• int sv_isa _((SV* sv, char* name));

• int sv_isobject _((SV* sv));

• STRLEN sv_len _((SV* sv));

• STRLEN sv_len _((SV* sv));

• void sv_magic _((SV* sv, SV* obj, int how, char* name, I32 namlen));

• void sv_setiv _((SV* sv, IV num));

• void sv_setnv _((SV* sv, double num));

• void sv_setpv _((SV* sv, char* ptr));

• void sv_setpvn _((SV* sv, char* ptr, STRLEN len));

Do not use with integral Perl types such as HV, AV, SV, CV, because those objects will become corrupted by the pointer copy process.

Note that sv_setref_pvn copies the string while this copies the pointer.

Note that sv_setref_pv copies the pointer while this copies the string.

• void sv_setsv _((SV* dsv, SV* ssv));

• svtype SvTYPE (SV* sv)

• void sv_usepvn _((SV* sv, char* ptr, STRLEN len));

AUTHOR

With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, and Spider Boardman.

API Listing by Dean Roehrich <roehrich@cray.com>.

DATE

If use locale is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale.

Return the square root of EXPR. If EXPR is omitted, returns square root of $_. Only works on non-negative operands, unless you've loaded the standard Math::Complex module.

    use Math::Complex;
    print sqrt(-2);    # prints 1.4142135623731i

Sets the random number seed for the rand() operator. If EXPR is omitted, uses a semi-random value supplied by the kernel (if it supports the /dev/urandom device) or based on the current time and process ID, among other things. In versions of Perl prior to 5.004 the default seed was just the current time(). This isn't a particularly good seed, so many old programs supply their own seed value (often time ^ $$ or time ^ ($$ + ($$ &lt;&lt; 15))), but that isn't necessary any more.

In fact, it's usually not necessary to call srand() at all, because if it is not called explicitly, it is called implicitly at the first use of the rand() operator. However, this was not the case in version of Perl before 5.004, so if your script will run under older Perl versions, it should call srand().

Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:

    srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);

If you're particularly concerned with this, see the Math::TrulyRandom module in CPAN.

Do not call srand() multiple times in your program unless you know exactly what you're doing and why you're doing it. The point of the function is to ``seed'' the rand() function so that rand() can produce a different sequence each time you run your program. Just do it once at the top of your program, or you won't get random numbers out of rand()!

Frequently called programs (like CGI scripts) that simply use

    time ^ $$

for a seed can fall prey to the mathematical property that

    a^b == (a+1)^(b+1)

one-third of the time. So don't do that.

Returns a 13-element list giving the status info for a file, either the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it stats $_. Returns a null list if the stat fails. Typically used as follows:

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types. Here are the meaning of the fields:

  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions)
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) since the epoch
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If 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:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }

(This works on machines only for which the device number is negative under NFS.)

Because the mode contains both the file type and its permissions, you should mask off the file type portion and (s)printf using a "%o" if you want to see the real permissions.

    $mode = (stat($filename))[2];
    printf "Permissions are %04o\n", $mode & 07777;

In scalar context, stat() returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle _.

The File::stat module provides a convenient, by-name access mechanism:

    use File::stat;
    $sb = stat($filename);
    printf "File is %s, size is %s, perm %04o, mtime %s\n", 
        $filename, $sb->size, $sb->mode & 07777,
        scalar localtime $sb->mtime;

Takes extra time to study SCALAR ($_ if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribution of character frequencies in the string to be searched -- you probably want to compare run times with and without it to see which runs faster. Those loops which scan for many short constant strings (including the constant parts of more complex patterns) will benefit most. You may have only one study() active at a time -- if you study a different scalar the first is ``unstudied''. (The way study() works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the 'k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this ``rarest'' character are examined.)

For example, here is a loop that inserts index producing entries before any line containing a certain pattern:

    while (<>) {
        study;
        print ".IX foo\n"       if /\bfoo\b/;
        print ".IX bar\n"       if /\bbar\b/;
        print ".IX blurfl\n"    if /\bblurfl\b/;
        # ...
        print;
    }

In searching for /\bfoo\b/, only those locations in $_ that contain "f" will be looked at, because "f" is rarer than "o". In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place.

Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval() that to avoid recompiling all your patterns all the time. Together with undefining $/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep(1). The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a match:

    $search = 'while (<>) { study;';
    foreach $word (@words) {
        $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;               # this screams
    $/ = "\n";          # put back to normal input delimiter
    foreach $file (sort keys(%seen)) {
        print $file, "\n";
    }

This is subroutine definition, not a real function per se. With just a NAME (and possibly prototypes), it's just a forward declaration. Without a NAME, it's an anonymous function declaration, and does actually return a value: the CODE ref of the closure you just created. See perlsub and perlref for details.

Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever you've set $[ to (but don't do that). If OFFSET is negative (or more precisely, less than $[), starts that far from the end of the string. If LEN is omitted, returns everything to the end of the string. If LEN is negative, leaves that many characters off the end of the string.

If you specify a substring that is partly outside the string, the part within the string is returned. If the substring is totally outside the string a warning is produced.

You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue. If you assign something shorter than LEN, the string will shrink, and if you assign something longer than LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using sprintf().

An alternative to using substr() as an lvalue is to specify the replacement string as the 4th argument. This allows you to replace parts of the EXPR and return what was there before in one operation, just as you can with splice().

Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't support symbolic links, produces a fatal error at run time. To check for that, use eval:

    $symlink_exists = eval { symlink("",""); 1 };

Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, produces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. You can't use a string literal (or other read-only string) as an argument to syscall() because Perl has to assume that any string pointer might be written through. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. This emulates the syswrite() function (or vice versa):

    require 'syscall.ph';               # may need to run h2ph
    $s = "hi there\n";
    syscall(&SYS_write, fileno(STDOUT), $s, length $s);

Note that Perl supports passing of up to only 14 arguments to your system call, which in practice should usually suffice.

Syscall returns whatever value returned by the system call it calls. If the system call fails, syscall() returns -1 and sets $! (errno). Note that some system calls can legitimately return -1. The proper way to handle such calls is to assign $!=0; before the call and check the value of $! if syscall returns -1.

There's a problem with syscall(&SYS_pipe): it returns the file number of the read end of the pipe it creates. There is no way to retrieve the file number of the other end. You can avoid this problem by using pipe() instead.

Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This function calls the underlying operating system's open() function with the parameters FILENAME, MODE, PERMS.

The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module Fcntl. For historical reasons, some values work on almost every system supported by perl: zero means read-only, one means write-only, and two means read/write. We know that these values do not work under OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to use them in new code.

If the file named by FILENAME does not exist and the open() call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to sysopen(), Perl uses the octal value 0666. These permission values need to be in octal, and are modified by your process's current umask.

You should seldom if ever use 0644 as argument to sysopen(), because that takes away the user's option to have a more permissive umask. Better to omit it. See the perlfunc(1) entry on umask for more on this.

See perlopentut for a kinder, gentler explanation of opening files.

Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the system call read(2). It bypasses stdio, so mixing this with other kinds of reads, print(), write(), seek(), tell(), or eof() can cause confusion because stdio usually buffers data. Returns the number of bytes actually read, 0 at end of file, or undef if there was an error. SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read.

An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many bytes counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended.

There is no syseof() function, which is ok, since eof() doesn't work very well on device files (like ttys) anyway. Use sysread() and check for a return value for 0 to decide whether you're done.

Sets FILEHANDLE's system position using the system call lseek(2). It bypasses stdio, so mixing this with reads (other than sysread()), print(), write(), seek(), tell(), or eof() may cause confusion. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to POSITION, 1 to set the it to the current position plus POSITION, and 2 to set it to EOF plus POSITION (typically negative). For WHENCE, you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable or the POSIX module.

Returns the new position, or the undefined value on failure. A position of zero is returned as the string ``0 but true''; thus sysseek() returns TRUE on success and FALSE on failure, yet you can still easily determine the new position.

Does exactly the same thing as ``exec LIST'', except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. If there is more than one argument in LIST, or if LIST is an array with more than one value, starts the program given by the first element of the list with arguments given by the rest of the list. If there is only one scalar argument, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to execvp(), which is more efficient.

The return value is the exit status of the program as returned by the wait() call. To get the actual exit value divide by 256. See also exec. This is NOT what you want to use to capture the output from a command, for that you should use merely backticks or qx//, as described in perlop.

