errno.h

The errno.h header provides macros for reporting error conditions in function calls of the standard library and for any program that includes it.

Excluding macro EILSEQ.

Macros

EDOM: Error: Domain
Expands to 33
Error when input argument to a mathematics function is outside the domain of the function.
POSIX.1-2017: Mathematics argument out of domain of function.
ERANGE: Error: Range
Expands to 34
Error when input argument to a mathematics function is outside the range of the function.
POSIX.1-2017: Result too large.
errno: Error number
Expands to (* __errno_location())
Reported error condition of function call (thread-local storage).
errno is initially 0 at program startup. errno should be set to 0 before calling a function, whether part of the standard library or the program itself, which is known to report error conditions. Once errno has been assessed, it should be reset to 0. errno has thread-local storage duration but for each subsequent thread after the initial thread, errno will have an indeterminate value and should therefore be set to 0 before calling a function as previously stated.
The __errno_location function returns an int * which is dereferenced.