perlfunc.1

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

\&\f(CW\*(C`binmode\*(C'\fR, \f(CW\*(C`close\*(C'\fR, \f(CW\*(C`closedir\*(C'\fR, \f(CW\*(C`dbmclose\*(C'\fR, \f(CW\*(C`dbmopen\*(C'\fR, \f(CW\*(C`die\*(C'\fR, \f(CW\*(C`eof\*(C'\fR,
\&\f(CW\*(C`fileno\*(C'\fR, \f(CW\*(C`flock\*(C'\fR, \f(CW\*(C`format\*(C'\fR, \f(CW\*(C`getc\*(C'\fR, \f(CW\*(C`print\*(C'\fR, \f(CW\*(C`printf\*(C'\fR, \f(CW\*(C`read\*(C'\fR,
\&\f(CW\*(C`readdir\*(C'\fR, \f(CW\*(C`rewinddir\*(C'\fR, \f(CW\*(C`say\*(C'\fR, \f(CW\*(C`seek\*(C'\fR, \f(CW\*(C`seekdir\*(C'\fR, \f(CW\*(C`select\*(C'\fR, \f(CW\*(C`syscall\*(C'\fR,
\&\f(CW\*(C`sysread\*(C'\fR, \f(CW\*(C`sysseek\*(C'\fR, \f(CW\*(C`syswrite\*(C'\fR, \f(CW\*(C`tell\*(C'\fR, \f(CW\*(C`telldir\*(C'\fR, \f(CW\*(C`truncate\*(C'\fR,
\&\f(CW\*(C`warn\*(C'\fR, \f(CW\*(C`write\*(C'\fR
.IP "Functions for fixed length data or records" 4
.IX Item "Functions for fixed length data or records"
\&\f(CW\*(C`pack\*(C'\fR, \f(CW\*(C`read\*(C'\fR, \f(CW\*(C`syscall\*(C'\fR, \f(CW\*(C`sysread\*(C'\fR, \f(CW\*(C`syswrite\*(C'\fR, \f(CW\*(C`unpack\*(C'\fR, \f(CW\*(C`vec\*(C'\fR
.IP "Functions for filehandles, files, or directories" 4
.IX Xref "file filehandle directory pipe link symlink"
.IX Item "Functions for filehandles, files, or directories"
\&\f(CW\*(C`\-\f(CIX\f(CW\*(C'\fR, \f(CW\*(C`chdir\*(C'\fR, \f(CW\*(C`chmod\*(C'\fR, \f(CW\*(C`chown\*(C'\fR, \f(CW\*(C`chroot\*(C'\fR, \f(CW\*(C`fcntl\*(C'\fR, \f(CW\*(C`glob\*(C'\fR,
\&\f(CW\*(C`ioctl\*(C'\fR, \f(CW\*(C`link\*(C'\fR, \f(CW\*(C`lstat\*(C'\fR, \f(CW\*(C`mkdir\*(C'\fR, \f(CW\*(C`open\*(C'\fR, \f(CW\*(C`opendir\*(C'\fR,
\&\f(CW\*(C`readlink\*(C'\fR, \f(CW\*(C`rename\*(C'\fR, \f(CW\*(C`rmdir\*(C'\fR, \f(CW\*(C`stat\*(C'\fR, \f(CW\*(C`symlink\*(C'\fR, \f(CW\*(C`sysopen\*(C'\fR,
\&\f(CW\*(C`umask\*(C'\fR, \f(CW\*(C`unlink\*(C'\fR, \f(CW\*(C`utime\*(C'\fR
.IP "Keywords related to the control flow of your Perl program" 4
.IX Xref "control flow"
.IX Item "Keywords related to the control flow of your Perl program"
\&\f(CW\*(C`caller\*(C'\fR, \f(CW\*(C`continue\*(C'\fR, \f(CW\*(C`die\*(C'\fR, \f(CW\*(C`do\*(C'\fR, \f(CW\*(C`dump\*(C'\fR, \f(CW\*(C`eval\*(C'\fR, \f(CW\*(C`exit\*(C'\fR,
\&\f(CW\*(C`goto\*(C'\fR, \f(CW\*(C`last\*(C'\fR, \f(CW\*(C`next\*(C'\fR, \f(CW\*(C`redo\*(C'\fR, \f(CW\*(C`return\*(C'\fR, \f(CW\*(C`sub\*(C'\fR, \f(CW\*(C`wantarray\*(C'\fR
.IP "Keywords related to switch" 4
.IX Item "Keywords related to switch"
\&\f(CW\*(C`break\*(C'\fR, \f(CW\*(C`continue\*(C'\fR, \f(CW\*(C`given\*(C'\fR, \f(CW\*(C`when\*(C'\fR, \f(CW\*(C`default\*(C'\fR
.Sp
(These are only available if you enable the \*(L"switch\*(R" feature.
