The fork-exec code in my program currently does some things that are supposedly unsafe, but I can't figure what to use instead of initgroups() which is not asyn

The fork-exec code in my program currently does some things that are supposedly unsafe, but I can't figure what to use instead of initgroups() which is not async-signal-safe. I want to run the child as a different user for which I do initgroups+setgid+setuid between fork and exec. The only solution I see is to run getgrouplist() before the fork then in the child use setgroups() instead of initgroups(), but both of these functions are non-standard. EDIT: Never mind, seems like initgroups() is also nonstandard but generally available on Unix-like systems.