cpp/error/errno

is a preprocessor macro used for error indication. It expands to a modifiable lvalue of type.

Several standard library functions indicate errors by writing positive integers to. Typically, the value of is set to one of the error codes, listed in  as macro constants that begin with the letter, followed by uppercase letters or digits.

The value of is  at program startup, and although library functions are allowed to write positive integers to  whether or not an error occurred, library functions never store  in.