Compile-time flags¶

Compile-time flags are boolean values provided through the compiler via a macro method. They allow to conditionally include or exclude code based on compile time conditions.

There are several default flags provided by the compiler with information about compiler options and the target platform. User-provided flags are passed to the compiler, which allow them to be used as feature flags.

Querying flags¶

A flag is just a named identifier which is either set or not. The status can be queried from code via the macro method flag?. It receives the name of a flag as a string or symbol literal and returns a bool literal indicating the flag's state.

The following program shows the use of compile-time flags by printing the target OS family.

{% if flag?(:unix) %}
  puts "This program is compiled for a UNIX-like operating system"
{% elsif flag?(:windows) %}
  puts "This program is compiled for Windows"
{% else %}
  # Currently, all supported targets are either UNIX or Windows platforms, so
  # this branch is practically unreachable.
  puts "Compiling for some other operating system"
{% end %}

There's also the macro method host_flag? which returns whether a flag is set for the host platform, which can differ from the target platform (queried by flag?) during cross-compilation.

Compiler-provided flags¶

The compiler defines a couple of implicit flags. They describe either the target platform or compiler options.

Target platform flags¶

Platform-specific flags derive from the target triple. See Platform Support for a list of supported target platforms.

crystal --version shows the default target triple of the compiler. It can be changed with the --target option.

The flags in each of the following tables are mutually exclusive, except for those marked as (derived).

Architecture¶

The target architecture is the first component of the target triple.

Flag name	Description
`aarch64`	AArch64 architecture
`arm`	ARM architecture
`i386`	x86 architecture (32-bit)
`wasm32`	WebAssembly
`x86_64`	x86-64 architecture
`bits32` (derived)	32-bit architecture
`bits64` (derived)	64-bit architecture

Vendor¶

The vendor is the second component of the target triple. This is typically unused, so the most common vendor is unknown.

Flag name	Description
`macosx`	Apple
`portbld`	FreeBSD variant
`unknown`	Unknown vendor

Operating System¶

The operating system is derived from the third component of a the target triple.

Flag name	Description
`bsd` (derived)	BSD family (DragonFlyBSD, FreeBSD, NetBSD, OpenBSD)
`darwin`	Darwin (MacOS)
`dragonfly`	DragonFlyBSD
`freebsd`	FreeBSD
`linux`	Linux
`netbsd`	NetBSD
`openbsd`	OpenBSD
`solaris`	Solaris/illumos
`unix` (derived)	UNIX-like (BSD, Darwin, Linux, Solaris)
`windows`	Windows

ABI¶

The ABI is derived from the last component of the target triple.

Flag name	Description
`android`	Android (Bionic C runtime)
`armhf` (derived)	ARM EABI with hard float
`gnu`	GNU
`gnueabihf`	GNU EABI with hard float
`msvc`	Microsoft Visual C++
`musl`	musl
`wasi`	Web Assembly System Interface
`win32` (derived)	Windows API

Compiler options¶

The compiler sets these flags based on compiler configuration.

Flag name	Description
`release`	Compiler operates in release mode (`--release` or `-O3 --single-module` CLI option)
`debug`	Compiler generates debug symbols (without `--no-debug` CLI option)
`static`	Compiler creates a statically linked executable (`--static` CLI option)
`docs`	Code is processed to generate API docs (`crystal docs` command)
`interpreted`	Running in the interpreter (`crystal i`)

User-provided flags¶

User-provided flags are not defined automatically. They can be passed to the compiler via the --define or -D command line options.

These flags usually enable certain features which activate breaking new or legacy functionality, a preview for a new feature, or entirely alternative behaviour (e.g. for debugging purposes).

$ crystal eval -Dfoo 'puts {{ flag?(:foo) }}'
true

Stdlib features¶

These flags enable or disable features in the standard library when building a Crystal program.

Flag name	Description
`gc_none`	Disables garbage collection (#5314)
`debug_raise`	Debugging flag for `raise` logic. Prints the backtrace before raising.
`preview_mt`	Enables multithreading preview. Introduced in 0.28.0 (#7546)
`skip_crystal_compiler_rt`	Exclude Crystal's native `compiler-rt` implementation.
`use_libiconv`	Use `libiconv` instead of the `iconv` system library
`use_pcre2`	Use PCRE2 as regex engine (instead of legacy PCRE). Introduced in 1.7.0.
`use_pcre`	Use PCRE as regex engine (instead of PCRE2). Introduced in 1.8.0.
`win7`	Use Win32 WinNT API for Windows 7
`without_iconv`	Do not link `iconv`/`libiconv`
`without_openssl`	Build without OpenSSL support
`without_zlib`	Build without Zlib support

Compiler features¶

These flags enable or disable compiler features when building a Crystal program.

Flag name	Description
`no_number_autocast`	Will not autocast numeric expressions, only literals
`no_restrictions_augmenter`	Disable enhanced restrictions augmenter. Introduced in 1.5 (#12103).
`preview_dll`	Enable dynamic linking on Windows; experimental
`preview_overload_order`	Enable more robust ordering between def overloads. Introduced in 1.6 (#10711).
`preview_win32_delay_load`	Delay-load all DLLs on Windows; experimental
`strict_multi_assign`	Enable strict semantics for one-to-many assignment. Introduced in 1.3.0 (#11145, #11545)

Compiler build features¶

These flags enable or disable features when building the Crystal compiler.

Flag name	Description
`without_ffi`	Build the compiler without `libffi`
`without_interpreter`	Build the compiler without interpreter support
`without_playground`	Build the compiler without playground (`crystal play`)
`i_know_what_im_doing`	Safety guard against involuntarily building the compiler

User code features¶

Custom flags can be freely used in user code as long as they don't collide with compiler-provided flags or other user-defined flags. When using a flag specific to a shard, it's recommended to use the shard name as a prefix.