[Dovecot] imap be killed (core dump disabled)

Hello everybody, When I modify the dovecot imap source ,in "src/imap/" dir, I meet the Error below: Error: child (imap) kill (core dump disabled).

In fact, I want to record the imap operation through dovecot. So, I modify every source file in "src/imap/cmd-IMAP_OPERATION" ,when the imap command function is executed, it will write to a file cur time and operation.

Could you tell me why the imap be killed?

Thank you.