c-mode and underscore

[geophile]

I am trying to get c-mode to treat underscore as a word, so that
forward-word backward-word don't stop on underscores.

My .emacs file includes:

    (modify-syntax-entry ?_ "w" c-mode-syntax-table)

which does not appear to be effective. But if I run this command
manually, it is effective.

I'm pretty sure that the line above is being reached in my .emacs
file, as later commands are effective.

Any idea what I'm doing wrong?

Jack Orenstein