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        <article>
            <div class="container">
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                        <main class="v-doc">
                            <h1 class="main">Conditional compilation</h1>
<p>The goal of this feature, is to tell V to <em>not compile</em> a function, and all its calls, in the final
executable, if a provided custom flag is not passed.</p>
<p>V will still type check the function and all its calls, <em>even</em> if they will not be present in the
final executable, due to the passed -d flags.</p>
<p>In order to see it in action, run the following example with <code>v run example.v</code> once,
and then a second time with <code>v -d trace_logs example.v</code>:</p>
<div class="language-v">@[if trace_logs ?]
fn elog(s string) {
    eprintln(s)
}

fn main() {
    elog('some expression: ${2 + 2}') // such calls will not be done *at all*, if `-d trace_logs` is not passed
    println('hi')
    elog('finish')
}
</div>
<p>Conditional compilation, based on custom flags, can also be used to produce slightly different
executables, which share the majority of the same code, but where some of the logic, is needed
only some of the time, for example a network server/client program can be written like so:</p>
<div class="language-v">fn act_as_client() { ... }
fn act_as_server() { ... }
fn main() {
    $if as_client ? {
        act_as_client()
    }
    $if as_server ? {
        act_as_server()
    }
}
</div>
<p>To generate a <code>client.exe</code> executable do: <code>v -d as_client -o client.exe .</code>
To generate a <code>server.exe</code> executable do: <code>v -d as_server -o server.exe .</code></p>
<h2 id="compile-time-pseudo-variables">Compile time pseudo variables <a href="conditional-compilation.html#compile-time-pseudo-variables" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V also gives your code access to a set of pseudo string variables,
that are substituted at compile time:</p>
<ul>
<li><code>@FN</code> =&gt; replaced with the name of the current V function.</li>
<li><code>@METHOD</code> =&gt; replaced with ReceiverType.MethodName.</li>
<li><code>@MOD</code> =&gt; replaced with the name of the current V module.</li>
<li><code>@STRUCT</code> =&gt; replaced with the name of the current V struct.</li>
<li><code>@FILE</code> =&gt; replaced with the absolute path of the V source file.</li>
<li><code>@LINE</code> =&gt; replaced with the V line number where it appears (as a string).</li>
<li><code>@FILE_LINE</code> =&gt; like <code>@FILE:@LINE</code>, but the file part is a relative path.</li>
<li><code>@LOCATION</code> =&gt; file, line and name of the current type + method; suitable for logging.</li>
<li><code>@COLUMN</code> =&gt; replaced with the column where it appears (as a string).</li>
<li><code>@VEXE</code> =&gt; replaced with the path to the V compiler.</li>
<li><code>@VEXEROOT</code>  =&gt; will be substituted with the <em>folder</em>,
where the V executable is (as a string).</li>
<li><code>@VHASH</code>  =&gt; replaced with the shortened commit hash of the V compiler (as a string).</li>
<li><code>@VCURRENTHASH</code> =&gt; Similar to <code>@VHASH</code>, but changes when the compiler is
recompiled on a different commit (after local modifications, or after
using git bisect etc).</li>
<li><code>@VMOD_FILE</code> =&gt; replaced with the contents of the nearest v.mod file (as a string).</li>
<li><code>@VMODHASH</code> =&gt; is replaced by the shortened commit hash, derived from the .git directory
next to the nearest v.mod file (as a string).</li>
<li><code>@VMODROOT</code> =&gt; will be substituted with the <em>folder</em>,
where the nearest v.mod file is (as a string).</li>
<li><code>@BUILD_DATE</code> =&gt; replaced with the build date, for example '2024-09-13' .</li>
<li><code>@BUILD_TIME</code> =&gt; replaced with the build time, for example '12:32:07' .</li>
<li><code>@BUILD_TIMESTAMP</code> =&gt; replaced with the build timestamp, for example '1726219885' .
Note: <code>@BUILD_DATE</code>, <code>@BUILD_TIME</code>, <code>@BUILD_TIMESTAMP</code> represent times in the UTC timezone.
By default, they are based on the current time of the compilation/build. They can be overriden
by setting the environment variable <code>SOURCE_DATE_EPOCH</code>. That is also useful while making
releases, since you can use the equivalent of this in your build system/script:
<code>export SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct) ;</code> , and then use <code>@BUILD_DATE</code> etc.,
inside your program, when you for example print your version information to users.
See also <a href="https://reproducible-builds.org/docs/source-date-epoch/">https://reproducible-builds.org/docs/source-date-epoch/</a> .</li>
</ul>
<p>The compile time pseudo variables allow you to do the following
example, which is useful while debugging/logging/tracing your code:</p>
<div class="language-v">eprintln(@LOCATION)
</div>
<p>Another example, is if you want to embed the version/name from v.mod <em>inside</em> your executable:</p>
<div class="language-v">import v.vmod
vm := vmod.decode( @VMOD_FILE ) or { panic(err) }
eprintln('${vm.name} ${vm.version}\n ${vm.description}')
</div>
<p>A program that prints its own source code (a quine):</p>
<div class="language-v">print($embed_file(@FILE).to_string())
</div>
<p>A program that prints the time when it was built:</p>
<div class="language-v">import time

