fonts - Accessing the Mono.Cairo FontMatrix? -

i'm trying write font exporter project working on. work need proper width of each glyph. complicated fact font bitmap generated @ user specified scale (e.g. cairocontet.setfontsize(...))

when try access cairocontext.fontmatrix scale extents, following sigsegv:

stacktrace:   native stacktrace:      0   mono                                0x000b6ea9 mono + 745129     1   mono                                0x00006e0e mono + 24078     2   libsystem_c.dylib                   0x949e259b _sigtramp + 43     3   ???                                 0xffffffff 0x0 + 4294967295     4   mono                                0x000b4e32 mono + 736818     5   mono                                0x000b7ac4 mono + 748228     6   mono                                0x000b8e18 mono + 753176     7   mono                                0x00107a02 mono + 1075714     8   ???                                 0x0035e2c1 0x0 + 3531457     9   ???                                 0x003d9a18 0x0 + 4037144     10  ???                                 0x0a20deb0 0x0 + 169926320  debug info gdb:  /tmp/mono-gdb-commands.ulvxab:1: error in sourced command file: unable debug self  ================================================================= got sigsegv while executing native code. indicates fatal error in mono runtime or 1 of native libraries  used application. ================================================================= 

this code triggers it, though in general attempt fontmatrix seems cause same error:

cr.fontmatrix.transformdistance(ref xadvance,ref yadvance); 

is there alternate/better way scale advances? cant math myself because don't know scale factor setfontsize set due how works (it sets size of em-quad).