There is no such guarantee in C99: 7.5 ¶2: [...] and `errno` which expands to a modifiable lvalue that has type `int`, the value of which is set to a positive

There is no such guarantee in C99: 7.5 ¶2: [...] and `errno` which expands to a modifiable lvalue that has type `int`, the value of which is set to a positive error number by several library functions. It is unspecified whether `errno` is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual object, or a program defines an identifier with the name `errno`, the behavior is undefined. 7.5 ¶3: The value of `errno` is zero at program startup, but is never set to zero by any library function. The value of `errno` may be set to nonzero by a library function call whether or not there is an error, provided the use of `errno` is not documented in the description of the function in this International Standard. The fact that `errno` can expand to an lvalue does reflect what is required for multithreading implementations among others, but that's about all.