Compile-time error on declaration of `errno`

Question

During the process of compilation of my C++ program on Linux, it gives the following warning:

warning #584: omission of exception specification is incompatible with previous function "__errno_location" (declared at line 43 of "/usr/include/bits/errno.h")
extern int errno;//error handling
         ^

The code is shown below:

#include <errno.h>    //for error handling
#include <cmath>
#include <cstring>
#include <ctime>

extern int errno;//error handling

errno is a global variables, it is used in other .cpp files. How can I solve this?

Answer 1

errno is required to be a "modifiable lvalue", not necessarily a declared object.

Apparently on your implementation, errno expands to an expression that includes a call to a function called __errno_location.

In your own declaration:

extern int errno;

errno expands to that expression, resulting in an error.

Since <errno.h> already declared errno for you, there's no need to declare it yourself, and as you've seen, you can't declare it yourself.

Just omit that extern declaration.