X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlport.pod;h=8d229d60b9ffad51d0bf5fe5c80c1f845a11e68d;hb=bfc3ae4fccf261f3ffcd77b5f339738dcd4ce290;hp=c977064ccdd5ce38d814dd4edf86bf88ca4d5a70;hpb=6d7381134aa8fec077b679a5dc9d49b6e324e1df;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlport.pod b/pod/perlport.pod index c977064..8d229d6 100644 --- a/pod/perlport.pod +++ b/pod/perlport.pod @@ -267,6 +267,13 @@ S perl can emulate Unix filenames with C as path separator, or go native and use C<.> for path separator and C<:> to signal filesystems and disk names. +Don't assume UNIX filesystem access semantics: that read, write, +and execute are all the permissions there are, and even if they exist, +that their semantics (for example what do r, w, and x mean on +a directory) are the UNIX ones. The various UNIX/POSIX compatibility +layers usually try to make interfaces like chmod() work, but sometimes +there simply is no good mapping. + If all this is intimidating, have no (well, maybe only a little) fear. There are modules that can help. The File::Spec modules provide methods to do the Right Thing on whatever platform happens @@ -311,30 +318,57 @@ the user to override the default location of the file. Don't assume a text file will end with a newline. They should, but people forget. -Do not have two files of the same name with different case, like -F and F, as many platforms have case-insensitive -filenames. Also, try not to have non-word characters (except for C<.>) -in the names, and keep them to the 8.3 convention, for maximum -portability, onerous a burden though this may appear. +Do not have two files or directories of the same name with different +case, like F and F, as many platforms have +case-insensitive (or at least case-forgiving) filenames. Also, try +not to have non-word characters (except for C<.>) in the names, and +keep them to the 8.3 convention, for maximum portability, onerous a +burden though this may appear. Likewise, when using the AutoSplit module, try to keep your functions to 8.3 naming and case-insensitive conventions; or, at the least, make it so the resulting files have a unique (case-insensitively) first 8 characters. -Whitespace in filenames is tolerated on most systems, but not all. +Whitespace in filenames is tolerated on most systems, but not all, +and even on systems where it might be tolerated, some utilities +might become confused by such whitespace. + Many systems (DOS, VMS) cannot have more than one C<.> in their filenames. Don't assume C<< > >> won't be the first character of a filename. -Always use C<< < >> explicitly to open a file for reading, -unless you want the user to be able to specify a pipe open. +Always use C<< < >> explicitly to open a file for reading, or even +better, use the three-arg version of open, unless you want the user to +be able to specify a pipe open. - open(FILE, "< $existing_file") or die $!; + open(FILE, '<', $existing_file) or die $!; If filenames might use strange characters, it is safest to open it with C instead of C. C is magic and can translate characters like C<< > >>, C<< < >>, and C<|>, which may be the wrong thing to do. (Sometimes, though, it's the right thing.) +Three-arg open can also help protect against this translation in cases +where it is undesirable. + +Don't use C<:> as a part of a filename since many systems use that for +their own semantics (MacOS Classic for separating pathname components, +many networking schemes and utilities for separating the nodename and +the pathname, and so on). For the same reasons, avoid C<@>, C<;> and +C<|>. + +The I as defined by ANSI C are + + a b c d e f g h i j k l m n o p q r t u v w x y z + A B C D E F G H I J K L M N O P Q R T U V W X Y Z + 0 1 2 3 4 5 6 7 8 9 + . _ - + +and the "-" shouldn't be the first character. If you want to be +hypercorrect, stay within the 8.3 naming convention (all the files and +directories have to be unique within one directory if their names are +lowercased and truncated to eight characters before the C<.>, if any, +and to three characters after the C<.>, if any). (And do not use +C<.>s in directory names.) =head2 System Interaction @@ -352,6 +386,25 @@ file already tied or opened; C or C it first. Don't open the same file more than once at a time for writing, as some operating systems put mandatory locks on such files. +Don't assume that write/modify permission on a directory gives the +right to add or delete files/directories in that directory. That is +filesystem specific: in some filesystems you need write/modify +permission also (or even just) in the file/directory itself. In some +filesystems (AFS, DFS) the permission to add/delete directory entries +is a completely separate permission. + +Don't assume that a single C completely gets rid of the file: +some filesystems (most notably the ones in VMS) have versioned +filesystems, and unlink() removes only the most recent one (it doesn't +remove all the versions because by default the native tools on those +platforms remove just the most recent version, too). The portable +idiom to remove all the versions of a file is + + 1 while unlink "file"; + +This will terminate if the file is undeleteable for some reason +(protected, not there, and so on). + Don't count on a specific environment variable existing in C<%ENV>. Don't count on C<%ENV> entries being case-sensitive, or even case-preserving. Don't try to clear %ENV by saying C<%ENV = ();>, or, @@ -404,6 +457,14 @@ simple, platform-independent mailing. The Unix System V IPC (C) is not available even on all Unix platforms. +Do not use either the bare result of C or +bare v-strings (such as C) to represent IPv4 addresses: +both forms just pack the four bytes into network order. That this +would be equal to the C language C struct (which is what the +socket code internally uses) is not