println('This program, was compiled at ${time.unix(@BUILD_TIMESTAMP.i64()).format_ss_milli()} .')
</div>
<blockquote class="tip">
<p>[!NOTE]
you can have arbitrary source code in the file, without problems, since the full file
will be embedded into the executable, produced by compiling it. Also note that printing
is done with <code>print</code> and not <code>println</code>, to not add another new line, missing in the
source code.</p>
</blockquote>
<h2 id="compile-time-reflection">Compile time reflection <a href="conditional-compilation.html#compile-time-reflection" class="header-anchor" aria-hidden="true">#</a></h2>
<p><code>$</code> is used as a prefix for compile time (also referred to as 'comptime') operations.</p>
<p>Having built-in JSON support is nice, but V also allows you to create efficient
serializers for any data format. V has compile time <code>if</code> and <code>for</code> constructs:</p>
<h3 id=".fields"><h4 id="comptime-fields">.fields</h4> <a href="#.fields" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can iterate over struct fields using <code>.fields</code>, it also works with generic types
(e.g. <code>T.fields</code>) and generic arguments (e.g. <code>param.fields</code> where <code>fn gen[T](param T) {</code>).</p>
<div class="language-v">struct User {
    name string
    age  int
}

fn main() {
    $for field in User.fields {
        $if field.typ is string {
            println('${field.name} is of type string')
        }
    }
}

// Output:
// name is of type string
</div>
<h3 id=".values"><h4 id="comptime-values">.values</h4> <a href="#.values" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can read <a href="type-declarations.html#enums">Enum</a> values and their attributes.</p>
<div class="language-v">enum Color {
    red   @[RED]  // first attribute
    blue  @[BLUE] // second attribute
}

fn main() {
    $for e in Color.values {
        println(e.name)
        println(e.attrs)
    }
}

// Output:
// red
// ['RED']
// blue
// ['BLUE']
</div>
<h3 id=".attributes"><h4 id="comptime-attrs">.attributes</h4> <a href="#.attributes" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can read <a href="structs.html">Struct</a> attributes.</p>
<div class="language-v">@[COLOR]
struct Foo {
    a int
}

fn main() {
    $for e in Foo.attributes {
        println(e)
    }
}

// Output:
// StructAttribute{
//    name: 'COLOR'
//    has_arg: false
//    arg: ''
//    kind: plain
// }
</div>
<h3 id=".variants"><h4 id="comptime-variants">.variants</h4> <a href="#.variants" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can read variant types from <a href="type-declarations.html#sum-types">Sum type</a>.</p>
<div class="language-v">type MySum = int | string

fn main() {
    $for v in MySum.variants {
        $if v.typ is int {
            println('has int type')
        } $else $if v.typ is string {
            println('has string type')
        }
    }
}

// Output:
// has int type
// has string type
</div>
<h3 id=".methods"><h4 id="comptime-methods">.methods</h4> <a href="#.methods" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can retrieve information about struct methods.</p>
<div class="language-v">struct Foo {
}

fn (f Foo) test() int {
    return 123
}

fn (f Foo) test2() string {
    return 'foo'
}

fn main() {
    foo := Foo{}
    $for m in Foo.methods {
        $if m.return_type is int {
            print('${m.name} returns int: ')
            println(foo.$method())
        } $else $if m.return_type is string {
            print('${m.name} returns string: ')
            println(foo.$method())
        }
    }
}

