Why a warning of "control reaches end of non-void function" for the main function?

Just put return 0 in your main(). Your function main returns an int (int main(void)) therefore you should add a return in the end of it.

Control reaches the end of a non-void function

Problem: I received the following warning:

warning: control reaches end of non-void function

Solution: This warning is similar to the warning described in Return with no value. If control reaches the end of a function and no return is encountered, GCC assumes a return with no return value. However, for this, the function requires a return value. At the end of the function, add a return statement that returns a suitable return value, even if control never reaches there.

source

Solution:

int main(void)
{
    my_strcpy(strB, strA);
    puts(strB);
    return 0;
}

As an alternative to the obvious solution of adding a return statement to main(), you can use a C99 compiler (“gcc -std=c99” if you are using GCC).

In C99 it is legal for main() not to have a return statement, and then the final } implicitly returns 0.

$ gcc -c -Wall t.c
t.c: In function ‘main’:
t.c:20: warning: control reaches end of non-void function
$ gcc -c -Wall -std=c99 t.c
$ 

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A note that purists would consider important: you should not fix the warning by declaring main() as returning type void.

The main function has a return-type of int, as indicated in

int main(void)

however your main function does not return anything, it closes after

puts(strB);

Add

return 0;

after that and it will work.