See feature and \*(L"Switch statements\*(R" in perlsyn.)
.IP "Keywords related to scoping" 4
.IX Item "Keywords related to scoping"
\&\f(CW\*(C`caller\*(C'\fR, \f(CW\*(C`import\*(C'\fR, \f(CW\*(C`local\*(C'\fR, \f(CW\*(C`my\*(C'\fR, \f(CW\*(C`our\*(C'\fR, \f(CW\*(C`state\*(C'\fR, \f(CW\*(C`package\*(C'\fR,
\&\f(CW\*(C`use\*(C'\fR
.Sp
(\f(CW\*(C`state\*(C'\fR is only available if the \*(L"state\*(R" feature is enabled. See
feature.)
.IP "Miscellaneous functions" 4
.IX Item "Miscellaneous functions"
\&\f(CW\*(C`defined\*(C'\fR, \f(CW\*(C`dump\*(C'\fR, \f(CW\*(C`eval\*(C'\fR, \f(CW\*(C`formline\*(C'\fR, \f(CW\*(C`local\*(C'\fR, \f(CW\*(C`my\*(C'\fR, \f(CW\*(C`our\*(C'\fR,
\&\f(CW\*(C`reset\*(C'\fR, \f(CW\*(C`scalar\*(C'\fR, \f(CW\*(C`state\*(C'\fR, \f(CW\*(C`undef\*(C'\fR, \f(CW\*(C`wantarray\*(C'\fR
.IP "Functions for processes and process groups" 4
.IX Xref "process pid process id"
.IX Item "Functions for processes and process groups"
\&\f(CW\*(C`alarm\*(C'\fR, \f(CW\*(C`exec\*(C'\fR, \f(CW\*(C`fork\*(C'\fR, \f(CW\*(C`getpgrp\*(C'\fR, \f(CW\*(C`getppid\*(C'\fR, \f(CW\*(C`getpriority\*(C'\fR, \f(CW\*(C`kill\*(C'\fR,
\&\f(CW\*(C`pipe\*(C'\fR, \f(CW\*(C`qx//\*(C'\fR, \f(CW\*(C`setpgrp\*(C'\fR, \f(CW\*(C`setpriority\*(C'\fR, \f(CW\*(C`sleep\*(C'\fR, \f(CW\*(C`system\*(C'\fR,
\&\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`wait\*(C'\fR, \f(CW\*(C`waitpid\*(C'\fR
.IP "Keywords related to perl modules" 4
.IX Xref "module"
.IX Item "Keywords related to perl modules"
\&\f(CW\*(C`do\*(C'\fR, \f(CW\*(C`import\*(C'\fR, \f(CW\*(C`no\*(C'\fR, \f(CW\*(C`package\*(C'\fR, \f(CW\*(C`require\*(C'\fR, \f(CW\*(C`use\*(C'\fR
.IP "Keywords related to classes and object-orientation" 4
.IX Xref "object class package"
.IX Item "Keywords related to classes and object-orientation"
\&\f(CW\*(C`bless\*(C'\fR, \f(CW\*(C`dbmclose\*(C'\fR, \f(CW\*(C`dbmopen\*(C'\fR, \f(CW\*(C`package\*(C'\fR, \f(CW\*(C`ref\*(C'\fR, \f(CW\*(C`tie\*(C'\fR, \f(CW\*(C`tied\*(C'\fR,
\&\f(CW\*(C`untie\*(C'\fR, \f(CW\*(C`use\*(C'\fR
.IP "Low-level socket functions" 4
.IX Xref "socket sock"
