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<a class="link subtopic" href="builtin-functions.html#println"> println</a>
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<a class="link subtopic" href="testing.html#test-files"> Test files</a>
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<a class="link" href="sizeof-and-__offsetof.html">sizeof and __offsetof</a>
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<a class="link" href="limited-operator-overloading.html">Limited operator overloading</a>
<a class="link subtopic" href="limited-operator-overloading.html#implicitly-generated-overloads"> Implicitly generated overloads</a>
<a class="link subtopic" href="limited-operator-overloading.html#restriction"> Restriction</a>
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<a class="link" href="performance-tuning.html">Performance tuning</a>
<a class="link subtopic" href="performance-tuning.html#tuning-operations-details"> Tuning operations details</a>
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<a class="link" href="atomics.html">Atomics</a>
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<a class="link" href="debugging.html">Debugging</a>
<a class="link subtopic" href="debugging.html#c-backend-binaries-(default)"> C Backend binaries (Default)</a>
<a class="link subtopic" href="debugging.html#native-backend-binaries"> Native Backend binaries</a>
<a class="link subtopic" href="debugging.html#javascript-backend"> Javascript Backend</a>
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<a class="link active" href="v-and-c.html">V and C</a>
<a class="link subtopic" href="v-and-c.html#calling-c-from-v"> Calling C from V</a>
<a class="link subtopic" href="v-and-c.html#calling-v-from-c"> Calling V from C</a>
<a class="link subtopic" href="v-and-c.html#passing-c-compilation-flags"> Passing C compilation flags</a>
<a class="link subtopic" href="v-and-c.html#pkgconfig"> pkgconfig</a>
<a class="link subtopic" href="v-and-c.html#including-c-code"> Including C code</a>
<a class="link subtopic" href="v-and-c.html#c-types"> C types</a>
<a class="link subtopic" href="v-and-c.html#c-declarations"> C Declarations</a>
<a class="link subtopic" href="v-and-c.html#export-to-shared-library"> Export to shared library</a>
<a class="link subtopic" href="v-and-c.html#translating-c-to-v"> Translating C to V</a>
<a class="link subtopic" href="v-and-c.html#working-around-c-issues"> Working around C issues</a>
<div class="items">
<a class="link" href="other-v-features.html">Other V Features</a>
<a class="link subtopic" href="other-v-features.html#inline-assembly"> Inline assembly</a>
<a class="link subtopic" href="other-v-features.html#hot-code-reloading"> Hot code reloading</a>
<a class="link subtopic" href="other-v-features.html#cross-platform-shell-scripts-in-v"> Cross-platform shell scripts in V</a>
<a class="link subtopic" href="other-v-features.html#vsh-scripts-with-no-extension"> Vsh scripts with no extension</a>
<div class="items">
<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
<div class="items">
<a class="link" href="appendix-ii-operators.html">Appendix II: Operators</a>
</div>
</section>
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<main>
<h1 class="main">V and C</h1>
<h2 id="calling-c-from-v">Calling C from V <a href="v-and-c.html#calling-c-from-v" class="header-anchor" aria-hidden="true">#</a></h2>
<p><strong>Example</strong></p>
<div class="language-v">#flag freebsd -I/usr/local/include -L/usr/local/lib
#flag -lsqlite3
#include "sqlite3.h"
// See also the example from https://www.sqlite.org/quickstart.html
pub struct C.sqlite3 {
}
pub struct C.sqlite3_stmt {
}
type FnSqlite3Callback = fn (voidptr, int, &&char, &&char) int
fn C.sqlite3_open(&char, &&C.sqlite3) int
fn C.sqlite3_close(&C.sqlite3) int
fn C.sqlite3_column_int(stmt &C.sqlite3_stmt, n int) int
// ... you can also just define the type of parameter and leave out the C. prefix
fn C.sqlite3_prepare_v2(&C.sqlite3, &char, int, &&C.sqlite3_stmt, &&char) int
fn C.sqlite3_step(&C.sqlite3_stmt)
fn C.sqlite3_finalize(&C.sqlite3_stmt)
fn C.sqlite3_exec(db &C.sqlite3, sql &char, cb FnSqlite3Callback, cb_arg voidptr, emsg &&char) int
fn C.sqlite3_free(voidptr)
fn my_callback(arg voidptr, howmany int, cvalues &&char, cnames &&char) int {
unsafe {
for i in 0 .. howmany {
print('| ${cstring_to_vstring(cnames[i])}: ${cstring_to_vstring(cvalues[i]):20} ')
}
}
println('|')
return 0
}
fn main() {
db := &C.sqlite3(unsafe { nil }) // this means `sqlite3* db = 0`
// passing a string literal to a C function call results in a C string, not a V string
C.sqlite3_open(c'users.db', &db)
// C.sqlite3_open(db_path.str, &db)
query := 'select count(*) from users'
stmt := &C.sqlite3_stmt(unsafe { nil })
// Note: You can also use the `.str` field of a V string,
// to get its C style zero terminated representation
C.sqlite3_prepare_v2(db, &char(query.str), -1, &stmt, 0)
C.sqlite3_step(stmt)
nr_users := C.sqlite3_column_int(stmt, 0)
C.sqlite3_finalize(stmt)
println('There are ${nr_users} users in the database.')
