For a successful call, the return value depends on the operation:
KEYCTL_GET_KEYRING_ID
The ID of the requested keyring.
KEYCTL_JOIN_SESSION_KEYRING
The ID of the joined session keyring.
KEYCTL_DESCRIBE
The size of the description (including the terminating
null byte), irrespective of the provided buffer size.
KEYCTL_SEARCH
The ID of the key that was found.
KEYCTL_READ
The amount of data that is available in the key,
irrespective of the provided buffer size.
KEYCTL_SET_REQKEY_KEYRING
The ID of the previous default keyring to which implicitly
requested keys were linked (one of
KEY_REQKEY_DEFL_USER_*).
KEYCTL_ASSUME_AUTHORITY
Either 0, if the ID given was 0, or the ID of the
authorization key matching the specified key, if a nonzero
key ID was provided.
KEYCTL_GET_SECURITY
The size of the LSM security label string (including the
terminating null byte), irrespective of the provided
buffer size.
KEYCTL_GET_PERSISTENT
The ID of the persistent keyring.
KEYCTL_DH_COMPUTE
The number of bytes copied to the buffer, or, if arg4 is
0, the required buffer size.
All other operations
Zero.
On error, -1 is returned, and errno is set to indicate the error.
