X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfunc.pod;h=b918cac80c86293d26e6a28469bcc77313458217;hb=938c8732ceb115a707f725327a631eb35319ba87;hp=7879e3493b33fd8ac5692c8029c214a277823928;hpb=be2f7487fc96cdcf159c9733c5c0dcd982e12af0;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod index 7879e34..b918cac 100644 --- a/pod/perlfunc.pod +++ b/pod/perlfunc.pod @@ -366,6 +366,12 @@ Example: 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 $file> as an argument to another filetest +operator, no special magic will happen.) + =item abs VALUE =item abs @@ -661,6 +667,10 @@ You can actually chomp anything that's an lvalue, including an assignment: If you chomp a list, each element is chomped, and the total number of characters removed is returned. +If the C pragma is in scope then the lengths returned are +calculated from the length of C<$/> 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 is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as @@ -766,10 +776,10 @@ another C on it, because C will close it for you. (See C.) However, an explicit C on an input file resets the line counter (C<$.>), while the implicit close done by C does not. -If the file handle came from a piped open C 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 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<$?>. @@ -1033,8 +1043,18 @@ In the case of an array, if the array elements happen to be at the end, the size of the array will shrink to the highest element that tests true for exists() (or 0 if no such element exists). -Returns each element so deleted or the undefined value if there was no such -element. Deleting from C<$ENV{}> modifies the environment. Deleting from +Returns a list with the same number of elements as the number of elements +for which deletion was attempted. Each element of that list consists of +either the value of the element deleted, or the undefined value. In scalar +context, this means that you get the value of the last element deleted (or +the undefined value if that element did not exist). + + %hash = (foo => 11, bar => 22, baz => 33); + $scalar = delete $hash{foo}; # $scalar is 11 + $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 + @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33) + +Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from a Cd hash or array may not necessarily return anything. @@ -2077,7 +2097,34 @@ IPs that the connection might have come in on. =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 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 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 (or C) 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 @@ -2131,22 +2178,13 @@ In scalar context, C returns the ctime(3) value: $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994" -Also see the C function provided by the C module, -and the strftime(3) function available via the POSIX module. - -This scalar value is B locale dependent (see L), but -is instead a Perl builtin. Also see the C 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) -and try for example: - - use POSIX qw(strftime); - $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; +If you need local time instead of GMT use the L builtin. +See also the C function provided by the C module, +and the strftime(3) and mktime(3) functions available via the L module. -Note that the C<%a> and C<%b> escapes, which represent the short forms -of the day of the week and the month of the year, may not necessarily -be three characters wide in all locales. +This scalar value is B locale dependent (see L), but is +instead a Perl builtin. To get somewhat similar but locale dependent date +strings, see the example in L. =item goto LABEL @@ -2216,6 +2254,11 @@ element of a list returned by grep (for example, in a C, C or another C) actually modifies the element in the original list. This is usually something to be avoided when writing clear code. +If C<$_> is lexical in the scope where the C appears (because it has +been declared with C) 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 for a list composed of the results of the BLOCK or EXPR. =item hex EXPR @@ -2325,7 +2368,8 @@ Perl for security reasons (see L). As a side effect, calling keys() resets the HASH's internal iterator, -see L. +see L. (In particular, calling keys() in void context resets +the iterator with no other overhead.) Here is yet another way to print your environment: @@ -2512,17 +2556,20 @@ In scalar context, C returns the ctime(3) value: $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994" -This scalar value is B locale dependent, see L, but -instead a Perl builtin. Also see the C 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) and try for example: +This scalar value is B locale dependent but is a Perl builtin. For GMT +instead of local time use the L builtin. See also the +C module (to convert the second, minutes, hours, ... back to +the integer value returned by time()), and the L 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) 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: + $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime; Note that the C<%a> and C<%b>, the short forms of the day of the week and the month of the year, may not necessarily be three characters wide. @@ -2600,6 +2647,11 @@ Using a regular C loop for this purpose would be clearer in most cases. See also L for an array composed of those items of the original list for which the BLOCK or EXPR evaluates to true. +If C<$_> is lexical in the scope where the C appears (because it has +been declared with C) then, in addition the be locally aliased to +the list elements, C<$_> keeps being lexical inside the block; i.e. it +can't be seen from the outside, avoiding any potential side-effects. + C<{> starts both hash references and blocks, so C could be either the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look ahead for the closing C<}> it has to take a guess at which its dealing with @@ -2883,7 +2935,9 @@ works for symmetry, but you really should consider writing something to the temporary file first. You will need to seek() to do the reading. -File handles can be opened to "in memory" files held in Perl scalars via: +Since v5.8.0, perl has built using PerlIO by default. Unless you've +changed this (ie Configure -Uuseperlio), you can open file handles to +"in memory" files held in Perl scalars via: open($fh, '>', \$variable) || .. @@ -2914,7 +2968,7 @@ Examples: 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 @@ -2946,6 +3000,8 @@ Examples: } } +See L 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 @@ -2972,9 +3028,6 @@ C using various methods: 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: $!"; @@ -3225,6 +3278,11 @@ fork() emulation on Windows platforms, or by embedding perl in a multi-threaded application. The C 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 +also has the effect of C. This may be +subject to change. + =item pack TEMPLATE,LIST Takes a LIST of values and converts it into a string using the rules @@ -3245,34 +3303,14 @@ of values, as follows: 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. @@ -3280,14 +3318,23 @@ of values, as follows: 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. @@ -3311,6 +3358,27 @@ of values, as follows: 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> and C> 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 @@ -3415,6 +3483,11 @@ The C

type packs a pointer to a structure of the size indicated by the length. A NULL pointer is created if the corresponding value for C

or C

is C, 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 @@ -3446,7 +3519,7 @@ which Perl does not regard as legal in numeric strings. =item * The integer types C, C, C, and C 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 does mean exactly 32 bits, the native C (as seen by the local C compiler) may be larger. This is an issue mainly in 64-bit platforms. You can @@ -3512,12 +3585,45 @@ via L: 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, C, -C, and C, their byte endianness and size are known. +If you want portable packed integers you can either use the formats +C, C, C, and C, or you can use the C> and C> +modifiers. These modifiers are only available as of perl 5.8.5. See also L. =item * +All integer and floating point formats as well as C

and C

and +C<()>-groups may be followed by the C> or C> modifiers +to force big- or little- endian byte-order, respectively. +This is especially useful, since C, C, C and C 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> or C> 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> or C> 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> or C> 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 @@ -3526,10 +3632,13 @@ may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). See also L. -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) will not in general -equal $foo). +If you know exactly what you're doing, you can use the C> or C> +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) +will not in general equal $foo). =item * @@ -3575,9 +3684,17 @@ both result in no-ops. =item * +C, C, C and C 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 * @@ -3637,6 +3754,15 @@ Examples: # 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 unless you're sure it'll return true otherwise. But it's better just to put the C<1;>, in case you add more @@ -4232,6 +4360,8 @@ in the opposite order. 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 @@ -4420,7 +4550,8 @@ You can effect a sleep of 250 milliseconds this way: select(undef, undef, undef, 0.25); Note that whether C. B: One should not attempt to mix buffered I/O (like C or ) with C