Open System Services System Calls Reference Manual (G06.28+, H06.05+)

System Functions (s and S) setregid(2)
STANDARDS CONFORMANCE
The following are HP extensions to the XPG4 Version 2 specication:
If both the real and effective group IDs are changed so that they differ from each other
and from the saved-set group ID, then the saved-set group ID is set to the value of the
effective group ID.
A process without appropriate privileges can set the effective group ID if the new
effective group ID matches a group ID in the group list of the process.
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