Like exec(), system() allows you to lie to a program about its name if you use the ``system PROGRAM LIST'' syntax. Again, see exec.

Because system() and backticks block SIGINT and SIGQUIT, killing the program they're running doesn't actually interrupt your program.

    @args = ("command", "arg1", "arg2");
    system(@args) == 0
         or die "system @args failed: $?"

You can check all the failure possibilities by inspecting $? like this:

    $exit_value  = $? >> 8;
    $signal_num  = $? & 127;
    $dumped_core = $? & 128;

When the arguments get executed via the system shell, results and return codes will be subject to its quirks and capabilities. See perlop and exec for details.

Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call write(2). If LENGTH is not specified, writes whole SCALAR. It bypasses stdio, so mixing this with reads (other than sysread()), print(), write(), seek(), tell(), or eof() may cause confusion because stdio usually buffers data. Returns the number of bytes actually written, or undef if there was an error. If the LENGTH is greater than the available data in the SCALAR after the OFFSET, only as much data as is available will be written.

An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many bytes counting backwards from the end of the string. In the case the SCALAR is empty you can use OFFSET but only zero offset.

Returns the current position for FILEHANDLE. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read.

There is no systell() function. Use sysseek(FH, 0, 1) for that.

Returns the current position of the readdir() routines on DIRHANDLE. Value may be given to seekdir() to access a particular location in a directory. Has the same caveats about possible directory compaction as the corresponding system library routine.

This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the ``new()'' method of the class (meaning TIESCALAR, TIEHANDLE, TIEARRAY, or TIEHASH). Typically these are arguments such as might be passed to the dbm_open() function of C. The object returned by the ``new()'' method is also returned by the tie() function, which would be useful if you want to access other methods in CLASSNAME.

Note that functions such as keys() and values() may return huge lists when used on large objects, like DBM files. You may prefer to use the each() function to iterate over such. Example:

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

A class implementing a hash should have the following methods:

    TIEHASH classname, LIST
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    CLEAR this
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey
    DESTROY this

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    FETCH this, key
    STORE this, key, value
    FETCHSIZE this
    STORESIZE this, count
    CLEAR this
    PUSH this, LIST
    POP this
    SHIFT this
    UNSHIFT this, LIST
    SPLICE this, offset, length, LIST
    EXTEND this, count
    DESTROY this

A class implementing a file handle should have the following methods:

    TIEHANDLE classname, LIST
    READ this, scalar, length, offset
    READLINE this
    GETC this
    WRITE this, scalar, length, offset
    PRINT this, LIST
    PRINTF this, format, LIST
    CLOSE this
    DESTROY this

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    FETCH this,
    STORE this, value
    DESTROY this

Not all methods indicated above need be implemented. See perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.

Unlike dbmopen(), the tie() function will not use or require a module for you--you need to do that explicitly yourself. See DB_File or the Config module for interesting tie() implementations.

For further details see perltie, tied VARIABLE.

Returns a reference to the object underlying VARIABLE (the same value that was originally returned by the tie() call that bound the variable to a package.) Returns the undefined value if VARIABLE isn't tied to a package.

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 MacOS, and 00:00:00 UTC, January 1, 1970 for most other systems). Suitable for feeding to gmtime() and localtime().

Returns a four-element list giving the user and system times, in seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;

The transliteration operator. Same as y///. See perlop.

Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system. Returns TRUE if successful, the undefined value otherwise.

Returns an uppercased version of EXPR. This is the internal function implementing the \U escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale. (It does not attempt to do titlecase mapping on initial letters. See ucfirst() for that.)

If EXPR is omitted, uses $_.

Returns the value of EXPR with the first character in uppercase. This is the internal function implementing the \u escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale.

If EXPR is omitted, uses $_.

Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask.

The Unix permission rwxr-x--- is represented as three sets of three bits, or three octal digits: 0750 (the leading 0 indicates octal and isn't one of the digits). The umask value is such a number representing disabled permissions bits. The permission (or ``mode'') values you pass mkdir or sysopen are modified by your umask, so even if you tell sysopen to create a file with permissions 0777, if your umask is 0022 then the file will actually be created with permissions 0755. If your umask were 0027 (group can't write; others can't read, write, or execute), then passing sysopen() 0666 would create a file with mode 0640 (0666 &~ 027 is 0640).

