Help with linking and symbol resolution

Help with linking and symbol resolution

[Lakshay Garg]

Hello everyone

I am taking a compilers course and have stumbled on a problem which I
am unable to resolve. Here is the code I am dealing with:

/* f1.c */
#include <stdio.h>

double c;
int main() {
  c = 1;
  printf("sizeof(c) = %lu\n", sizeof(c));
  return 0;
}

/* f2.c */
int c = 0;

I compile the program using gcc f1.c f2.c. From what I have learnt, I
believe that the output of the file should be sizeof(c) = 4 since the
declaration of c in f1.c is weak and would be resolved to the symbol c
in file f2.c which is strong. I also checked the symbol table using
the readelf utility which also shows that the size of c is 4. Here is
a snippet of the output from readelf -s ./a.out:

Num: Value           Size Type   Bind   Vis     Ndx Name
--------------------------------------------------------------
61:  0000000000400430 42  FUNC   GLOBAL DEFAULT 14 _start
62:  000000000060103c  4  OBJECT GLOBAL DEFAULT 26 c
63:  0000000000601038  0  NOTYPE GLOBAL DEFAULT 26 __bss_start
--------------------------------------------------------------

But the output from the program is sizeof(c) = 8. Could someone please
explain this behavior? I am using Ubuntu 16.04, gcc version 5.4.0

If the question is unclear, please see slides 22,23,24 from here:
https://www.cs.cmu.edu/afs/cs/academic/class/15213-f10/www/lectures/11-linking.pdf

Thank You

--
Lakshay G.

Re: Help with linking and symbol resolution

[Alan Modra]

On Sat, Jan 28, 2017 at 08:59:35PM +0530, Lakshay Garg wrote:
> From what I have learnt, I
> believe that the output of the file should be sizeof(c) = 4 since the
> declaration of c in f1.c is weak and would be resolved to the symbol c
> in file f2.c which is strong.

No.  (Also don't ignore linker warnings).

Read the C standard regarding sizeof.  What is the type of "c" in f1.c?

-- 
Alan Modra
Australia Development Lab, IBM

Re: Help with linking and symbol resolution

[Nick Clifton]

Hi Lakshay,

  Alan has already given you the correct answer to this quandary.

  I thought however that I should also point out that there is an 
  error in the slides you referenced:

> If the question is unclear, please see slides 22,23,24 from here:
> https://www.cs.cmu.edu/afs/cs/academic/class/15213-f10/www/lectures/11-linking.pdf

  Slide 22 is wrong.  The definition of foo in p2.c creates a 
  *common* symbol, not a *weak* one.  Common symbols can be
  combined with strong symbols of the same name, provided that
  their size and alignment are the same.

  Weak symbols are created using an attribute, at least when 
  using gcc.  To use your example:
  
/* f1.c */
#include <stdio.h>

double c __attribute__((weak));
int main() {
  c = 1;
  printf("sizeof(c) = %lu\n", sizeof(c));
  return 0;
}

/* f2.c */
int c = 0;

  This should compile and link without any warning messages.  
  You will still get the size being reported as 8 however, because
  of the way the sizeof operator works.

  Note - the assignment "c = 1" at the start of main() is actually
  quite dangerous since it will be storing an 8-byte value into a
  4-byte sized area of memory.  See the "puzzles" on slide 24.

Cheers
  Nick