7.4 Writing portable code

Free Pascal is designed to be cross-platform. This means that the basic RTL units are usable on all platforms, and the compiler behaves the same on all platforms (as far as possible). The Object Pascal language is the same on all platforms. Nevertheless, FPC comes with a lot of units that are not portable, but provide access to all possibilities that a platform provides.

The following are some guidelines to consider when writing portable code:

The system unit contains some constants which describe file access on a system:

AllFilesMask
a file mask that will return all files in a directory. This is * on Unix-like platforms, and *.* on dos and windows like platforms.
LineEnding
A character or string which describes the end-of-line marker used on the current platform. Commonly, this is one of #10, #13#10 or #13.
LFNSupport
A boolean that indicates whether the system supports long filenames (i.e. is not limited to MS-DOS 8.3 filenames).
DirectorySeparator
The character which acts as a separator between directory parts of a path.
DriveSeparator
For systems that support drive letters, this is the character that is used to separate the drive indication from the path.
PathSeparator
The character used to separate items in a list (notably, a PATH).
maxExitCode
The maximum value for a process exitcode.
MaxPathLen
The maximum length of a filename, including a path.
FileNameCaseSensitive
A boolean that indicates whether filenames are handled case sensitively.
UnusedHandle
A value used to indicate an unused/invalid file handle.
StdInputHandle
The value of the standard input file handle. This is not always 0 (zero), as is commonly the case on Unices.
StdOutputHandle
The value of the standard output file handle. This is not always 1, as is commonly the case on Unices.
StdErrorHandle
The value of the standard diagnostics output file handle. This is not always 2, as is commonly the case on Unices.
CtrlZMarksEOF
A boolean that indicates whether the #26 character marks the end of a file (an old MS-DOS convention).

To ease writing portable filesystem code, the Free Pascal file routines in the system unit and sysutils unit treat the common directory separator characters (/ and \) as equivalent. That means that if you use / on a Windows system, it will be transformed to a backslash, and vice versa.

This feature is controlled by 2 (pre-initialized) variables in the system unit:

AllowDirectorySeparators
A set of characters which, when used in filenames, are treated as directory separators. They are transformed to the DirectorySeparator character.
AllowDriveSeparators
A set of characters which, when used in filenames, are treated as drive separator characters. They are transformed to the DriveSeparator character.