Not so Ubuntu related C question

[dave walker]

I have a question, and just thought someone here may have an answer for me.

I am writing a library, with plugins in Thunderbird and Gaim (for right 
now).  The library is an API to edit an XML file.  So both, TB and GAIM, 
should not be able to issue a write command to the file at the same 
time.  My question is, am I able to use mutex in the library to allow 
only one process to write at a time, OR, will I need to use a file lock?

Thanks

-- 
Dave Walker