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I have a test.c file which contains main() function and some test cases and it cannot be modified it(such as adding "include *.h"). Then I have a foo.c file which contains some functions(no main() function). These functions will be tested through test cases in test.c file. What I'm going to do is use foo.c as a library and link it to test.c file. And here is the simple code.

test.c

//cannot modify int main(){ ... bar(); ... }

foo.c

#include "foo.h" //I will explain this below. int bar(){ ... }

I'm trying to implement an interface using .h file, such as

foo.h

#ifndef _FOO_H_ #define _FOO_H_ extern int bar(); #endif

Then using cmd line

gcc -c foo.c gcc -o output test.c foo.o ./output

You may guess the result. There is a warning that "implicit declaration of function 'bar' is invalid in C99 [-Wimplicit-function-declaration]". And the test.c file cannot run correctly.

Could someone help me about this? Thank you so much!

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  • 1
    What the heck is main.c? Commented May 2, 2016 at 2:27
  • sry, it's a typo. It should be test.c. I have corrected it Commented May 2, 2016 at 2:29

2 Answers 2

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Your problem is:

  1. test.c has a call to bar() in it.
  2. test.c doesn't have any declaration for bar, nor does it have an #include for a .h file that declares bar.
  3. You are not allowed to change test.c in any way to add either a declaration or an #include.

This is a hard problem. The C language requires there be a prototype/declaration for bar in test.c! It can be written directly in the test.c file (write extern int bar(); before you call it), or the declaration can come in from another file with an #include statement, but you must have it.

Luckily, GCC has a way to force an #include statement into a file while it's compiling the file. You don't have to change test.c in order to make it start with #include "foo.h". This will solve your problem:

gcc -c -include foo.h test.c
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Many people do not think about that one
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You need to include the declaration of bar in the test.c file:

#include "foo.h"

So that the compiler have the prototype in the translation unit, of test.c.

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3 Comments

As I said above, test.c file cannot be modified.
@AATRSong The warning is because test.c called a function that it does not know its prototype. The only way is add it to test.c. "test.c cannot be modified" sounds a weird restriction, can you tell me why?
You assume I don't understand the warning? Good Job. Listen, the test file isn't implement by me. The person who have the test file just use it to test the foo.c file.

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