// Output:
// test returns int: 123
// test2 returns string: foo
</div>
<h3 id=".params"><h4 id="comptime-method-params">.params</h4> <a href="#.params" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can retrieve information about struct method params.</p>
<div class="language-v">struct Test {
}

fn (t Test) foo(arg1 int, arg2 string) {
}

fn main() {
    $for m in Test.methods {
        $for param in m.params {
            println('${typeof(param.typ).name}: ${param.name}')
        }
    }
}

// Output:
// int: arg1
// string: arg2
</div>
<p>See <a href="https://github.com/vlang/v/blob/master/examples/compiletime/reflection.v"><code>examples/compiletime/reflection.v</code></a>
for a more complete example.</p>
<h2 id="compile-time-code">Compile time code <a href="conditional-compilation.html#compile-time-code" class="header-anchor" aria-hidden="true">#</a></h2>
<h3 id="$if-condition"><code>$if</code> condition <a href="conditional-compilation.html#$if-condition" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">fn main() {
    // Support for multiple conditions in one branch
    $if ios || android {
        println('Running on a mobile device!')
    }
    $if linux &amp;&amp; x64 {
        println('64-bit Linux.')
    }
    // Usage as expression
    os := $if windows { 'Windows' } $else { 'UNIX' }
    println('Using ${os}')
    // $else-$if branches
    $if tinyc {
        println('tinyc')
    } $else $if clang {
        println('clang')
    } $else $if gcc {
        println('gcc')
    } $else {
        println('different compiler')
    }
    $if test {
        println('testing')
    }
    // v -cg ...
    $if debug {
        println('debugging')
    }
    // v -prod ...
    $if prod {
        println('production build')
    }
    // v -d option ...
    $if option ? {
        println('custom option')
    }
}
</div>
<p>If you want an <code>if</code> to be evaluated at compile time it must be prefixed with a <code>$</code> sign.
Right now it can be used to detect an OS, compiler, platform or compilation options.
<code>$if debug</code> is a special option like <code>$if windows</code> or <code>$if x32</code>, it's enabled if the program
is compiled with <code>v -g</code> or <code>v -cg</code>.
If you're using a custom ifdef, then you do need <code>$if option ? {}</code> and compile with<code>v -d option</code>.
Full list of builtin options:</p>
<table>
<thead>
<tr>
<th>OS</th>
<th>Compilers</th>
<th>Platforms</th>
<th>Other</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>windows</code>, <code>linux</code>, <code>macos</code></td>
<td><code>gcc</code>, <code>tinyc</code></td>
<td><code>amd64</code>, <code>arm64</code>, <code>aarch64</code></td>
<td><code>debug</code>, <code>prod</code>, <code>test</code></td>
</tr>
<tr>
<td><code>darwin</code>, <code>ios</code>, <code>bsd</code></td>
<td><code>clang</code>, <code>mingw</code></td>
<td><code>i386</code>, <code>arm32</code></td>
<td><code>js</code>, <code>glibc</code>, <code>prealloc</code></td>
</tr>
<tr>
<td><code>freebsd</code>, <code>openbsd</code>, <code>netbsd</code></td>
<td><code>msvc</code></td>
<td><code>rv64</code>, <code>rv32</code></td>
<td><code>no_bounds_checking</code>, <code>freestanding</code></td>
</tr>
<tr>
<td><code>android</code>, <code>mach</code>, <code>dragonfly</code></td>
<td><code>cplusplus</code></td>
<td><code>x64</code>, <code>x32</code></td>
<td><code>no_segfault_handler</code>, <code>no_backtrace</code></td>
</tr>
<tr>
<td><code>gnu</code>, <code>hpux</code>, <code>haiku</code>, <code>qnx</code></td>
<td></td>
<td><code>little_endian</code>, <code>big_endian</code></td>
<td><code>no_main</code>, <code>fast_math</code>, <code>apk</code>, <code>threads</code></td>
</tr>
<tr>
<td><code>solaris</code>, <code>termux</code></td>
<td></td>
<td></td>
<td><code>js_node</code>, <code>js_browser</code>, <code>js_freestanding</code></td>
</tr>
<tr>
<td><code>serenity</code>, <code>vinix</code>, <code>plan9</code></td>
<td></td>
<td></td>
<td><code>interpreter</code>, <code>es5</code>, <code>profile</code>, <code>wasm32</code></td>
</tr>
<tr>
<td></td>
<td></td>
<td></td>
<td><code>wasm32_emscripten</code>, <code>wasm32_wasi</code></td>
</tr>
<tr>
<td></td>
<td></td>
<td></td>
<td><code>native</code>, <code>autofree</code></td>
</tr>
</tbody>
</table>
<h3 id="$embed_file"><code>$embed_file</code> <a href="conditional-compilation.html#$embed_file" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">import os
fn main() {
    embedded_file := $embed_file('v.png')
    os.write_file('exported.png', embedded_file.to_string())!
}
</div>
<p>V can embed arbitrary files into the executable with the <code>$embed_file(&lt;path&gt;)</code>
compile time call. Paths can be absolute or relative to the source file.</p>
<p>Note that by default, using <code>$embed_file(file)</code>, will always embed the whole content
of the file, but you can modify that behaviour by passing: <code>-d embed_only_metadata</code>
when compiling your program. In that case, the file will not be embedded. Instead,
it will be loaded <em>the first time</em> your program calls <code>embedded_file.data()</code> at runtime,
making it easier to change in external editor programs, without needing to recompile
your program.</p>
<p>Embedding a file inside your executable, will increase its size, but
it will make it more self contained and thus easier to distribute.
When that happens (the default), <code>embedded_file.data()</code> will cause <em>no IO</em>,
and it will always return the same data.</p>
<p><code>$embed_file</code> supports compression of the embedded file when compiling with <code>-prod</code>.
Currently only one compression type is supported: <code>zlib</code>.</p>
<div class="language-v">import os
fn main() {
    embedded_file := $embed_file('x.css', .zlib) // compressed using zlib
    os.write_file('exported.css', embedded_file.to_string())!
}
</div>
<p>Note: compressing binary assets like png or zip files, usually will not gain you much,
and in some cases may even take more space in the final executable, since they are
already compressed.</p>
<p><code>$embed_file</code> returns
<a href="https://modules.vlang.io/v.embed_file.html#EmbedFileData">EmbedFileData</a>
which could be used to obtain the file contents as <code>string</code> or <code>[]u8</code>.</p>
<h3 id="$tmpl-for-embedding-and-parsing-v-template-files"><code>$tmpl</code> for embedding and parsing V template files <a href="conditional-compilation.html#$tmpl-for-embedding-and-parsing-v-template-files" class="header-anchor" aria-hidden="true">#</a></h3>
<p>V has a simple template language for text and html templates, and they can easily
be embedded via <code>$tmpl('path/to/template.txt')</code>:</p>
<div class="language-v">fn build() string {
    name := 'Peter'
    age := 25
    numbers := [1, 2, 3]
    return $tmpl('1.txt')
}

