errno - number of last error
The <errno.h> header file defines the integer variable errno, which is set by system calls and some library functions in the event of an error to indicate what went wrong. Its value is significant only when the return value of the call indicated an error (i.e., -1 from most system calls; -1 or NULL from most library functions); a function that succeeds is allowed to change errno.
Valid error numbers are all nonzero; errno is never set to zero by any system call or library function.
Impairment of goodwill(MSFT)