Here's some advice: supply a creation mode of 0666 for regular files (in sysopen()) and one of 0777 for directories (in mkdir()) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.

If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a fatal error at run time. If umask(2) is not implemented and you are not trying to restrict access for yourself, returns undef.

Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also oct, if all you have is a string.

Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an array (using ``@''), a hash (using ``%''), a subroutine (using ``&''), or a typeglob (using ``<*>''). (Saying undef $hash{$key} will probably not do what you expect on most predefined variables or DBM list values, so don't do that; see delete.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable or pass as a parameter. Examples:

    undef $foo;
    undef $bar{'blurfl'};      # Compare to: delete $bar{'blurfl'};
    undef @ary;
    undef %hash;
    undef &mysub;
    undef *xyz;       # destroys $xyz, @xyz, %xyz, &xyz, etc.
    return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
    select undef, undef, undef, 0.25;
    ($a, $b, undef, $c) = &foo;       # Ignore third value returned

Note that this is a unary operator, not a list operator.

Deletes a list of files. Returns the number of files successfully deleted.

    $cnt = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink <*.bak>;

Note: unlink() will not delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use rmdir() instead.

If LIST is omitted, uses $_.

Unpack() does the reverse of pack(): it takes a string representing a structure and expands it out into a list value, returning the array value. (In scalar context, it returns merely the first value produced.) The TEMPLATE has the same format as in the pack() function. Here's a subroutine that does substring:

    sub substr {
        my($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum program:

    while (<>) {
        $checksum += unpack("%32C*", $_);
    }
    $checksum %= 65535;

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);

See pack for more examples.

Breaks the binding between a variable and a package. (See tie().)

Does the opposite of a shift(). Or the opposite of a push(), depending on how you look at it. Prepends list to the front of the array, and returns the new number of elements in the array.

    unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. Use reverse() to do the reverse.

Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

except that Module must be a bareword.

If the first argument to use is a number, it is treated as a version number instead of a module name. If the version of the Perl interpreter is less than VERSION, then an error message is printed and Perl exits immediately. This is often useful if you need to check the current Perl version before useing library modules that have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.)

The BEGIN forces the require and import() to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import() is not a builtin--it's just an ordinary static method call into the ``Module'' package to tell the module to import the list of features back into the current package. The module can implement its import() method any way it likes, though most modules just choose to derive their import() method via inheritance from the Exporter class that is defined in the Exporter module. See Exporter. If no import() method can be found then the error is currently silently ignored. This may change to a fatal error in a future version.

If you don't want your namespace altered, explicitly supply an empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module }

If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument. The default VERSION method, inherited from the Universal class, croaks if the given version is larger than the value of the variable $Module::VERSION. (Note that there is not a comma after VERSION!)

Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:

    use integer;
    use diagnostics;
    use sigtrap qw(SEGV BUS);
    use strict  qw(subs vars refs);
    use subs    qw(afunc blurfl);

Some of these these pseudo-modules import semantics into the current block scope (like strict or integer, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).

There's a corresponding ``no'' command that unimports meanings imported by use, i.e., it calls unimport Module LIST instead of import().

    no integer;
    no strict 'refs';

If no unimport() method can be found the call fails with a fatal error.

See perlmod for a list of standard modules and pragmas.

Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modification times, in that order. Returns the number of files successfully changed. The inode modification time of each file is set to the current time. This code has the same effect as the ``touch'' command if the files already exist:

    #!/usr/bin/perl
    $now = time;
    utime $now, $now, @ARGV;

Returns a list consisting of all the values of the named hash. (In a scalar context, returns the number of values.) The values are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be the same order as either the keys() or each() function would produce on the same (unmodified) hash.

Note that you cannot modify the values of a hash this way, because the returned list is just a copy. You need to use a hash slice for that, since it's lvaluable in a way that values() is not.

    for (values %hash)      { s/foo/bar/g }   # FAILS!
    for (@hash{keys %hash}) { s/foo/bar/g }   # ok

As a side effect, calling values() resets the HASH's internal iterator. See also keys(), each(), and sort().