fn main() {
    println(build())
}
</div>
<p>1.txt:</p>
<pre><code>name: @name

age: @age

numbers: @numbers

@for number in numbers
  @number
@end
</code></pre>
<p>output:</p>
<pre><code>name: Peter

age: 25

numbers: [1, 2, 3]

1
2
3
</code></pre>
<p>See more <a href="https://github.com/vlang/v/blob/master/vlib/v/TEMPLATES.md">details</a></p>
<h3 id="$env"><code>$env</code> <a href="conditional-compilation.html#$env" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">module main

fn main() {
    compile_time_env := $env('ENV_VAR')
    println(compile_time_env)
}
</div>
<p>V can bring in values at compile time from environment variables.
<code>$env('ENV_VAR')</code> can also be used in top-level <code>#flag</code> and <code>#include</code> statements:
<code>#flag linux -I $env('JAVA_HOME')/include</code>.</p>
<h3 id="$d"><code>$d</code> <a href="conditional-compilation.html#$d" class="header-anchor" aria-hidden="true">#</a></h3>
<p>V can bring in values at compile time from <code>-d ident=value</code> flag defines, passed on
the command line to the compiler. You can also pass <code>-d ident</code>, which will have the
same meaning as passing <code>-d ident=true</code>.</p>
<p>To get the value in your code, use: <code>$d('ident', default)</code>, where <code>default</code>
can be <code>false</code> for booleans, <code>0</code> or <code>123</code> for i64 numbers, <code>0.0</code> or <code>113.0</code>
for f64 numbers, <code>'a string'</code> for strings.</p>
<p>When a flag is not provided via the command line, <code>$d()</code> will return the <code>default</code>
value provided as the <em>second</em> argument.</p>
<div class="language-v">module main