.IX Item "Low-level socket functions"
\&\f(CW\*(C`accept\*(C'\fR, \f(CW\*(C`bind\*(C'\fR, \f(CW\*(C`connect\*(C'\fR, \f(CW\*(C`getpeername\*(C'\fR, \f(CW\*(C`getsockname\*(C'\fR,
\&\f(CW\*(C`getsockopt\*(C'\fR, \f(CW\*(C`listen\*(C'\fR, \f(CW\*(C`recv\*(C'\fR, \f(CW\*(C`send\*(C'\fR, \f(CW\*(C`setsockopt\*(C'\fR, \f(CW\*(C`shutdown\*(C'\fR,
\&\f(CW\*(C`socket\*(C'\fR, \f(CW\*(C`socketpair\*(C'\fR
.IP "System V interprocess communication functions" 4
.IX Xref "IPC System V semaphore shared memory memory message"
.IX Item "System V interprocess communication functions"
\&\f(CW\*(C`msgctl\*(C'\fR, \f(CW\*(C`msgget\*(C'\fR, \f(CW\*(C`msgrcv\*(C'\fR, \f(CW\*(C`msgsnd\*(C'\fR, \f(CW\*(C`semctl\*(C'\fR, \f(CW\*(C`semget\*(C'\fR, \f(CW\*(C`semop\*(C'\fR,
\&\f(CW\*(C`shmctl\*(C'\fR, \f(CW\*(C`shmget\*(C'\fR, \f(CW\*(C`shmread\*(C'\fR, \f(CW\*(C`shmwrite\*(C'\fR
.IP "Fetching user and group info" 4
.IX Xref "user group password uid gid passwd  etc passwd"
.IX Item "Fetching user and group info"
\&\f(CW\*(C`endgrent\*(C'\fR, \f(CW\*(C`endhostent\*(C'\fR, \f(CW\*(C`endnetent\*(C'\fR, \f(CW\*(C`endpwent\*(C'\fR, \f(CW\*(C`getgrent\*(C'\fR,
\&\f(CW\*(C`getgrgid\*(C'\fR, \f(CW\*(C`getgrnam\*(C'\fR, \f(CW\*(C`getlogin\*(C'\fR, \f(CW\*(C`getpwent\*(C'\fR, \f(CW\*(C`getpwnam\*(C'\fR,
\&\f(CW\*(C`getpwuid\*(C'\fR, \f(CW\*(C`setgrent\*(C'\fR, \f(CW\*(C`setpwent\*(C'\fR
.IP "Fetching network info" 4
.IX Xref "network protocol host hostname IP address service"
.IX Item "Fetching network info"
\&\f(CW\*(C`endprotoent\*(C'\fR, \f(CW\*(C`endservent\*(C'\fR, \f(CW\*(C`gethostbyaddr\*(C'\fR, \f(CW\*(C`gethostbyname\*(C'\fR,
\&\f(CW\*(C`gethostent\*(C'\fR, \f(CW\*(C`getnetbyaddr\*(C'\fR, \f(CW\*(C`getnetbyname\*(C'\fR, \f(CW\*(C`getnetent\*(C'\fR,
\&\f(CW\*(C`getprotobyname\*(C'\fR, \f(CW\*(C`getprotobynumber\*(C'\fR, \f(CW\*(C`getprotoent\*(C'\fR,
\&\f(CW\*(C`getservbyname\*(C'\fR, \f(CW\*(C`getservbyport\*(C'\fR, \f(CW\*(C`getservent\*(C'\fR, \f(CW\*(C`sethostent\*(C'\fR,
\&\f(CW\*(C`setnetent\*(C'\fR, \f(CW\*(C`setprotoent\*(C'\fR, \f(CW\*(C`setservent\*(C'\fR
