intro — introduction to system calls
This section describes all of the system calls.
All of these calls return a function result.
This result indicates the status of the call.
Typically, a zero or positive result indicates that the call
completed successfully, and -1 indicates an error.
The individual descriptions specify the details.
An error number is also made available
in the external variable
is not cleared on successful calls.
Therefore, it should be tested only after an error has been indicated.