Why redefinition of a function which is already present in dynamic or static library does not throw any error?

Question

Wy redefinition of function already present in dynamic library does not throws any compilation and linking error?

In the below function

#include "calc_mean.h"
#include <stdio.h>

int mean(int t, int v) {
  return 0;
}

int main () {
  int theMean = mean(3,6);
  printf("\n  %d\n",theMean);
}

Inside the shared library Definition of mean function already present as below.

#include <stdio.h>
#include "calc_mean.h"

int mean(int a, int b) {
  return (a+b)/2;
}

The definition of mean function is already present in the shared library libmean.so. But during compilation I don't see any redefinition error and compilation is successful.

And on successful execution the o/p I see is 0 instead of 4 so the function definition of mean inside the shared library is not getting executed but the one inside the main module is getting executed.

Why is this happening so?

You always can redifined your shared lib functions, the source code is prio on compilation. — Yann, Jun 24 '14 at 15:14
But as the definition of the function is already present in the shared library and we are linking the main program along with the shared library .So redefining a function already defined in the library should throw a error . — Abhishek parikshya, Jun 24 '14 at 15:18
related: http://stackoverflow.com/questions/13089166/how-to-make-gcc-link-strong-symbol-in-static-library-to-overwittren-weak-symbol — Ciro Santilli OurBigBook.com, May 27 '15 at 09:49

alk · Answer 1 · 2019-10-11T11:12:19.430

7

The linker only links in a function from a library if the function had not yet been found during the compilation/linking process.

The reason for the difference in functionality is that there are different types of symbols. A library function is a weak symbol. It is only included if it is not already defined. nm is a tool for listing the symbols in an object or executable. In its man-page you can find a list of the types of symbols.

There is also a wikipedia page on weak symbols.

edited Oct 11 '19 at 11:12

answered Jun 24 '14 at 15:16

alk

69,737
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Why is it that `nm -D /lib/x86_64-linux-gnu/libc.so.6`tells me that `puts` is `W`, `printf` is `T`, but I can redefined both in a program with GCC? This makes it look like being weak (in the ELF sense) is not the question. – Ciro Santilli OurBigBook.com May 27 '15 at 08:36

score 1 · Answer 2 · answered Jun 25 '14 at 07:43

Having two definitions of one externally-visible function (even if the definitions are identical, for non-inline functions) causes undefined behaviour, with no diagnostic required. (Ref: C99 6.9#5 and Annex J.2)

In C, some illegal code requires a compiler diagnostic and some doesn't. Typically the ones that do not require a diagnostic are because:

it would be considered too prohibitive to require all compilers to detect and report the error
there were existing systems in use that did not diagnose it and the Standard committee did not want to render an existing implementation non-conforming.

In this case, my guess would be that this is a case of the first one; they wanted to leave open the option for compilers/linkers to implement weak symbols as an extension, so they did not specify that the compiler must give a warning here. Or possibly it is actually difficult to detect this in general, I've never tried to write a linker!

It should be considered a quality-of-implementation issue if no diagnostic is given. Perhaps it is possible to pass different flags to your linker so that it does reject this code; if not then you could put a in bug report or a feature request.

score -2 · Answer 3 · answered Jun 24 '14 at 15:18

-2

Did you link correctly the shared library because the compiler should give the error :

multiple definition of 'mean'

answered Jun 24 '14 at 15:18

Chemseddine

7

Why redefinition of a function which is already present in dynamic or static library does not throw any error?

3 Answers3