Treats the string in EXPR as a vector of unsigned integers, and returns the value of the bit field specified by OFFSET. BITS specifies the number of bits that are reserved for each entry in the bit vector. This must be a power of two from 1 to 32. vec() may also be assigned to, in which case parentheses are needed to give the expression the correct precedence as in

    vec($image, $max_x * $x + $y, 8) = 3;

Vectors created with vec() can also be manipulated with the logical operators |, &, and ^, which will assume a bit vector operation is desired when both operands are strings. See perlop.

The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the string after each step. Note that this code works in the same way on big-endian or little-endian machines.

    my $foo = '';
    vec($foo,  0, 32) = 0x5065726C;     # 'Perl'
    vec($foo,  2, 16) = 0x5065;         # 'PerlPe'
    vec($foo,  3, 16) = 0x726C;         # 'PerlPerl'
    vec($foo,  8,  8) = 0x50;           # 'PerlPerlP'
    vec($foo,  9,  8) = 0x65;           # 'PerlPerlPe'
    vec($foo, 20,  4) = 2;              # 'PerlPerlPe'   . "\x02"
    vec($foo, 21,  4) = 7;              # 'PerlPerlPer'
                                        # 'r' is "\x72"
    vec($foo, 45,  2) = 3;              # 'PerlPerlPer'  . "\x0c"
    vec($foo, 93,  1) = 1;              # 'PerlPerlPer'  . "\x2c"
    vec($foo, 94,  1) = 1;              # 'PerlPerlPerl'
                                        # 'l' is "\x6c"

To transform a bit vector into a string or array of 0's and 1's, use these:

    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the *.

Behaves like the wait(2) system call on your system: it waits for a child process to terminate and returns the pid of the deceased process, or -1 if there are no child processes. The status is rketurned in $?. Note that a return value of -1 could mean that child processes are being automatically reaped, as described in perlipc.

Waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there is no such child process. On some systems, a value of 0 indicates that there are processes still running. The status is returned in $?. If you say

    use POSIX ":sys_wait_h";
    #...
    do { 
        $kid = waitpid(-1,&WNOHANG);
    } until $kid == -1;

then you can do a non-blocking wait for all pending zombie processes. Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) system calls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)

Note that on some systems, a return value of -1 could mean that child processes are being automatically reaped. See perlipc for details, and for other examples.

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).

    return unless defined wantarray;    # don't bother doing more
    my @a = complex_calculation();
    return wantarray ? @a : "@a";

Produces a message on STDERR just like die(), but doesn't exit or throw an exception.

If LIST is empty and $@ already contains a value (typically from a previous eval) that value is used after appending "\t...caught" to $@. This is useful for staying almost, but not entirely similar to die().

If $@ is empty then the string "Warning: Something's wrong" is used.

No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's responsibility to deal with the message as it sees fit (like, for instance, converting it into a die()). Most handlers must therefore make arrangements to actually display the warnings that they are not prepared to deal with, by calling warn() again in the handler. Note that this is quite safe and will not produce an endless loop, since __WARN__ hooks are not called from inside one.

You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call die() again to change it).

Using a __WARN__ handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:

    # wipe out *all* compile-time warnings
    BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
    my $foo = 10;
    my $foo = 20;          # no warning about duplicate my $foo,
                           # but hey, you asked for it!
    # no compile-time or run-time warnings before here
    $DOWARN = 1;

    # run-time warnings enabled after here
    warn "\$foo is alive and $foo!";     # does show up

See perlvar for details on setting %SIG entries, and for more examples. See the Carp module for other kinds of warnings using its carp() and cluck() functions.

Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the format associated with that file. By default the format for a file is the one having the same name as the filehandle, but the format for the current output channel (see the select() function) may be set explicitly by assigning the name of the format to the $~ variable.

Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed, a special top-of-page format is used to format the new page header, and then the record is written. By default the top-of-page format is the name of the filehandle with ``_TOP'' appended, but it may be dynamically set to the format of your choice by assigning the name to the $^ variable while the filehandle is selected. The number of lines remaining on the current page is in variable $-, which can be set to 0 to force a new page.

If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as STDOUT but may be changed by the select() operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see perlform.

Note that write is NOT the opposite of read(). Unfortunately.

The transliteration operator. Same as tr///. See perlop.