//
error_msg := &char(0)
query_all_users := 'select * from users'
rc := C.sqlite3_exec(db, &char(query_all_users.str), my_callback, voidptr(7), &error_msg)
if rc != C.SQLITE_OK {
eprintln(unsafe { cstring_to_vstring(error_msg) })
C.sqlite3_free(error_msg)
}
C.sqlite3_close(db)
}
</div>
<h2 id="calling-v-from-c">Calling V from C <a href="v-and-c.html#calling-v-from-c" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Since V can compile to C, calling V code from C is very easy, once you know how.</p>
<p>Use <code>v -o file.c your_file.v</code> to generate a C file, corresponding to the V code.</p>
<p>More details in <a href="https://github.com/vlang/v/blob/master/doc/../examples/call_v_from_c">call_v_from_c example</a>.</p>
<h2 id="passing-c-compilation-flags">Passing C compilation flags <a href="v-and-c.html#passing-c-compilation-flags" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Add <code>#flag</code> directives to the top of your V files to provide C compilation flags like:</p>
<ul>
<li><code>-I</code> for adding C include files search paths</li>
<li><code>-l</code> for adding C library names that you want to get linked</li>
<li><code>-L</code> for adding C library files search paths</li>
<li><code>-D</code> for setting compile time variables</li>
</ul>
<p>You can (optionally) use different flags for different targets.
Currently the <code>linux</code>, <code>darwin</code> , <code>freebsd</code>, and <code>windows</code> flags are supported.</p>
<blockquote class="tip">
<p>[!NOTE]
Each flag must go on its own line (for now)</p>
</blockquote>
<div class="language-v">#flag linux -lsdl2
#flag linux -Ivig
#flag linux -DCIMGUI_DEFINE_ENUMS_AND_STRUCTS=1
#flag linux -DIMGUI_DISABLE_OBSOLETE_FUNCTIONS=1
#flag linux -DIMGUI_IMPL_API=
</div>
<p>In the console build command, you can use:</p>
<ul>
<li><code>-cc</code> to change the default C backend compiler.</li>
<li><code>-cflags</code> to pass custom flags to the backend C compiler (passed before other C options).</li>
<li><code>-ldflags</code> to pass custom flags to the backend C linker (passed after every other C option).</li>
<li>For example: <code>-cc gcc-9 -cflags -fsanitize=thread</code>.</li>
</ul>
<p>You can define a <code>VFLAGS</code> environment variable in your terminal to store your <code>-cc</code>
and <code>-cflags</code> settings, rather than including them in the build command each time.</p>
<h2 id="#pkgconfig">#pkgconfig <a href="##pkgconfig" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Add <code>#pkgconfig</code> directives to tell the compiler which modules should be used for compiling
and linking using the pkg-config files provided by the respective dependencies.</p>
<p>As long as backticks can't be used in <code>#flag</code> and spawning processes is not desirable for security
and portability reasons, V uses its own pkgconfig library that is compatible with the standard
freedesktop one.</p>
<p>If no flags are passed it will add <code>--cflags</code> and <code>--libs</code> to pkgconfig (not to V).
