c-default-style is a variable defined in `cc-vars.el'.
Its value is

((java-mode . "java") (awk-mode . "awk") (other . "gnu"))

*Style which gets installed by default when a file is visited.

The value of this variable can be any style defined in
`c-style-alist', including styles you add. The value can also be an
association list of major mode symbols to style names.

When the value is a string, all CC Mode major modes will install this
style by default.

When the value is an alist, the major mode symbol is looked up in it
and the associated style is installed. If the major mode is not
listed in the alist, then the symbol `other' is looked up in it, and
if found, the style in that entry is used. If `other' is not found in
the alist, then "gnu" style is used.

The default style gets installed before your mode hooks run, so you
can always override the use of `c-default-style' by making calls to
`c-set-style' in the appropriate mode hook.

You can customize this variable.