What defines control CONDITIONAL compilation in Delphi for platform-specific code?

Core Platform Defines

Delphi automatically defines a mutually exclusive set of top-level platform symbols based on the active target platform. These are the primary controls for conditional compilation:

• MSWINDOWS — Windows 32-bit and 64-bit targets
• POSIX — All non-Windows platforms (macOS, iOS, Android, Linux)
• MACOS — macOS targets (Intel and Apple Silicon)
• IOS — iOS device and simulator targets
• ANDROID — Android targets
• LINUX — Linux 64-bit targets (x64 in Delphi 11.0–11.2; ARM64 added in 11.3)

Framework-level defines further distinguish the UI library: VCL for Windows-only VCL applications and FMX for FireMonkey cross-platform projects. The legacy CLX define (Kylix/Delphi 6–7) is obsolete after Delphi 2009.

Sub-Platform and CPU Defines (Delphi 10.4+ and 11)

Each platform target adds more specific symbols:

CONDITIONAIREDITOR

CONDITIONAIREDITOR is an IDE-internal define set only when the conditional compilation editor UI is active. It does not affect compiled output and must not be used in source code for platform logic. Relying on it causes unpredictable IDE behavior and is unsupported.

Custom Defines vs. Automatic Defines

Defines entered in Project Options → Compiler → Defines are additive with the automatic platform symbols. Conflicts occur only if a custom define explicitly overrides an automatic one (e.g., {$UNDEF MSWINDOWS} or manually defining MSWINDOWS on a non-Windows target), which breaks platform detection. The safe pattern is to only add custom symbols (e.g., MYFEATURE_ENABLED) and never redefine platform symbols.

Verification Steps

1. Create a test project for each target platform.
2. Build the project.
3. Open Project Options → Compiler → Defines to see the merged list, or run the compiler with --defines (e.g., dcciosarm64 --defines).
4. Optionally, insert {$IFDEF SYMBOL}{$MESSAGE 'DEFINE: SYMBOL'}{$ENDIF} in a unit to emit active defines during compilation.

Version Sensitivity

Define sets change between Delphi versions (10.4, 11.0, 11.3, 12). The RAD Studio documentation for your specific version and the installed System.SysUtils.inc/System.Types.inc in source/rtl/sys/ are the authoritative sources. For production code, verify the active defines on each targeted Delphi version.

One diagnostic detail needed: Which exact Delphi version (e.g., 11.0, 11.3, 12.0) and target platforms are you compiling for? The Linux ARM64 define (LINUXARM64) and iOS simulator CPU define differ across 11.x releases.