/**Configuration options for druntime. The default way to configure the runtime is by passing command line argumentsstarting with `--DRT-` and followed by the option name, e.g. `--DRT-gcopt` toconfigure the GC.When command line parsing is enabled, command line options startingwith `--DRT-` are filtered out before calling main, so the programwill not see them. They are still available via `rt_args()`. Configuration via the command line can be disabled by declaring a variable for thelinker 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 theenvironment 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 anarray 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 commandline arguments, i.e. if command line arguments are not disabled, they can overrideoptions specified through the environment or embedded in the executable. Copyright: Copyright Digital Mars 2014.License: Distributed under the     $(LINK2 http://www.boost.org/LICENSE_1_0.txt, Boost Software License 1.0).   (See accompanying file LICENSE)Authors:   Rainer SchuetzeSource: $(DRUNTIMESRC rt/_config.d)*/ module rt.config; // put each variable in its own COMDAT by making them template instancestemplate rt_envvars_enabled(){    extern(C) pragma(mangle, "rt_envvars_enabled") __gshared bool rt_envvars_enabled = false;}template rt_cmdline_enabled(){    extern(C) pragma(mangle, "rt_cmdline_enabled") __gshared bool rt_cmdline_enabled = true;}template rt_options(){    extern(C) pragma(mangle, "rt_options") __gshared string[] rt_options = [];} import core.stdc.ctype : toupper;import core.stdc.stdlib : getenv;import core.stdc.string : strlen; extern extern(C) string[] rt_args() @nogc nothrow @system; alias rt_configCallBack = string delegate(string) @nogc nothrow; /*** 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-<opt>=value" (rt_cmdline_enabled enabled)*  - the environment variable "DRT_<OPT>" is set (rt_envvars_enabled enabled)*  - rt_options[] contains an entry "<opt>=value"*  - null otherwise*/string rt_configOption(string opt, scope rt_configCallBack dg = null, bool reverse = false) @nogc nothrow{    if (!dg)        dg = (string s) => s;     string s = (reverse ? rt_linkOption(opt, dg) : rt_cmdlineOption(opt, dg));    if (s != null)        return s;    s = rt_envvarsOption(opt, dg);    if (s != null)        return s;    s = (reverse ? rt_cmdlineOption(opt, dg) : rt_linkOption(opt, dg));    return s;} string rt_cmdlineOption(string opt, scope rt_configCallBack dg) @nogc nothrow{    if (rt_cmdline_enabled!())    {        foreach (a; rt_args)        {            if (a == "--")                break;             if (a.length >= opt.length + 7 && a[0..6] == "--DRT-" &&                a[6 .. 6 + opt.length] == opt && a[6 + opt.length] == '=')            {                string s = dg(a[7 + opt.length .. $]);                if (s != null)                    return s;            }        }    }    return null;} string rt_envvarsOption(string opt, scope rt_configCallBack dg) @nogc nothrow{    if (rt_envvars_enabled!())    {        if (opt.length >= 32)            assert(0);         char[40] var = void;        var[0 .. 4] = "DRT_";        foreach (i, c; opt)            var[4 + i] = cast(char) toupper(c);        var[4 + opt.length] = 0;         if (auto p = getenv(var.ptr))        {            string s = dg(cast(string) p[0 .. strlen(p)]);            if (s != null)                return s;        }    }    return null;} string rt_linkOption(string opt, scope rt_configCallBack dg) @nogc nothrow{    foreach (a; rt_options!())    {        if (a.length > opt.length && a[0..opt.length] == opt && a[opt.length] == '=')        {            string s = dg(a[opt.length + 1 .. $]);            if (s != null)                return s;        }    }    return null;}