.
+
You may also, in the Bourne shell tradition, specify an EXPR beginning
with C<< '>&' >>, in which case the rest of the string is interpreted
as the name of a filehandle (or file descriptor, if numeric) to be
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
- close STDOUT;
- close STDERR;
-
open STDOUT, ">&", $oldout or die "Can't dup \$oldout: $!";
open STDERR, ">&OLDERR" or die "Can't dup OLDERR: $!";
multi-threaded application. The C<unique> attribute does nothing in
all other environments.
+Warning: the current implementation of this attribute operates on the
+typeglob associated with the variable; this means that C<our $x : unique>
+also has the effect of C<our @x : unique; our %x : unique>. This may be
+subject to change.
+
=item pack TEMPLATE,LIST
Takes a LIST of values and converts it into a string using the rules
h A hex string (low nybble first).
H A hex string (high nybble first).
- c A signed char value.
+ c A signed char (8-bit) value.
C An unsigned char value. Only does bytes. See U for Unicode.
- s A signed short value.
+ s A signed short (16-bit) value.
S An unsigned short value.
- (This 'short' is _exactly_ 16 bits, which may differ from
- what a local C compiler calls 'short'. If you want
- native-length shorts, use the '!' suffix.)
-
- i A signed integer value.
- I An unsigned integer value.
- (This 'integer' is _at_least_ 32 bits wide. Its exact
- size depends on what a local C compiler calls 'int',
- and may even be larger than the 'long' described in
- the next item.)
- l A signed long value.
+ l A signed long (32-bit) value.
L An unsigned long value.
- (This 'long' is _exactly_ 32 bits, which may differ from
- what a local C compiler calls 'long'. If you want
- native-length longs, use the '!' suffix.)
-
- n An unsigned short in "network" (big-endian) order.
- N An unsigned long in "network" (big-endian) order.
- v An unsigned short in "VAX" (little-endian) order.
- V An unsigned long in "VAX" (little-endian) order.
- (These 'shorts' and 'longs' are _exactly_ 16 bits and
- _exactly_ 32 bits, respectively.)
q A signed quad (64-bit) value.
Q An unsigned quad value.
integer values _and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
- j A signed integer value (a Perl internal integer, IV).
- J An unsigned integer value (a Perl internal unsigned integer, UV).
+ i A signed integer value.
+ I A unsigned integer value.
+ (This 'integer' is _at_least_ 32 bits wide. Its exact
+ size depends on what a local C compiler calls 'int'.)
+
+ n An unsigned short (16-bit) in "network" (big-endian) order.
+ N An unsigned long (32-bit) in "network" (big-endian) order.
+ v An unsigned short (16-bit) in "VAX" (little-endian) order.
+ V An unsigned long (32-bit) in "VAX" (little-endian) order.
+
+ j A Perl internal signed integer value (IV).
+ J A Perl internal unsigned integer value (UV).
f A single-precision float in the native format.
d A double-precision float in the native format.
- F A floating point value in the native native format
- (a Perl internal floating point value, NV).
+ F A Perl internal floating point value (NV) in the native format
D A long double-precision float in the native format.
(Long doubles are available only if your system supports long
double values _and_ if Perl has been compiled to support those.
the innermost ()-group.
( Start of a ()-group.
+Some letters in the TEMPLATE may optionally be followed by one or
+more of these modifiers (the second column lists the letters for
+which the modifier is valid):
+
+ ! sSlLiI Forces native (short, long, int) sizes instead
+ of fixed (16-/32-bit) sizes.
+
+ xX Make x and X act as alignment commands.
+
+ nNvV Treat integers as signed instead of unsigned.
+
+ > sSiIlLqQ Force big-endian byte-order on the type.
+ jJfFdDpP (The "big end" touches the construct.)
+
+ < sSiIlLqQ Force little-endian byte-order on the type.
+ jJfFdDpP (The "little end" touches the construct.)
+
+The C<E<gt>> and C<E<lt>> modifiers can also be used on C<()>-groups,
+in which case they force a certain byte-order on all components of
+that group, including subgroups.
+
The following rules apply:
=over 8
length. A NULL pointer is created if the corresponding value for C<p> or
C<P> is C<undef>, similarly for unpack().
+If your system has a strange pointer size (i.e. a pointer is neither as
+big as an int nor as big as a long), it may not be possible to pack or
+unpack pointers in big- or little-endian byte order. Attempting to do
+so will result in a fatal error.