In other words, both lines below do the same:</p>
<div class="language-v">#pkgconfig r_core
#pkgconfig --cflags --libs r_core
</div>
<p>The <code>.pc</code> files are looked up into a hardcoded list of default pkg-config paths, the user can add
extra paths by using the <code>PKG_CONFIG_PATH</code> environment variable. Multiple modules can be passed.</p>
<p>To check the existence of a pkg-config use <code>$pkgconfig('pkg')</code> as a compile time "if" condition to
check if a pkg-config exists. If it exists the branch will be created. Use <code>$else</code> or <code>$else $if</code>
to handle other cases.</p>
<div class="language-v">$if $pkgconfig('mysqlclient') {
#pkgconfig mysqlclient
} $else $if $pkgconfig('mariadb') {
#pkgconfig mariadb
}
</div>
<h2 id="including-c-code">Including C code <a href="v-and-c.html#including-c-code" class="header-anchor" aria-hidden="true">#</a></h2>
<p>You can also include C code directly in your V module.
For example, let's say that your C code is located in a folder named 'c' inside your module folder.
Then:</p>
<ul>
<li>Put a v.mod file inside the toplevel folder of your module (if you
created your module with <code>v new</code> you already have v.mod file). For example:</li>
</ul>
<div class="language-v">Module {
name: 'mymodule',
description: 'My nice module wraps a simple C library.',
version: '0.0.1'
dependencies: []
}
</div>
<ul>
<li>Add these lines to the top of your module:</li>
</ul>
<div class="language-v">#flag -I @VMODROOT/c
#flag @VMODROOT/c/implementation.o
#include "header.h"
</div>
<blockquote class="tip">
<p>[!NOTE]
@VMODROOT will be replaced by V with the <em>nearest parent folder,
where there is a v.mod file</em>.
Any .v file beside or below the folder where the v.mod file is,
can use <code>#flag @VMODROOT/abc</code> to refer to this folder.
The @VMODROOT folder is also <em>prepended</em> to the module lookup path,
so you can <em>import</em> other modules under your @VMODROOT, by just naming them.</p>
</blockquote>
<p>The instructions above will make V look for an compiled .o file in
your module <code>folder/c/implementation.o</code>.
If V finds it, the .o file will get linked to the main executable, that used the module.
If it does not find it, V assumes that there is a <code>@VMODROOT/c/implementation.c</code> file,
and tries to compile it to a .o file, then will use that.</p>
<p>This allows you to have C code, that is contained in a V module, so that its distribution is easier.
You can see a complete minimal example for using C code in a V wrapper module here:
<a href="https://github.com/vlang/v/tree/master/vlib/v/tests/project_with_c_code">project_with_c_code</a>.
Another example, demonstrating passing structs from C to V and back again:
<a href="https://github.com/vlang/v/tree/master/vlib/v/tests/project_with_c_code_2">interoperate between C to V to C</a>.</p>
<h2 id="c-types">C types <a href="v-and-c.html#c-types" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Ordinary zero terminated C strings can be converted to V strings with
<code>unsafe { &char(cstring).vstring() }</code> or if you know their length already with
<code>unsafe { &char(cstring).vstring_with_len(len) }</code>.</p>
<blockquote class="tip">
<p>[!NOTE]
The <code>.vstring()</code> and <code>.vstring_with_len()</code> methods do NOT create a copy of the <code>cstring</code>,
so you should NOT free it after calling the method <code>.vstring()</code>.
If you need to make a copy of the C string (some libc APIs like <code>getenv</code> pretty much require that,
since they return pointers to internal libc memory), you can use <code>cstring_to_vstring(cstring)</code>.</p>
</blockquote>
<p>On Windows, C APIs often return so called <code>wide</code> strings (utf16 encoding).
These can be converted to V strings with <code>string_from_wide(&u16(cwidestring))</code> .</p>
<p>V has these types for easier interoperability with C:</p>
<ul>
<li><code>voidptr</code> for C's <code>void*</code>,</li>
<li><code>&u8</code> for C's <code>byte*</code> and</li>
<li><code>&char</code> for C's <code>char*</code>.</li>
<li><code>&&char</code> for C's <code>char**</code></li>
</ul>
<p>To cast a <code>voidptr</code> to a V reference, use <code>user := &User(user_void_ptr)</code>.</p>
<p><code>voidptr</code> can also be dereferenced into a V struct through casting: <code>user := User(user_void_ptr)</code>.</p>
<p><a href="https://github.com/vlang/v/blob/master/vlib/v/tests/project_with_c_code/mod1/wrapper.c.v">an example of a module that calls C code from V</a></p>
<h2 id="c-declarations">C Declarations <a href="v-and-c.html#c-declarations" class="header-anchor" aria-hidden="true">#</a></h2>
<p>C identifiers are accessed with the <code>C</code> prefix similarly to how module-specific
identifiers are accessed. Functions must be redeclared in V before they can be used.