.IP "Time-related functions" 4
.IX Xref "time date"
.IX Item "Time-related functions"
\&\f(CW\*(C`gmtime\*(C'\fR, \f(CW\*(C`localtime\*(C'\fR, \f(CW\*(C`time\*(C'\fR, \f(CW\*(C`times\*(C'\fR
.IP "Functions new in perl5" 4
.IX Xref "perl5"
.IX Item "Functions new in perl5"
\&\f(CW\*(C`abs\*(C'\fR, \f(CW\*(C`bless\*(C'\fR, \f(CW\*(C`break\*(C'\fR, \f(CW\*(C`chomp\*(C'\fR, \f(CW\*(C`chr\*(C'\fR, \f(CW\*(C`continue\*(C'\fR, \f(CW\*(C`default\*(C'\fR, 
\&\f(CW\*(C`exists\*(C'\fR, \f(CW\*(C`formline\*(C'\fR, \f(CW\*(C`given\*(C'\fR, \f(CW\*(C`glob\*(C'\fR, \f(CW\*(C`import\*(C'\fR, \f(CW\*(C`lc\*(C'\fR, \f(CW\*(C`lcfirst\*(C'\fR,
\&\f(CW\*(C`lock\*(C'\fR, \f(CW\*(C`map\*(C'\fR, \f(CW\*(C`my\*(C'\fR, \f(CW\*(C`no\*(C'\fR, \f(CW\*(C`our\*(C'\fR, \f(CW\*(C`prototype\*(C'\fR, \f(CW\*(C`qr//\*(C'\fR, \f(CW\*(C`qw//\*(C'\fR, \f(CW\*(C`qx//\*(C'\fR,
\&\f(CW\*(C`readline\*(C'\fR, \f(CW\*(C`readpipe\*(C'\fR, \f(CW\*(C`ref\*(C'\fR, \f(CW\*(C`sub\*(C'\fR*, \f(CW\*(C`sysopen\*(C'\fR, \f(CW\*(C`tie\*(C'\fR, \f(CW\*(C`tied\*(C'\fR, \f(CW\*(C`uc\*(C'\fR,
\&\f(CW\*(C`ucfirst\*(C'\fR, \f(CW\*(C`untie\*(C'\fR, \f(CW\*(C`use\*(C'\fR, \f(CW\*(C`when\*(C'\fR
.Sp
* \- \f(CW\*(C`sub\*(C'\fR was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.
.IP "Functions obsoleted in perl5" 4
.IX Item "Functions obsoleted in perl5"
\&\f(CW\*(C`dbmclose\*(C'\fR, \f(CW\*(C`dbmopen\*(C'\fR
.Sh "Portability"
.IX Xref "portability Unix portable"
.IX Subsection "Portability"
Perl was born in Unix and can therefore access all common Unix
system calls.  In non-Unix environments, the functionality of some
Unix system calls may not be available, or details of the available
functionality may differ slightly.  The Perl functions affected
by this are:
.PP
\&\f(CW\*(C`\-X\*(C'\fR, \f(CW\*(C`binmode\*(C'\fR, \f(CW\*(C`chmod\*(C'\fR, \f(CW\*(C`chown\*(C'\fR, \f(CW\*(C`chroot\*(C'\fR, \f(CW\*(C`crypt\*(C'\fR,
\&\f(CW\*(C`dbmclose\*(C'\fR, \f(CW\*(C`dbmopen\*(C'\fR, \f(CW\*(C`dump\*(C'\fR, \f(CW\*(C`endgrent\*(C'\fR, \f(CW\*(C`endhostent\*(C'\fR,