+
=item *
The C</> template character allows packing and unpacking of strings where
=item *
The integer types C<s>, C<S>, C<l>, and C<L> may be
-immediately followed by a C<!> suffix to signify native shorts or
+followed by a C<!> modifier to signify native shorts or
longs--as you can see from above for example a bare C<l> does mean
exactly 32 bits, the native C<long> (as seen by the local C compiler)
may be larger. This is an issue mainly in 64-bit platforms. You can
Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
and C<'87654321'> are big-endian.
-If you want portable packed integers use the formats C<n>, C<N>,
-C<v>, and C<V>, their byte endianness and size are known.
+If you want portable packed integers you can either use the formats
+C<n>, C<N>, C<v>, and C<V>, or you can use the C<E<gt>> and C<E<lt>>
+modifiers. These modifiers are only available as of perl 5.9.2.
See also L<perlport>.
=item *
+All integer and floating point formats as well as C<p> and C<P> and
+C<()>-groups may be followed by the C<E<gt>> or C<E<lt>> modifiers
+to force big- or little- endian byte-order, respectively.
+This is especially useful, since C<n>, C<N>, C<v> and C<V> don't cover
+signed integers, 64-bit integers and floating point values. However,
+there are some things to keep in mind.
+
+Exchanging signed integers between different platforms only works
+if all platforms store them in the same format. Most platforms store
+signed integers in two's complement, so usually this is not an issue.
+
+The C<E<gt>> or C<E<lt>> modifiers can only be used on floating point
+formats on big- or little-endian machines. Otherwise, attempting to
+do so will result in a fatal error.
+
+Forcing big- or little-endian byte-order on floating point values for
+data exchange can only work if all platforms are using the same
+binary representation (e.g. IEEE floating point format). Even if all
+platforms are using IEEE, there may be subtle differences. Being able
+to use C<E<gt>> or C<E<lt>> on floating point values can be very useful,
+but also very dangerous if you don't know exactly what you're doing.
+It is definetely not a general way to portably store floating point
+values.
+
+When using C<E<gt>> or C<E<lt>> on an C<()>-group, this will affect
+all types inside the group that accept the byte-order modifiers,
+including all subgroups. It will silently be ignored for all other
+types. You are not allowed to override the byte-order within a group
+that already has a byte-order modifier suffix.
+
+=item *
+
Real numbers (floats and doubles) are in the native machine format only;
due to the multiplicity of floating formats around, and the lack of a
standard "network" representation, no facility for interchange has been
arithmetic (as the endian-ness of the memory representation is not part
of the IEEE spec). See also L<perlport>.
-Note that Perl uses doubles internally for all numeric calculation, and
-converting from double into float and thence back to double again will
-lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
-equal $foo).
+If you know exactly what you're doing, you can use the C<E<gt>> or C<E<lt>>
+modifiers to force big- or little-endian byte-order on floating point values.
+
+Note that Perl uses doubles (or long doubles, if configured) internally for
+all numeric calculation, and converting from double into float and thence back
+to double again will lose precision (i.e., C<unpack("f", pack("f", $foo)>)
+will not in general equal $foo).
=item *
=item *
+C<n>, C<N>, C<v> and C<V> accept the C<!> modifier. In this case they
+will represent signed 16-/32-bit integers in big-/little-endian order.
+This is only portable if all platforms sharing the packed data use the
+same binary representation for signed integers (e.g. all platforms are
+using two's complement representation).
+
+=item *
+
A comment in a TEMPLATE starts with C<#> and goes to the end of line.
White space may be used to separate pack codes from each other, but
-a C<!> modifier and a repeat count must follow immediately.
+modifiers and a repeat count must follow immediately.
=item *
# short 12, zero fill to position 4, long 34
# $foo eq $bar
+ $foo = pack('nN', 42, 4711);
+ # pack big-endian 16- and 32-bit unsigned integers
+ $foo = pack('S>L>', 42, 4711);
+ # exactly the same
+ $foo = pack('s<l<', -42, 4711);
+ # pack little-endian 16- and 32-bit signed integers
+ $foo = pack('(sl)<', -42, 4711);
+ # exactly the same
+
The same template may generally also be used in unpack().