Linux Will Become Indistinguishable From Unix

Finally, Forrester predicts that by 2004, Linux and proprietary Unix will have so much in common that many binaries will run on either platform.

"IBM, HP and Sun are grafting Linux interoperability features onto upcoming releases of AIX, HP-UX and Solaris -- and engineers from SGI, HP and IBM are working on the Trillian project to solve Linux's scalability problems," the report concludes. 

[Jan 3, 2000] ZDNet Not a Monopoly, Marketing -- John C. Dvorak PC Magazine Opinions -- I believe that the autor is wrong and Microsoft does not control the PC as a platform. But Microsoft is a very influential player in this game (probably second after Intel) and here the article attract attention to this subtle fact.

Microsoft is not a monopoly; it just pretends to be one. In fact, what's gotten the company into trouble is its scheme to cover up the fact that the PC is a proprietary platform designed and owned by Microsoft. Windows is part of a proprietary platform and competes with other pure platforms, including Mac and Sun's Solaris. Why else would Scott McNealy be so obsessed with Microsoft?

Microsoft's model for marketing a proprietary PC (which I'll call the Microsoft Windows PC) is unusual, because Microsoft doesn't manufacture the hardware. The concept is similar to the fab-less semiconductor company. Microsoft does control the specifications of the computers and essentially outsources the manufacturing of the computers on a no-fee basis. This is a remarkably generous approach. The company expects that this generosity be rewarded with some modest amount of loyalty. Why should Microsoft put all this effort into the design (for example, PC-100 specifications) if a company is going to put Linux on the PC? Use a new legacy-free box or something else for that.

It's for marketing purposes that this proprietary Microsoft Windows PC platform is not portrayed as such. Microsoft is playing into the sentimental notion that the PC itself is an open platform that offers more choices. Third-party development and faux competition with Microsoft was encouraged to deceive the buying public.

With the PC, Microsoft took control of the platform immediately and is largely responsible for its development. The DOJ testimony that Microsoft was pushing Intel around reflects this fact. IBM got early credit for the platform, but when IBM rolled out its PS/2 machines and the Micro Channel, IBM found Microsoft owned the platform. Once Windows fully arrived, Microsoft took brutal control.

This way of looking at Microsoft and Windows explains everything, including Microsoft's recent whining — especially about innovation. To many of us Microsoft seems to have made no innovations that weren't borrowed or bought. But if you throw in the hardware part of the equation, you'll find things such as the USB port and other true innovations. Microsoft's annual WinHEC conference addresses the hardware aspect of the Windows platform. Why would such a conference even exist if Microsoft weren't in full control of hardware specifications?

Seeing the Windows PC as a proprietary platform also explains why Microsoft would want Netscape out of the picture. I'm certain that nobody at Netscape visualized the situation accurately. They obviously believed the Microsoft marketing nonsense and made bad assumptions. By siding with Sun and making noise about having a Net OS running on the Microsoft Windows PC hardware, Netscape violated the proprietary nature of the Microsoft Windows PC.

Microsoft owns the specifications and essentially licenses them on a no-fee basis, so the company feels no obligation to let other companies do whatever they want on the system, which would be like going into a fancy restaurant and setting up your own hot- dog stand. Once Microsoft decided to shut Netscape out, it had to go around and remind the gratis licensees (Dell, Compaq, and others) of their obligations as manufacturers of the Microsoft machine! These companies make money off Microsoft's hard work — at the behest of Microsoft. Seen this way, the Microsoft "threats" to pull the OS from any company bundling Netscape were reasonable.

The most laughable irony is that Microsoft has marketed around the fact that the Microsoft Windows PC is a proprietary computer. The marketing machine should have said: "Hey, this whole thing is nonsense, since we own a closed proprietary system. It was only for the purposes of marketing that we made it appear to be open, so we could sucker people into buying our software and fool them into believing they actually had a real choice. Marketing, marketing, marketing." The fact that other operating systems can run on the Microsoft Windows PC is actually amazing. But the fact that few people use these other systems is no surprise.

Seen in this light, Microsoft is no more a monopoly than any other company selling a single product in a complex marketplace. Unfortunately, the foundation of Microsoft's success — an overly complex marketing scheme — is what led to this recent humiliation.