\&\f(CW\*(C`endnetent\*(C'\fR, \f(CW\*(C`endprotoent\*(C'\fR, \f(CW\*(C`endpwent\*(C'\fR, \f(CW\*(C`endservent\*(C'\fR, \f(CW\*(C`exec\*(C'\fR,
\&\f(CW\*(C`fcntl\*(C'\fR, \f(CW\*(C`flock\*(C'\fR, \f(CW\*(C`fork\*(C'\fR, \f(CW\*(C`getgrent\*(C'\fR, \f(CW\*(C`getgrgid\*(C'\fR, \f(CW\*(C`gethostbyname\*(C'\fR,
\&\f(CW\*(C`gethostent\*(C'\fR, \f(CW\*(C`getlogin\*(C'\fR, \f(CW\*(C`getnetbyaddr\*(C'\fR, \f(CW\*(C`getnetbyname\*(C'\fR, \f(CW\*(C`getnetent\*(C'\fR,
\&\f(CW\*(C`getppid\*(C'\fR, \f(CW\*(C`getpgrp\*(C'\fR, \f(CW\*(C`getpriority\*(C'\fR, \f(CW\*(C`getprotobynumber\*(C'\fR,
\&\f(CW\*(C`getprotoent\*(C'\fR, \f(CW\*(C`getpwent\*(C'\fR, \f(CW\*(C`getpwnam\*(C'\fR, \f(CW\*(C`getpwuid\*(C'\fR,
\&\f(CW\*(C`getservbyport\*(C'\fR, \f(CW\*(C`getservent\*(C'\fR, \f(CW\*(C`getsockopt\*(C'\fR, \f(CW\*(C`glob\*(C'\fR, \f(CW\*(C`ioctl\*(C'\fR,
\&\f(CW\*(C`kill\*(C'\fR, \f(CW\*(C`link\*(C'\fR, \f(CW\*(C`lstat\*(C'\fR, \f(CW\*(C`msgctl\*(C'\fR, \f(CW\*(C`msgget\*(C'\fR, \f(CW\*(C`msgrcv\*(C'\fR,
\&\f(CW\*(C`msgsnd\*(C'\fR, \f(CW\*(C`open\*(C'\fR, \f(CW\*(C`pipe\*(C'\fR, \f(CW\*(C`readlink\*(C'\fR, \f(CW\*(C`rename\*(C'\fR, \f(CW\*(C`select\*(C'\fR, \f(CW\*(C`semctl\*(C'\fR,
\&\f(CW\*(C`semget\*(C'\fR, \f(CW\*(C`semop\*(C'\fR, \f(CW\*(C`setgrent\*(C'\fR, \f(CW\*(C`sethostent\*(C'\fR, \f(CW\*(C`setnetent\*(C'\fR,
\&\f(CW\*(C`setpgrp\*(C'\fR, \f(CW\*(C`setpriority\*(C'\fR, \f(CW\*(C`setprotoent\*(C'\fR, \f(CW\*(C`setpwent\*(C'\fR,
\&\f(CW\*(C`setservent\*(C'\fR, \f(CW\*(C`setsockopt\*(C'\fR, \f(CW\*(C`shmctl\*(C'\fR, \f(CW\*(C`shmget\*(C'\fR, \f(CW\*(C`shmread\*(C'\fR,
\&\f(CW\*(C`shmwrite\*(C'\fR, \f(CW\*(C`socket\*(C'\fR, \f(CW\*(C`socketpair\*(C'\fR,
\&\f(CW\*(C`stat\*(C'\fR, \f(CW\*(C`symlink\*(C'\fR, \f(CW\*(C`syscall\*(C'\fR, \f(CW\*(C`sysopen\*(C'\fR, \f(CW\*(C`system\*(C'\fR,
\&\f(CW\*(C`times\*(C'\fR, \f(CW\*(C`truncate\*(C'\fR, \f(CW\*(C`umask\*(C'\fR, \f(CW\*(C`unlink\*(C'\fR,
\&\f(CW\*(C`utime\*(C'\fR, \f(CW\*(C`wait\*(C'\fR, \f(CW\*(C`waitpid\*(C'\fR
.PP
For more information about the portability of these functions, see
perlport and other available platform-specific documentation.