=item package NAMESPACE
=item pos
Returns the offset of where the last C<m//g> search left off for the variable
-in question (C<$_> is used when the variable is not specified). May be
-modified to change that offset. Such modification will also influence
-the C<\G> zero-width assertion in regular expressions. See L<perlre> and
+in question (C<$_> is used when the variable is not specified). Note that
+0 is a valid match offset, while C<undef> indicates that the search position
+is reset (usually due to match failure, but can also be because no match has
+yet been performed on the scalar). C<pos> directly accesses the location used
+by the regexp engine to store the offset, so assigning to C<pos> will change
+that offset, and so will also influence the C<\G> zero-width assertion in
+regular expressions. Because a failed C<m//gc> match doesn't reset the offset,
+the return from C<pos> won't change either in this case. See L<perlre> and
L<perlop>.
=item print FILEHANDLE LIST
undef $/; # for efficiency of <>
print scalar reverse <>; # character tac, last line tsrif
+Used without arguments in scalar context, reverse() reverses C<$_>.
+
This operator is also handy for inverting a hash, although there are some
caveats. If a value is duplicated in the original hash, only one of those
can be represented as a key in the inverted hash. Also, this has to
select(undef, undef, undef, 0.25);
Note that whether C<select> gets restarted after signals (say, SIGALRM)
-is implementation-dependent.
+is implementation-dependent. See also L<perlport> for notes on the
+portability of C<select>.
B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
or <FH>) with C<select>, except as permitted by POSIX, and even
=item split
-Splits a string into a list of strings and returns that list. By default,
-empty leading fields are preserved, and empty trailing ones are deleted.
+Splits the string EXPR into a list of strings and returns that list. By
+default, empty leading fields are preserved, and empty trailing ones are
+deleted. (If all fields are empty, they are considered to be trailing.)
In scalar context, returns the number of fields found and splits into
the C<@_> array. Use of split in scalar context is deprecated, however,
= stat($filename);
Not all fields are supported on all filesystem types. Here are the
-meaning of the fields:
+meanings of the fields:
0 dev device number of filesystem
1 ino inode number
(The epoch was at 00:00 January 1, 1970 GMT.)
-(*) The ctime field is non-portable, in particular you cannot expect
-it to be a "creation time", see L<perlport/"Files and Filesystems">
-for details.
+(*) Not all fields are supported on all filesystem types. Notably, the
+ctime field is non-portable. In particular, you cannot expect it to be a
+"creation time", see L<perlport/"Files and Filesystems"> for details.
-If stat is passed the special filehandle consisting of an underline, no
+If C<stat> is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
-last stat or filetest are returned. Example:
+last C<stat>, C<lstat>, or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
$is_setgid = S_ISDIR($mode);
You could write the last two using the C<-u> and C<-d> operators.
-The commonly available S_IF* constants are
+The commonly available C<S_IF*> constants are
# Permissions: read, write, execute, for user, group, others.
S_IREAD S_IWRITE S_IEXEC
-and the S_IF* functions are
+and the C<S_IF*> functions are
S_IMODE($mode) the part of $mode containing the permission bits
and the setuid/setgid/sticky bits
which can be bit-anded with e.g. S_IFREG
or with the following functions
- # The operators -f, -d, -l, -b, -c, -p, and -s.
+ # The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
S_ISENFMT($mode) S_ISWHT($mode)
See your native chmod(2) and stat(2) documentation for more details
-about the S_* constants. To get status info for a symbolic link
+about the C<S_*> constants. To get status info for a symbolic link
instead of the target file behind the link, use the C<lstat> function.
=item study SCALAR
parts of the EXPR and return what was there before in one operation,
just as you can with splice().
-If the lvalue returned by substr is used after the EXPR is changed in
-any way, the behaviour may not be as expected and is subject to change.
-This caveat includes code such as C<print(substr($foo,$a,$b)=$bar)> or
-C<(substr($foo,$a,$b)=$bar)=$fud> (where $foo is changed via the
-substring assignment, and then the substr is used again), or where a
-substr() is aliased via a C<foreach> loop or passed as a parameter or
-a reference to it is taken and then the alias, parameter, or deref'd
-reference either is used after the original EXPR has been changed or
-is assigned to and then used a second time.
+Note that the lvalue returned by by the 3-arg version of substr() acts as
+a 'magic bullet'; each time it is assigned to, it remembers which part
+of the original string is being modified; for example:
+
+ $x = '1234';
+ for (substr($x,1,2)) {
+ $_ = 'a'; print $x,"\n"; # prints 1a4
+ $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
+ $x = '56789';
+ $_ = 'pq'; print $x,"\n"; # prints 5pq9
+ }
+
+
+Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
+unspecified.
