It is 4 spaces, no tabs, preferrably at 80 columns per line.

The preferred coding style is explained by example.

Source file example:

    /*
       Copyright
       LICENSE TEXT
    */

    #include "foo.h"

    #include "bar.h"

    #include <glib.h>

    void foobar(FooEnum bar, const gchar* foo)
    {
        if(!foo)
            return;

        #ifdef BAR_STRICT
        if(bar == FOO_N)
        {
            g_print("illegal value for 'bar'.\n");
            return;
        }
        #endif

        // this is an example
        gint n;
        switch(bar)
        {
        case FOO_FOO:
            n = bar + 1;
            break;
        case FOO_BAR:
            n = bar + 10;
            break;
        default:
            n = 1;
        }

        guint i;
        for(i = 0; i < n; i++)
        {
            g_print("%s\n", foo);
        }
    }

Header file example:

    /*
       Copyright
       LICENSE TEXT
    */

    #ifndef __FOO_H__
    #define __FOO_H__ 1

    #ifdef HAVE_BAR_H
        #define BAR_STRICT
    #endif

    // -- Types

    typedef enum
    {
        FOO_FOO,
        FOO_BAR,
        FOO_N
    } FooEnum;

    typedef struct
    {
        FooEnum fooBar;
    } FooStruct;

    // -- Declarations

    void
    foobar(FooEnum, const gchar*);

    #endif /* !__FOO_H__ */