.Sh "Alphabetical Listing of Perl Functions"
.IX Subsection "Alphabetical Listing of Perl Functions"
.IP "\-X \s-1FILEHANDLE\s0" 8
.IX Xref "-r -w -x -o -R -W -X -O -e -z -s -f -d -l -p -S -b -c -t -u -g -k -T -B -M -A -C"
.IX Item "-X FILEHANDLE"
.PD 0
.IP "\-X \s-1EXPR\s0" 8
.IX Item "-X EXPR"
.IP "\-X \s-1DIRHANDLE\s0" 8
.IX Item "-X DIRHANDLE"
.IP "\-X" 8
.IX Item "-X"
.PD
A file test, where X is one of the letters listed below.  This unary
operator takes one argument, either a filename, a filehandle, or a dirhandle, 
and tests the associated file to see if something is true about it.  If the
argument is omitted, tests \f(CW$_\fR, except for \f(CW\*(C`\-t\*(C'\fR, which tests \s-1STDIN\s0.
Unless otherwise documented, it returns \f(CW1\fR for true and \f(CW\*(Aq\*(Aq\fR for false, or
the undefined value if the file doesn't exist.  Despite the funny
names, precedence is the same as any other named unary operator.  The
operator may be any of:
.Sp
.Vb 4
\&    \-r  File is readable by effective uid/gid.
\&    \-w  File is writable by effective uid/gid.
\&    \-x  File is executable by effective uid/gid.
\&    \-o  File is owned by effective uid.
\&
\&    \-R  File is readable by real uid/gid.
\&    \-W  File is writable by real uid/gid.
\&    \-X  File is executable by real uid/gid.
\&    \-O  File is owned by real uid.
\&
\&    \-e  File exists.
\&    \-z  File has zero size (is empty).
\&    \-s  File has nonzero size (returns size in bytes).
\&
\&    \-f  File is a plain file.
\&    \-d  File is a directory.
\&    \-l  File is a symbolic link.
\&    \-p  File is a named pipe (FIFO), or Filehandle is a pipe.
\&    \-S  File is a socket.
\&    \-b  File is a block special file.
\&    \-c  File is a character special file.
\&    \-t  Filehandle is opened to a tty.
\&
\&    \-u  File has setuid bit set.
\&    \-g  File has setgid bit set.
\&    \-k  File has sticky bit set.
\&
\&    \-T  File is an ASCII text file (heuristic guess).
\&    \-B  File is a "binary" file (opposite of \-T).
\&
\&    \-M  Script start time minus file modification time, in days.
\&    \-A  Same for access time.
\&    \-C  Same for inode change time (Unix, may differ for other platforms)
.Ve
.Sp
Example:
.Sp
.Vb 5
\&    while (<>) {
\&        chomp;
\&        next unless \-f $_;      # ignore specials
\&        #...
\&    }
.Ve
.Sp
The interpretation of the file permission operators \f(CW\*(C`\-r\*(C'\fR, \f(CW\*(C`\-R\*(C'\fR,
\&\f(CW\*(C`\-w\*(C'\fR, \f(CW\*(C`\-W\*(C'\fR, \f(CW\*(C`\-x\*(C'\fR, and \f(CW\*(C`\-X\*(C'\fR is by default based solely on the mode
of the file and the uids and gids of the user.  There may be other
reasons you can't actually read, write, or execute the file: for
example network filesystem access controls, ACLs (access control lists),
read-only filesystems, and unrecognized executable formats.  Note
that the use of these six specific operators to verify if some operation
is possible is usually a mistake, because it may be open to race
conditions.
.Sp
Also note that, for the superuser on the local filesystems, the \f(CW\*(C`\-r\*(C'\fR,
\&\f(CW\*(C`\-R\*(C'\fR, \f(CW\*(C`\-w\*(C'\fR, and \f(CW\*(C`\-W\*(C'\fR tests always return 1, and \f(CW\*(C`\-x\*(C'\fR and \f(CW\*(C`\-X\*(C'\fR return 1
if any execute bit is set in the mode.  Scripts run by the superuser
may thus need to do a \fIstat()\fR to determine the actual mode of the file,
or temporarily set their effective uid to something else.
