Shared library and GObject introspection

Build a tiny shared library in Vala, call it from another Vala program, then expose it to GObject introspection so runtimes such as GJS can call into it. Adapted from the archived GNOME Wiki page Projects/Vala/SharedLibSample.

Library source (test_shared.vala)

namespace MyMath {
    public int sum (int a, int b) {
        return a + b;
    }

    public int square (int a) {
        return a * a;
    }
}

Build the .so and VAPI for direct Vala reuse

valac --library=test_shared -H test_shared.h test_shared.vala -X -fPIC -X -shared -o test_shared.so

--library tells the compiler you are building a library. Add --vapi=name.vapi if you need a specific VAPI file name. -H emits a C header for consumers that include C code. -fPIC and -shared are forwarded to the C compiler so the result is a position-independent shared object.

Vala client (main.vala)

using MyMath;

int main (string[] args) {
    stdout.printf ("\nTesting shared library\n");
    stdout.printf ("\t2 + 3 is %d\n", sum (2, 3));
    stdout.printf ("\t8 squared is %d\n", square (8));
    return 0;
}

valac test_shared.vapi main.vala -X test_shared.so -X -I. -o valatest
export LD_LIBRARY_PATH=.:${LD_LIBRARY_PATH}
./valatest

LD_LIBRARY_PATH is only for local testing before make install places the library on the system loader path.

GObject introspection metadata

Generate a GIR and typelib so other language bindings can load the library:

valac test_shared.vala -X -fPIC -X -shared -o test_shared.so --library=testShared --gir testShared-0.1.gir
g-ir-compiler --shared-library=test_shared.so --output=testShared-0.1.typelib testShared-0.1.gir

GJS client (client.js)

imports.gi.versions.testShared = '0.1';
const testShared = imports.gi.testShared;
print('Result: ' + testShared.square(42));

GI_TYPELIB_PATH=. LD_LIBRARY_PATH=. gjs client.js

Further language examples live in community repositories such as antono/vala-object.

Modern projects usually wrap these valac flags inside Meson (shared_library(), vala.generate_gir(), and gnome.generate_gir()), which tracks dependencies and install locations for you.