const my_i64 = $d('my_i64', 1024)

fn main() {
    compile_time_value := $d('my_string', 'V')
    println(compile_time_value)
    println(my_i64)
}
</div>
<p>Running the above with <code>v run .</code> will output:</p>
<pre><code>V
1024
</code></pre>
<p>Running the above with <code>v -d my_i64=4096 -d my_string=&quot;V rocks&quot; run .</code> will output:</p>
<pre><code>V rocks
4096
</code></pre>
<p>Here is an example of how to use the default values, which have to be <em>pure</em> literals:</p>
<div class="language-v">fn main() {
    val_str := $d('id_str', 'value') // can be changed by providing `-d id_str=&quot;my id&quot;`
    val_f64 := $d('id_f64', 42.0) // can be changed by providing `-d id_f64=84.0`
    val_i64 := $d('id_i64', 56) // can be changed by providing `-d id_i64=123`
    val_bool := $d('id_bool', false) // can be changed by providing `-d id_bool=true`
    val_char := $d('id_char', `f`) // can be changed by providing `-d id_char=v`
    println(val_str)
    println(val_f64)
    println(val_i64)
    println(val_bool)
    println(rune(val_char))
}
</div>
<p><code>$d('ident','value')</code> can also be used in top-level statements like <code>#flag</code> and <code>#include</code>:
<code>#flag linux -I $d('my_include','/usr')/include</code>. The default value for <code>$d</code> when used in these
statements should be literal <code>string</code>s.</p>
<p><code>$d('ident', false)</code> can also be used inside <code>$if $d('ident', false) {</code> statements,
granting you the ability to selectively turn on/off certain sections of code, at compile
time, without modifying your source code, or keeping different versions of it.</p>
<h3 id="$compile_error-and-$compile_warn"><code>$compile_error</code> and <code>$compile_warn</code> <a href="conditional-compilation.html#$compile_error-and-$compile_warn" class="header-anchor" aria-hidden="true">#</a></h3>
<p>These two comptime functions are very useful for displaying custom errors/warnings during
compile time.</p>
<p>Both receive as their only argument a string literal that contains the message to display:</p>
<div class="language-v">// x.v
module main

