June 23, 2011, 9:42 p.m.
posted by tyler
10.9. kill and raise Functions
The kill function sends a signal to a process or a group of processes. The raise function allows a process to send a signal to itself.
is equivalent to the call
There are four different conditions for the pid argument to kill.
As we've mentioned, a process needs permission to send a signal to another process. The superuser can send a signal to any process. For other users, the basic rule is that the real or effective user ID of the sender has to equal the real or effective user ID of the receiver. If the implementation supports _POSIX_SAVED_IDS (as POSIX.1 now requires), the saved set-user-ID of the receiver is checked instead of its effective user ID. There is also one special case for the permission testing: if the signal being sent is SIGCONT, a process can send it to any other process in the same session.
POSIX.1 defines signal number 0 as the null signal. If the signo argument is 0, then the normal error checking is performed by kill, but no signal is sent. This is often used to determine if a specific process still exists. If we send the process the null signal and it doesn't exist, kill returns 1 and errno is set to ESRCH. Be aware, however, that UNIX systems recycle process IDs after some amount of time, so the existence of a process with a given process ID does not mean that it's the process that you think it is.
Also understand that the test for process existence is not atomic. By the time that kill returns the answer to the caller, the process in question might have exited, so the answer is of limited value.
If the call to kill causes the signal to be generated for the calling process and if the signal is not blocked, either signo or some other pending, unblocked signal is delivered to the process before kill returns. (Additional conditions occur with threads; see Section 12.8 for more information.)