error_log confusion!

[dscsvsv vdvvdfsvv]

I am a bit confused about what is exactly the correct syntax and place for having error_log set in mysql:

1- should I define the entry in [mysqld_safe] section or [mysqld] section or both? what is the difference?

2- should I define it as: log-error = /var/log/mysql/error_mysql.log
or log_error = /var/log/mysql/error_mysql.log
( is it under-score correct or hypen or doesn't matter? )