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<a class="nav-entry-link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<main class="v-doc">
<h1 class="main">V and C</h1>
<p>The basic mapping between C and V types is described in
<a href="https://github.com/vlang/v/blob/master/doc/c_and_v_type_interoperability.md">C and V Type Interoperability</a>.</p>
<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>V currently does not have a parser for C code. That means that even
though it allows you to <code>#include</code> existing C header and source files,
it will not know anything about the declarations in them. The <code>#include</code>
statement will only appear in the generated C code, to be used by the
C compiler backend itself.</p>
<p><strong>Example of #include</strong></p>
<div class="language-v">#include <stdio.h>
</div>
<p>After this statement, V will <em>not</em> know anything about the functions and
structs declared in <code>stdio.h</code>, but if you try to compile the .v file,
it will add the include in the generated C code, so that if that header file
is missing, you will get a C error (you will not in this specific case, if you
have a proper C compiler setup, since <code><stdio.h></code> is part of the
standard C library).</p>
<p>To overcome that limitation (that V does not have a C parser), V needs you to
redeclare the C functions and structs, on the V side, in your <code>.c.v</code> files.
Note that such redeclarations only need to have enough details about the
functions/structs that you want to use.
Note also that they <em>do not have</em> to be complete, unlike the ones in the .h files.</p>
<p><strong>C. struct redeclarations</strong>
For example, if a struct has 3 fields on the C side, but you want to only
refer to 1 of them, you can declare it like this:</p>
<p><strong>Example of C struct redeclaration</strong></p>
<div class="language-v">struct C.NameOfTheStruct {
a_field int
}
</div>
<p>Another feature, that is very frequently needed for C interoperability,
is the <code>@[typedef]</code> attribute. It is used for marking <code>C.</code> structs,
that are defined with <code>typedef struct SomeName { ..... } TypeName;</code> in the C headers.</p>
<p>For that case, you will have to write something like this in your .c.v file:</p>
<div class="language-v">@[typedef]
pub struct C.TypeName {
}
</div>
<p>Note that the name of the <code>C.</code> struct in V, is the one <em>after</em> the <code>struct SomeName {...}</code>.</p>
<p><strong>C. function redeclarations</strong>
The situation is similar for <code>C.</code> functions. If you are going to call just 1 function in a
library, but its .h header declares dozens of them, you will only need to declare that single
function, for example:</p>
<p><strong>Example of C function redeclaration</strong></p>
<div class="language-v">fn C.name_of_the_C_function(param1 int, const_param2 &char, param3 f32) f64
</div>
<p>... and then later, you will be able to call the same way you would V function:</p>
<div class="language-v">f := C.name_of_the_C_function(123, c'here is some C style string', 1.23)
dump(f)
</div>
<p><strong>Example of using a C function from stdio, by redeclaring it on the V side</strong></p>
<div class="language-v">#include <stdio.h>
// int dprintf(int fd, const char *format, ...)
fn C.dprintf(fd int, const_format &char, ...voidptr) int
value := 12345
x := C.dprintf(0, c'Hello world, value: %d\n', value)
dump(x)
</div>
<p>If your C backend compiler is properly setup, you should see something like this, when you try
to run it:</p>
<pre><code class="language-console">#0 10:42:32 /v/examples> v run a.v
Hello world, value: 12345
[a.v:8] x: 26
#0 10:42:33 /v/examples>
</code></pre>
<p>Note, that the C function redeclarations look very simillar to the V ones, with some differences:</p>
<ol>
<li>They lack a body (they are defined on the C side) .</li>
<li>Their names start with <code>C.</code> .</li>
<li>Their names can have capital letters (unlike V ones, that are required to use snake_case) .</li>
</ol>
<p>Note also the second parameter <code>const char *format</code>, which was redeclared as <code>const_format &char</code> .
The <code>const_</code> prefix in that redeclaration may seem arbitrary, but it is important, if you want
to compile your code with <code>-cstrict</code> or thirdparty C static analysis tools. V currently does not
have another way to express that this parameter is a const (this will probably change in V 1.0).</p>
<p>For some C functions, that use variadics (<code>...</code>) as parameters, V supports a special syntax for
the parameters - <code>...voidptr</code>, that is not available for ordinary V functions (V's variadics are
<em>required</em> to have the same exact type). Usually those are functions of the printf/scanf family
i.e for <code>printf</code>, <code>fprintf</code>, <code>scanf</code>, <code>sscanf</code>, etc, and other formatting/parsing/logging
functions.</p>
<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>