Any C types may be used behind the <code>C</code> prefix, but types must be redeclared in V in
order to access type members.</p>
<p>To redeclare complex types, such as in the following C code:</p>
<div class="language-c">struct SomeCStruct {
uint8_t implTraits;
uint16_t memPoolData;
union {
struct {
void* data;
size_t size;
};
DataView view;
};
};
</div>
<p>members of sub-data-structures may be directly declared in the containing struct as below:</p>
<div class="language-v">pub struct C.SomeCStruct {
implTraits u8
memPoolData u16
// These members are part of sub data structures that can't currently be represented in V.
// Declaring them directly like this is sufficient for access.
// union {
// struct {
data voidptr
size usize
// }
view C.DataView
// }
}
</div>
<p>The existence of the data members is made known to V, and they may be used without
re-creating the original structure exactly.</p>
<p>Alternatively, you may <a href="structs.html#embedded-structs">embed</a> the sub-data-structures to maintain
a parallel code structure.</p>
<h2 id="export-to-shared-library">Export to shared library <a href="v-and-c.html#export-to-shared-library" class="header-anchor" aria-hidden="true">#</a></h2>
<p>By default all V functions have the following naming scheme in C: <code>[module name]__[fn_name]</code>.</p>
<p>For example, <code>fn foo() {}</code> in module <code>bar</code> will result in <code>bar__foo()</code>.</p>
<p>To use a custom export name, use the <code>[export]</code> attribute:</p>
<pre><code>@[export: 'my_custom_c_name']
fn foo() {
}
</code></pre>
<h2 id="translating-c-to-v">Translating C to V <a href="v-and-c.html#translating-c-to-v" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V can translate your C code to human readable V code, and generating V wrappers
on top of C libraries.</p>
<p>C2V currently uses Clang's AST to generate V, so to translate a C file to V
you need to have Clang installed on your machine.</p>
<p>Let's create a simple program <code>test.c</code> first:</p>
<div class="language-c">#include "stdio.h"
int main() {
for (int i = 0; i < 10; i++) {
printf("hello world\n");
}
return 0;
}
</div>
<p>Run <code>v translate test.c</code>, and V will generate <code>test.v</code>:</p>
<div class="language-v">fn main() {
for i := 0; i < 10; i++ {
println('hello world')
}
}
</div>
<p>To generate a wrapper on top of a C library use this command:</p>
<pre><code class="language-bash">v translate wrapper c_code/libsodium/src/libsodium
</code></pre>
<p>This will generate a directory <code>libsodium</code> with a V module.</p>
<p>Example of a C2V generated libsodium wrapper:</p>
<p><a href="https://github.com/vlang/libsodium">https://github.com/vlang/libsodium</a></p>
<br>
<p>When should you translate C code and when should you simply call C code from V?</p>
<p>If you have well-written, well-tested C code,
then of course you can always simply call this C code from V.</p>
<p>Translating it to V gives you several advantages:</p>
<ul>
<li>If you plan to develop that code base, you now have everything in one language,
which is much safer and easier to develop in than C.</li>
<li>Cross-compilation becomes a lot easier. You don't have to worry about it at all.</li>
<li>No more build flags and include files either.</li>
</ul>
<h2 id="working-around-c-issues">Working around C issues <a href="v-and-c.html#working-around-c-issues" class="header-anchor" aria-hidden="true">#</a></h2>
<p>In some cases, C interop can be extremely difficult.
One of these such cases is when headers conflict with each other.
For example, V needs to include the Windows header libraries in order for your V binaries to work
seamlessly across all platforms.</p>
<p>However, since the Windows header libraries use extremely generic names such as <code>Rectangle</code>,
this will cause a conflict if you wish to use C code that also has a name defined as <code>Rectangle</code>.</p>
<p>For very specific cases like this, we have <code>#preinclude</code>.</p>
<p>This will allow things to be configured before V adds in its built in libraries.</p>
<p>Example usage:</p>
<div class="language-v">// This will include before built in libraries are used.
#preinclude "pre_include.h"
// This will include after built in libraries are used.
#include "include.h"
</div>
<p>An example of what might be included in <code>pre_include.h</code>
can be <a href="https://github.com/irishgreencitrus/raylib.v/blob/main/include/pre.h">found here</a></p>
<p>This is an advanced feature, and will not be necessary
outside of very specific cases with C interop,
meaning it could cause more issues than it solves.</p>
<p>Consider it last resort!</p>
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