The importance of Declaration in c language

Let’s create two c language files,

and compile & link them:

But after run “./test”, the result is

Why the result from myNumber() become different in main() function?

Let’s see the assembly language source of test.c (gcc -S test.c)

It only get the 32-bits result from function ‘myNumber’ (The size of %eax register is just 32-bits, smaller than the size of “long long”). Actually, we missed the declaration of myNumber() in test.c file so it only consider the result of myNumber() as 32-bits size.
After adding the declaration of myNumber() into test.c, we could check the assembly language source has changed:

(The size of %rax register is 64-bits)
And the result of running ‘./test’ is correct now.