=item symlink OLDFILE,NEWFILE
Some of the most common values are C<O_RDONLY> for opening the file in
read-only mode, C<O_WRONLY> for opening the file in write-only mode,
-and C<O_RDWR> for opening the file in read-write mode, and.
+and C<O_RDWR> for opening the file in read-write mode.
For historical reasons, some values work on almost every system
supported by perl: zero means read-only, one means write-only, and two
In many systems the C<O_EXCL> flag is available for opening files in
exclusive mode. This is B<not> locking: exclusiveness means here that
-if the file already exists, sysopen() fails. The C<O_EXCL> wins
-C<O_TRUNC>.
+if the file already exists, sysopen() fails. C<O_EXCL> may not work
+on network filesystems, and has no effect unless the C<O_CREAT> flag
+is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
+being opened if it is a symbolic link. It does not protect against
+symbolic links in the file's path.
-Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
+Sometimes you may want to truncate an already-existing file. This
+can be done using the C<O_TRUNC> flag. The behavior of
+C<O_TRUNC> with C<O_RDONLY> is undefined.
You should seldom if ever use C<0644> as argument to C<sysopen>, because
that takes away the user's option to have a more permissive umask.
that would render sysseek() very slow).
sysseek() bypasses normal buffered IO, so mixing this with reads (other
-than C<sysread>, for example >< or read()) C<print>, C<write>,
+than C<sysread>, for example C<< <> >> or read()) C<print>, C<write>,
C<seek>, C<tell>, or C<eof> may cause confusion.
For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
Like C<exec>, C<system> allows you to lie to a program about its name if
you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
-Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>,
-killing the program they're running doesn't actually interrupt
-your program.
+Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
+C<system>, if you expect your program to terminate on receipt of these
+signals you will need to arrange to do so yourself based on the return
+value.
@args = ("command", "arg1", "arg2");
system(@args) == 0
printf "child exited with value %d\n", $? >> 8;
}
-
or more portably by using the W*() calls of the POSIX extension;
see L<perlport> for more information.
There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
-Do not use tell() on a filehandle that has been opened using
-sysopen(), use sysseek() for that as described above. Why? Because
-sysopen() creates unbuffered, "raw", filehandles, while open() creates
-buffered filehandles. sysseek() make sense only on the first kind,
-tell() only makes sense on the second kind.
+Do not use tell() (or other buffered I/O operations) on a file handle
+that has been manipulated by sysread(), syswrite() or sysseek().
+Those functions ignore the buffering, while tell() does not.
=item telldir DIRHANDLE
=item time
Returns the number of non-leap seconds since whatever time the system
-considers to be the epoch (that's 00:00:00, January 1, 1904 for Mac OS,
-and 00:00:00 UTC, January 1, 1970 for most other systems).
-Suitable for feeding to C<gmtime> and C<localtime>.
+considers to be the epoch, suitable for feeding to C<gmtime> and
+C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
+a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
+1904 in the current local time zone for its epoch.
For measuring time in better granularity than one second,
you may use either the Time::HiRes module (from CPAN, and starting from
and modification times, in that order. Returns the number of files
successfully changed. The inode change time of each file is set
to the current time. For example, this code has the same effect as the
-Unix touch(1) command when the files I<already exist>.
+Unix touch(1) command when the files I<already exist> and belong to
+the user running the program:
#!/usr/bin/perl
$atime = $mtime = time;
the utime(2) function in the C library will be called with a null second
argument. On most systems, this will set the file's access and
modification times to the current time (i.e. equivalent to the example
-above.)
+above) and will even work on other users' files where you have write
+permission:
utime undef, undef, @ARGV;
=item wantarray
-Returns true if the context of the currently executing subroutine is
-looking for a list value. Returns false if the context is looking
-for a scalar. Returns the undefined value if the context is looking
-for no value (void context).
+Returns true if the context of the currently executing subroutine or
+C<eval> is looking for a list value. Returns false if the context is
+looking for a scalar. Returns the undefined value if the context is
+looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
+C<wantarray()>'s result is unspecified in the top level of a file,
+in a C<BEGIN>, C<CHECK>, C<INIT> or C<END> block, or in a C<DESTROY>
+method.
+
This function should have been named wantlist() instead.
=item warn LIST
projects / p5sagit/p5-mst-13.2.git / blobdiff