8.1. Debugging

For demonstration purposes we will create a buggy program by intentionally dereferencing a null reference, which will result in a segmentation fault:

class Foo : Object {
    public int field;
}

void main() {
    Foo? foo = null;
    stdout.printf("%d\n", foo.field);
}

$ valac debug-demo.vala
$ ./debug-demo
Segmentation fault

So how do we debug this program? The -g command line option tells the Vala compiler to include Vala source code line information in the compiled binary, --save-temps keeps the temporary C source files:

$ valac -g --save-temps debug-demo.vala

Vala programs can be debugged with the GNU Debugger gdb or one of its graphical front-ends, e.g. gdbgui.

$ nemiver debug-demo

A sample gdb session:

$ gdb debug-demo
(gdb) run
Starting program: /home/valacoder/debug-demo

Program received signal SIGSEGV, Segmentation fault.
0x0804881f in _main () at debug-demo.vala:7
7           stdout.printf("%d\n", foo.field);
(gdb)