Like variables, function names should speak for themselves. The above arguments for naming variables apply also to functions. Instead of _, use mixed uppercase and lowercase.
It is also helpful to distinguish between global and local functions. Local functions should start with a lowercase letter, while global functions should start with an uppercase letter.
Do
// this is a local function
writeStringToFile (a_string, file_name);
// this is a global function
global_settings = ReadGlobalSettings ();Don't
f1 (a_string, file_name);
gs = rgs();