the last sentence of the first paragraph of 4.2.1. SysV IPC says: The difference between a message and a signal is that the message only consists of the signal number. i think the corrected sentence could read: The difference between a message and a signal is that a message contains a message id and a message pointer, whereas a signal contains only a signal id. https://www.freebsd.org/doc/en/books/arch-handbook/jail-restrictions.html