guaranteed. To be portable use +the routines of the Socket extension, such as C, +C, and C. + The rule of thumb for portable code is: Do it all in portable Perl, or use a module (that may internally implement it with platform-specific code, but expose a common interface). @@ -468,15 +529,20 @@ to get what should be the proper value on any system. =head2 Character sets and character encoding -Assume little about character sets. Assume nothing about -numerical values (C, C) of characters. Do not -assume that the alphabetic characters are encoded contiguously (in -the numeric sense). Do not assume anything about the ordering of the -characters. The lowercase letters may come before or after the -uppercase letters; the lowercase and uppercase may be interlaced so -that both `a' and `A' come before `b'; the accented and other -international characters may be interlaced so that E comes -before `b'. +Assume very little about character sets. + +Assume nothing about numerical values (C, C) of characters. +Do not use explicit code point ranges (like \xHH-\xHH); use for +example symbolic character classes like C<[:print:]>. + +Do not assume that the alphabetic characters are encoded contiguously +(in the numeric sense). There may be gaps. + +Do not assume anything about the ordering of the characters. +The lowercase letters may come before or after the uppercase letters; +the lowercase and uppercase may be interlaced so that both `a' and `A' +come before `b'; the accented and other international characters may +be interlaced so that E comes before `b'. =head2 Internationalisation @@ -511,13 +577,31 @@ more efficient that the first. Most multi-user platforms provide basic levels of security, usually implemented at the filesystem level. Some, however, do -not--unfortunately. Thus the notion of user id, or "home" directory, +not-- unfortunately. Thus the notion of user id, or "home" directory, or even the state of being logged-in, may be unrecognizable on many platforms. If you write programs that are security-conscious, it is usually best to know what type of system you will be running under so that you can write code explicitly for that platform (or class of platforms). +Don't assume the UNIX filesystem access semantics: the operating +system or the filesystem may be using some ACL systems, which are +richer languages than the usual rwx. Even if the rwx exist, +their semantics might be different. + +(From security viewpoint testing for permissions before attempting to +do something is silly anyway: if one tries this, there is potential +for race conditions-- someone or something might change the +permissions between the permissions check and the actual operation. +Just try the operation.) + +Don't assume the UNIX user and group semantics: especially, don't +expect the C<< $< >> and C<< $> >> (or the C<$(> and C<$)>) to work +for switching identities (or memberships). + +Don't assume set-uid and set-gid semantics. (And even if you do, +think twice: set-uid and set-gid are a known can of security worms.) + =head2 Style For those times when it is necessary to have platform-specific code, @@ -532,7 +616,7 @@ often happens when tests spawn off other processes or call external programs to aid in the testing, or when (as noted above) the tests assume certain things about the filesystem and paths. Be careful not to depend on a specific output style for errors, such as when -checking C<$!> after an system call. Some platforms expect a certain +checking C<$!> after a system call. Some platforms expect a certain output format, and perl on those platforms may have been adjusted accordingly. Most specifically, don't anchor a regex when testing an error value. @@ -663,17 +747,22 @@ often assume nothing about their data. The C<$^O> variable and the C<$Config{archname}> values for various DOSish perls are as follows: - OS $^O $Config{'archname'} - -------------------------------------------- - MS-DOS dos - PC-DOS dos - OS/2 os2 - Windows 95 MSWin32 MSWin32-x86 - Windows 98 MSWin32 MSWin32-x86 - Windows NT MSWin32 MSWin32-x86 - Windows NT MSWin32 MSWin32-ALPHA - Windows NT MSWin32 MSWin32-ppc - Cygwin cygwin + OS $^O $Config{archname} ID Version + -------------------------------------------------------- + MS-DOS dos ? + PC-DOS dos ? + OS/2 os2 ? + Windows 3.1 ? ? 0 3 01 + Windows 95 MSWin32 MSWin32-x86 1 4 00 + Windows 98 MSWin32 MSWin32-x86 1 4 10 + Windows ME MSWin32 MSWin32-x86 1 ? + Windows NT MSWin32 MSWin32-x86 2 4 xx + Windows NT MSWin32 MSWin32-ALPHA 2 4 xx + Windows NT MSWin32 MSWin32-ppc 2 4 xx + Windows 2000 MSWin32 MSWin32-x86 2 5 xx + Windows XP MSWin32 MSWin32-x86 2 ? + Windows CE MSWin32 ? 3 + Cygwin cygwin ? The various MSWin32 Perl's can distinguish the OS they are running on via the value of the fifth element of the list returned from @@ -1384,6 +1473,17 @@ Implemented via Spawn. (VM/ESA) Does not automatically flush output handles on some platforms. (SunOS, Solaris, HP-UX) +=item exit EXPR + +=item exit + +Emulates UNIX exit() (which considers C to indicate an error) by +mapping the C<1> to SS$_ABORT (C<44>). This behavior may be overridden +with the pragma C. As with the CRTL's exit() +function, C is also mapped to an exit status of SS$_NORMAL +(C<1>); this mapping cannot be overridden. Any other argument to exit() +is used directly as Perl's exit status. (VMS) + =item fcntl FILEHANDLE,FUNCTION,SCALAR Not implemented. (Win32, VMS) @@ -1529,7 +1629,7 @@ Not implemented. (Plan9, Win32) =item getsockopt SOCKET,LEVEL,OPTNAME -Not implemented. (S, Plan9) +Not implemented. (Plan9) =item glob EXPR @@ -1615,11 +1715,11 @@ Not implemented. (Win32, VMS, S) =item select RBITS,WBITS,EBITS,TIMEOUT -Only implemented on sockets. (Win32) +Only implemented on sockets. (Win32, VMS) Only reliable on sockets. (S) -Note that the C form is generally portable. +Note that the C