The sig signal is sent to the process whose ID is equal to pid.
If pid is zero:
The sig signal is sent to all processes whose group ID is equal to the process group ID of the sender, and for which the process has permission; this is a variant of killpg(2).
If pid is -1:
If the user has super-user privileges, the signal is sent to all processes excluding system processes (with P_SYSTEM flag set), process with ID 1 (usually init(8)), and the process sending the signal. If the user is not the super user, the signal is sent to all processes with the same uid as the user excluding the process sending the signal. No error is returned if any process could be signaled.
For compatibility with System V, if the process number is negative but not -1, the signal is sent to all processes whose process group ID is equal to the absolute value of the process number. This is a variant of killpg(2).
No process can be found corresponding to that specified by pid.
The process id was given as 0 but the sending process does not have a process group.
The sending process is not the super-user and its effective user id does not match the effective user-id of the receiving process. When signaling a process group, this error is returned if any members of the group could not be signaled.