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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).


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

Flag name Description
aarch64 AArch64 architecture
avr AVR 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


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


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) }}'

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.
tracing Build with support for runtime tracing.
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.