exit codes to shell from main? exit()?

What is the correct way to pass exit codes from a FORTRAN main to the calling shell?
In the past we've used:
call exit(icode) where icode is an integer byte.

call exit() seems to work fine but there is no function I could find in the documentation.
Seems to be a supported extension?
What library is this function in?

EXIT is a reserved word for exiting out of DO loops.

Thanks for any info.