HP 9000 Series 300 Tutorials Manual page 116

To determine which of these conditions caused the error, your program should
check for the following values of errno:
errno Value
EBADF
ENOTTY
EIO
Error Condition
eid does not refer to an open file.
eid does not refer to a raw interface file.
Interface is not System Controller.
In addition, hpib_abort can return the following values for errno:
errno Value Error Condition
ETIMEDOUT A timeout occurred (EIO on Series 300)
EACCES The interface associated with this eid was locked by another process
and
D_NDELAY
was set for this eid
3-56 Controlling the HP-IB Interface