Question

I have got a C function in a static library, let’s call it A, with the following interface :

int A(unsigned int a, unsigned long long b, unsigned int *y, unsigned char *z); 

This function will change the value of y an z (this is for sure). I use it from within a dynamic C++ library, using extern ‘C’.

Now, here is what stune me :

• y is properly set, z is not changed. What I exactly mean is that if both are initialized with a (pointed) value of 666, the value pointed by y will have changed after the call but not the value pointed by z (still 666).
• when called from a C binary, this function works seamlessly (value pointed by z is modified).
• if I create a dummy C library with a function having the same prototype, and I use it from within my dynamic C++ library, it works very well. If I re-use the same variables to call A(..), I get the same result as before, z is not changed.

I think that the above points show that it is not a stupid mistake with the declaration of my variables.

I am clearly stuck, and I can’t change the C library. Do you have any clue on what can be the problem ? I was thinking about a problem on the C/C++ interface, per instance the way a char* is interpreted.

Edit : I finally found out what was the problem. See below my answer.

Answer 1

First of all, I am very grateful to everyone for your help. Thanks to the numerous ideas and clues you gave me, I have been able to finally sort out this problem. Your advices helped me to question what I took for granted.

Short answer to my problem : The problem was that my C++ library used an old version of the C library. This old version missed the 4th argument. As a consequence, the 4th argument was obviously never changed.

I am a bit ashamed now that I realised this was the problem. However, I was misslead by the fact that my code was compiling fine. This was due to the fact that the C++ library compiled against the correct version of the C lib, but at runtime it used the old version statically linked with another library that I was using.

C++ Lib (M) ---> dyn C++ lib (N) ---> C lib (P) v.1.0      |      ------> C lib (P) v.1.1 

(N) is a dynamic library which is statically linked with (P) version 1.0. The compiler accepted the call from (M) to the function with 4 arguments because I linked against (P) version 1.1, but at runtime it used the old version of (P).

Feel free to edit this answer or the question or to ask me to do so.