.Sp
If you are using ACLs, there is a pragma called \f(CW\*(C`filetest\*(C'\fR that may
produce more accurate results than the bare \fIstat()\fR mode bits.
When under the \f(CW\*(C`use filetest \*(Aqaccess\*(Aq\*(C'\fR the above-mentioned filetests
will test whether the permission can (not) be granted using the
\&\fIaccess()\fR family of system calls.  Also note that the \f(CW\*(C`\-x\*(C'\fR and \f(CW\*(C`\-X\*(C'\fR may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs).  This strangeness is
due to the underlying system calls' definitions. Note also that, due to
the implementation of \f(CW\*(C`use filetest \*(Aqaccess\*(Aq\*(C'\fR, the \f(CW\*(C`_\*(C'\fR special
filehandle won't cache the results of the file tests when this pragma is
in effect.  Read the documentation for the \f(CW\*(C`filetest\*(C'\fR pragma for more
information.
.Sp
Note that \f(CW\*(C`\-s/a/b/\*(C'\fR does not do a negated substitution.  Saying
\&\f(CW\*(C`\-exp($foo)\*(C'\fR still works as expected, however\*(--only single letters
following a minus are interpreted as file tests.
.Sp
The \f(CW\*(C`\-T\*(C'\fR and \f(CW\*(C`\-B\*(C'\fR switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many strange characters (>30%)
are found, it's a \f(CW\*(C`\-B\*(C'\fR file; otherwise it's a \f(CW\*(C`\-T\*(C'\fR file.  Also, any file
containing null in the first block is considered a binary file.  If \f(CW\*(C`\-T\*(C'\fR
or \f(CW\*(C`\-B\*(C'\fR is used on a filehandle, the current \s-1IO\s0 buffer is examined
rather than the first block.  Both \f(CW\*(C`\-T\*(C'\fR and \f(CW\*(C`\-B\*(C'\fR return true on a null
file, or a file at \s-1EOF\s0 when testing a filehandle.  Because you have to
read a file to do the \f(CW\*(C`\-T\*(C'\fR test, on most occasions you want to use a \f(CW\*(C`\-f\*(C'\fR
against the file first, as in \f(CW\*(C`next unless \-f $file && \-T $file\*(C'\fR.
.Sp
If any of the file tests (or either the \f(CW\*(C`stat\*(C'\fR or \f(CW\*(C`lstat\*(C'\fR operators) are given
the special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call.  (This doesn't work with \f(CW\*(C`\-t\*(C'\fR, and you need to remember
that \fIlstat()\fR and \f(CW\*(C`\-l\*(C'\fR will leave values in the stat structure for the
symbolic link, not the real file.)  (Also, if the stat buffer was filled by
an \f(CW\*(C`lstat\*(C'\fR call, \f(CW\*(C`\-T\*(C'\fR and \f(CW\*(C`\-B\*(C'\fR will reset it with the results of \f(CW\*(C`stat _\*(C'\fR).
Example:
.Sp
.Vb 1
\&    print "Can do.\en" if \-r $a || \-w _ || \-x _;
\&
\&    stat($filename);
\&    print "Readable\en" if \-r _;
\&    print "Writable\en" if \-w _;
\&    print "Executable\en" if \-x _;
\&    print "Setuid\en" if \-u _;
\&    print "Setgid\en" if \-g _;
\&    print "Sticky\en" if \-k _;
\&    print "Text\en" if \-T _;
\&    print "Binary\en" if \-B _;
.Ve
.Sp
As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
test operators, in a way that \f(CW\*(C`\-f \-w \-x $file\*(C'\fR is equivalent to
\&\f(CW\*(C`\-x $file && \-w _ && \-f _\*(C'\fR. (This is only syntax fancy: if you use
the return value of \f(CW\*(C`\-f $file\*(C'\fR as an argument to another filetest
operator, no special magic will happen.)
.IP "abs \s-1VALUE\s0" 8
.IX Xref "abs absolute"
.IX Item "abs VALUE"