c programming

Question

Will it result in to an error if a header file is included twice?

[A]. Yes [B]. No [C]. It is compiler dependent

Answer 1

Edit: It depends on contents of the header file.

Header files are literally copied and pasted into the body of your C files

So may be it will give an error if the same function is there twice. 

In compiler like GCC

If a header file happens to be included twice, the compiler will process its contents twice. This is very likely to cause an error, e.g. when the compiler sees the same structure definition twice. Even if it does not, it will certainly waste time.

Source: https://gcc.gnu.org/onlinedocs/gcc-3.0.2/cpp_2.html

So the answer is "Yes" though not for all header files.

Comments

Then why it is compiler dependent? It should be "Yes" though not for all header files.

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Sir because some c/c++ compilers have include guard (same header file cannot be included twice). So in that for all header file, there will be no issue. Correct me if am wrong/

Source: https://en.wikipedia.org/wiki/Include_guard

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include guards are part of include files -- so not really part of compiler. A compiler will just include the file contents as iit is -- because that is what C language says.

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Ok, sir. Thanks for pointing this out. Have made required changes to answer.

Answer 2

Header file is nothing but the file which contains the declarations(or prototypes) of the functions which we use in our code..For example  , for using input - output functions , we need "stdio.h" header file..

For string manipulation functions , like strlen() we need "string.h" header file..So including a header twice will have no problem ; just a redundancy as the function in main() will get its prototype from any one of the include statement..

Hence it should not give an error..

Answer 3

No error will be printed