Tcl-extension c++, Umlaute get lost from Tcl to c++

[bychan]

When I pass a string from TCL to C++ the Umlaute get lost.
I am using
gcc version 4.0.2 20050901 (prerelease) (SUSE Linux)

I am using the following functions:
new_name = utf8to_iso(Tcl_GetStringFromObj(objv[2],
&new_name_len));

This mechanism worked well until
gcc version 3.3.5 20050117 (prerelease) (SUSE Linux)

I hope that somebody can help me!