rt.config - D Programming Language
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# rt.config

Configuration options for druntime.
The default way to configure the runtime is by passing command line arguments starting with --DRT- and followed by the option name, e.g. --DRT-gcopt to configure the GC. When command line parsing is enabled, command line options starting with --DRT- are filtered out before calling main, so the program will not see them. They are still available via rt_args().
Configuration via the command line can be disabled by declaring a variable for the linker to pick up before using it's default from the runtime:

    extern(C) __gshared bool rt_cmdline_enabled = false;

Likewise, declare a boolean rt_envvars_enabled to enable configuration via the environment variable DRT_ followed by the option name, e.g. DRT_GCOPT:

    extern(C) __gshared bool rt_envvars_enabled = true;

Setting default configuration properties in the executable can be done by specifying an array of options named rt_options:

    extern(C) __gshared string[] rt_options = [ "gcopt=precise:1 profile:1"];

Evaluation order of options is rt_options, then environment variables, then command line arguments, i.e. if command line arguments are not disabled, they can override options specified through the environment or embedded in the executable.
License:
Distributed under the Boost Software License 1.0. (See accompanying file LICENSE)
Authors:
Rainer Schuetze

Source rt/config.d

nothrow @nogc string `rt_configOption`(string `opt`, scope rt_configCallBack `dg` = null, bool `reverse` = false);
  get a druntime config option using standard configuration options opt name of the option to retrieve dg if non-null, passes the option through this delegate and only returns its return value if non-null reverse reverse the default processing order cmdline/envvar/rt_options to allow overwriting settings in the delegate with values from higher priority
  returns the options' value if
  - set on the command line as "--DRT-=value" (rt_cmdline_enabled enabled)
  - the environment variable "DRT_" is set (rt_envvars_enabled enabled)
  - rt_options[] contains an entry "=value"
  - null otherwise

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