没有为多个定义生成编译器警告

Question

problem I am facing is function with same signature is defined in two .c files and is not giving compile time error. I have included declaration in .h file, which is included to both .c files.

For example:

int add(int x, int y) { return x+y;}

same definition is given in two .c files (Say Ac and Bc) and declaration in one .h file which is included in both Ac and Bc But why this is not giving compile time error or How can I make to give them compile error

Even Linker is not giving any error, it looks it is taking first definition

I am using GCC compiler mingw

I found another pattern in this. if I am using this in header file

#ifndef H_H_
#define H_H_

linker is not giving warning warning but If i don't use this Linker gives warning which is expected.

Answer 1

The compiler doesn't analyze your program as a whole. It simply processes one .c file at a time. If the declaration in the .h file matches the definition in the .c file, then everything is good as far as the compiler is concerned.

The linker will detect that the function was defined twice and will generate a "duplicate symbol" error.

Answer 2

Compiler sees each source file apart from the other. Compiler includes the content of header file(s) into Ac then geneates an object file A.obj from AcAobj file will contain symbols of the variables and functions defined in Ac On the other hand, compiler will process Bc apart without checking Ac, or any other source file, content. It will start by including header file(s) into Bc then it generates B.obj which also includes symbols of the variables and functions defined in Bc

As a result, you will not get errors at compile time as the function duplication is not detected by the compiler. It is the linker job to check the symbols consistency and that there are no duplication present. Linker will get all generated object files in order to generate an executable. Linker must assign a unique memory address to each symbol. For example, in your code if there is a point (let's say in main function) where a function of Ac is called, actually, this is translated into a jump to an address in memory where that function is located. Now, imagine if two functions with the same signature coexist in the executable and each symbol has a different address. Then, how can the processor figure out which function exactly do you intend to call in your program. For that reason, if linker finds a symbol which is duplicated it will signal an error.

Answer 3

This situation is undefined behaviour with no diagnostic required.

Consult your linker's documentation to see if it has any options to report multiple definition of functions.

Answer 4

As @Matt-McNabb says: consult your linker documentation.

The only other cause I can come up with is that the linker binary compares the two functions, finds they are idenical, and ignores one. You can check this by slightly changing the code, for example by 'return y+x'.