4.58.1 errno
The variable errno holds the last error code encountered by a library function. This variable must be cleared by the user prior to calling a library function.
CAUTION:
The errno global variable is not safe to use in a threaded or multi-task system. A race condition can occur if a task is interrupted between the call which sets error and when the task examines errno. If another task changes errno during this time, the result will be incorrect for the interrupted task.
int errno