$if linux {
    $compile_error('Linux is not supported')
}

fn main() {
}

$ v run x.v
x.v:4:5: error: Linux is not supported
    2 |
    3 | $if linux {
    4 |     $compile_error('Linux is not supported')
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    5 | }
    6 |
</div>
<h2 id="compile-time-types">Compile time types <a href="conditional-compilation.html#compile-time-types" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Compile time types group multiple types into a general higher-level type. This is useful in
functions with generic parameters, where the input type must have a specific property, for example
the <code>.len</code> attribute in arrays.</p>
<p>V supports the following compile time types:</p>
<ul>
<li><code>$alias</code> =&gt; matches <a href="type-declarations.html#type-aliases">Type aliases</a>.</li>
<li><code>$array</code> =&gt; matches <a href="v-types.html#arrays">Arrays</a> and <a href="v-types.html#fixed-size-arrays">Fixed Size Arrays</a>.</li>
<li><code>$array_dynamic</code> =&gt; matches <a href="v-types.html#arrays">Arrays</a>, but not <a href="v-types.html#fixed-size-arrays">Fixed Size Arrays</a>.</li>
<li><code>$array_fixed</code> =&gt; matches <a href="v-types.html#fixed-size-arrays">Fixed Size Arrays</a>, but not <a href="v-types.html#arrays">Arrays</a></li>
<li><code>$enum</code> =&gt; matches <a href="type-declarations.html#enums">Enums</a>.</li>
<li><code>$float</code> =&gt; matches <code>f32</code>, <code>f64</code> and float literals.</li>
<li><code>$function</code> =&gt; matches <a href="type-declarations.html#function-types">Function Types</a>.</li>
<li><code>$int</code> =&gt; matches <code>int</code>, <code>i8</code>, <code>i16</code>, <code>i32</code>, <code>i64</code>, <code>u8</code>, <code>u16</code>, <code>u32</code>, <code>u64</code>, <code>isize</code>, <code>usize</code>
and integer literals.</li>
<li><code>$interface</code> =&gt; matches <a href="type-declarations.html#interfaces">Interfaces</a>.</li>
<li><code>$map</code> =&gt; matches <a href="v-types.html#maps">Maps</a>.</li>
<li><code>$option</code> =&gt; matches <a href="type-declarations.html#optionresult-types-and-error-handling">Option Types</a>.</li>
<li><code>$struct</code> =&gt; matches <a href="structs.html">Structs</a>.</li>
<li><code>$sumtype</code> =&gt; matches <a href="type-declarations.html#sum-types">Sum Types</a>.</li>
<li><code>$string</code> =&gt; matches <a href="v-types.html#strings">Strings</a>.</li>
</ul>
<h2 id="environment-specific-files">Environment specific files <a href="conditional-compilation.html#environment-specific-files" class="header-anchor" aria-hidden="true">#</a></h2>
<p>If a file has an environment-specific suffix, it will only be compiled for that environment.</p>
<ul>
<li><code>.js.v</code> =&gt; will be used only by the JS backend. These files can contain JS. code.</li>
<li><code>.c.v</code> =&gt; will be used only by the C backend. These files can contain C. code.</li>
<li><code>.native.v</code> =&gt; will be used only by V's native backend.</li>
<li><code>_nix.c.v</code> =&gt; will be used only on Unix systems (non Windows).</li>
<li><code>_${os}.c.v</code> =&gt; will be used only on the specific <code>os</code> system.
For example, <code>_windows.c.v</code> will be used only when compiling on Windows, or with <code>-os windows</code>.</li>
<li><code>_default.c.v</code> =&gt; will be used only if there is NOT a more specific platform file.
For example, if you have both <code>file_linux.c.v</code> and <code>file_default.c.v</code>,
and you are compiling for linux, then only <code>file_linux.c.v</code> will be used,
and <code>file_default.c.v</code> will be ignored.</li>
</ul>
<p>Here is a more complete example:</p>
<p><code>main.v</code>:</p>
<div class="language-v">module main
fn main() { println(message) }
</div>
<p><code>main_default.c.v</code>:</p>
<div class="language-v">module main
const message = 'Hello world'
</div>
<p><code>main_linux.c.v</code>:</p>
<div class="language-v">module main
const message = 'Hello linux'
</div>
<p><code>main_windows.c.v</code>:</p>
<div class="language-v">module main
const message = 'Hello windows'
</div>
<p>With the example above:</p>
<ul>
<li><p>when you compile for Windows, you will get <code>Hello windows</code></p>
</li>
<li><p>when you compile for Linux, you will get <code>Hello linux</code></p>
</li>
<li><p>when you compile for any other platform, you will get the
non specific <code>Hello world</code> message.</p>
</li>
<li><p><code>_d_customflag.v</code> =&gt; will be used <em>only</em> if you pass <code>-d customflag</code> to V.
That corresponds to <code>$if customflag ? {}</code>, but for a whole file, not just a
single block. <code>customflag</code> should be a snake_case identifier, it can not
contain arbitrary characters (only lower case latin letters + numbers + <code>_</code>).</p>
<blockquote class="tip">
<p><strong>Note</strong></p>
<p>A combinatorial <code>_d_customflag_linux.c.v</code> postfix will not work.
If you do need a custom flag file, that has platform dependent code, use the
postfix <code>_d_customflag.v</code>, and then use platform dependent compile time
conditional blocks inside it, i.e. <code>$if linux {}</code> etc.</p>
</blockquote>
</li>
<li><p><code>_notd_customflag.v</code> =&gt; similar to _d_customflag.v, but will be used
<em>only</em> if you do NOT pass <code>-d customflag</code> to V.</p>
</li>
</ul>
<p>See also <a href="cross-compilation.html">Cross Compilation</a>.</p>

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