read_registers
should probably be an instance method (on Modbus.modbus_t) instead of a static method, and Modbus.modbus_t should probably be renamed to something like Modbus.Context, create_tcp should probably be a constructor, and Modbus.close should be a free function on the Modbus.Context compact class, but that's beside the point of this question (if you stop by #vala on irc.gnome.org you can get help with that stuff).
You probably want to make it an array:
public static int read_registers (Modbus.modbus_t ctx, int addr, [CCode (array_length_pos = 2.5)] uint16[] dest);
Then you would do something like this in Vala:
public void run () {
uint16 reg[2];
ctx = create_tcp ("10.0.1.77", 502);
Modbus.read_registers (ctx, 0x20, reg);
message ("reg = %d (0x%X)", reg, reg);
Modbus.close(ctx);
}
For a port more faithful to the original C (where tab_reg has 16 elements instead of 2), you could use array slicing:
public void run () {
uint16 reg[16];
ctx = create_tcp ("10.0.1.77", 502);
Modbus.read_registers (ctx, 0x20, reg[0:2]);
stdout.printf ("reg = %d (0x%X)\n", reg, reg);
Modbus.close(ctx);
}
Note that if you make it an instance method you'll need to change the array_length_pos to 1.5.