Question

I have 2 questions regarding different declarations of the same function and global variable in two files in case of C and C++ as well.

1. Different function declarations

   Consider the following code fragments:

   file_1.c

   void foo(int a);
   
   int main(void)
   {
       foo('A');
   }
   

   file_2.c

   #include <stdio.h>
   
   void foo(char a)
   {
       printf("%c", a); //prints 'A' (gcc)
   }
   

   As we can see, the prototype differs from the definition located in
   file_2.c, however, the function prints expected value.

   If it comes to C++, the above program is invalid due to undefined
   reference to foo(int) at link time. It’s probably caused by
   presence of other function signatures – in comparison with C, where
   a function name doesn’t contain any extra characters indicating the
   type of function arguments.

   But when it comes to C then what? Since the prototypes with the same
   name have the same signature regardless of the number of arguments
   and its types, linker won’t issue an error. But which type
   conversions are performed in here? Does it look like this: 'A' ->
   int -> back to char? Or maybe this behavior is
   undefined/implementation-defined ?

2. Different declarations of a global variable

   We’ve got two files and two different declarations of the same
   global variable:

   file_1.c

   #include <stdio.h>
   
   extern int a;
   
   int main(void)
   {
       printf("%d", a); //prints 65 (g++ and gcc)
   }
   

   file_2.c

   char a = 'A';
   

   Both in C and C++ the output is 65.

   Though I’d like to know what both standards say about that kind of
   situation.

   In the C11 standard I’ve found the following fragment:

   J.5.11 Multiple external definitions (Annex J.5 Common extensions)
   There may be more than one external definition for the identifier of
   an object, with or without the explicit use of the keyword extern; if
   the definitions disagree, or more than one is initialized, the
   behavior is undefined (6.9.2).

   Notice that it refers to presence of two and more definitions, in
   my code there is only one, so I’m not sure whether this article is a good point of reference in
   this case…

Answer 1

Q1. According to C99 specification, section 6.5.2.2.9, it is an undefined behavior in C:

If the function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function, the behavior is undefined.

The expression “points to” a function taking an int, while the function is defined as taking a char.

Q2. The case with variables is also undefined behavior, because you are reading or assigning an int to/from char. Assuming 4-byte integers, this will access three bytes past the memory location where it is valid. You can test this by declaring more variables, like this:

char a = 'A';
char b = 'B';
char c = 'C';
char d = 'D';