The Return_code parameter is referred to as the errno in the POSIX C interface. The Return_code is returned only if the service fails.

In the callable service description, some of the possible return codes are listed for services that have return codes. The return codes are described in each service if they help describe its function.

Reason codes are listed with the return codes they describe.

The return codes and their descriptions are found in z/OS UNIX System Services Messages and Codes.

Some Return_code values may occur for any callable service: the ones that are unique to z/OS UNIX. They are not always listed under each callable service. See z/OS UNIX System Services Messages and Codes for a description of these return codes.