Re: other/4372: #pragma weak pthread* inclusion causes applications to crash without a linker error when one forgets to link with -lpthread

[ehrhardt at mathematik dot uni-ulm dot de]

Synopsis: #pragma weak pthread* inclusion causes applications to crash without a linker error when one forgets to link with -lpthread

State-Changed-From-To: closed->analyzed
State-Changed-By: cae
State-Changed-When: Wed Jan 15 06:29:01 2003
State-Changed-Why:
    
    This testcase boils down to the following:
    
    extern void f (); 
    #pragma weak f
    int main(void)
    { 
    	f();
    }
    
    This compiles and links fine despite the missing symbol for f.
    This is correct (documented) behaviour as analyzed below.
    
    BUT: This construct (external declaration combined with weak pragma) is
    used in header files provided by gcc (gthr-default.h). The effect
    is that programs that use this header file can be linked without error
    messages even if they aren't linked against libpthread. Hoever these
    programs crash when  executed. Is there really any need for these pragmas
    at all? As far as I can see the situation is this:
    
    With pragmas:
    * If the functions aren't used everything works fine.
    * If the functions are used but there is no implementation for them
      the program compiles and links but crashes at run time.
    Without pragmas:
    * If the functions aren't used everything works fine.
      If the functions are used but there is no implementation for them
      the program compiles (to a .o) but it doesn't link.
    
    I'd highly prefer the latter, i.e. we get an error message instead of
    a broken executable. If the intention is to make the implementation
    of these functions weak, why can't this be done in the corresponding .c
    file that implements the function?
    
    Here are the exceprts form the manual to prove that the current
    behaviour of gcc is as documented:
    
    From gccs info file:
    | `weak'
    |      The `weak' attribute causes the declaration to be emitted as a weak
    |      symbol rather than a global.  This is primarily useful in defining
    |      library functions which can be overridden in user code, though it
    |      can also be used with non-function declarations.  Weak symbols are
    |      supported for ELF targets, and also for a.out targets when using
    |      the GNU assembler and linker.
    
    Note that this explicitly mentions declarations, not just definitions.
    The nm manual page describes the behaviour of weak symbols as follows:
    
    |          "W" The symbol is  a  weak  symbol  that  has  not  been
    |              specifically tagged as a weak object symbol.  When a
    |              weak defined symbol is linked with a normal  defined
    |              symbol,  the  normal  defined symbol is used with no
    |              error.  When a weak undefined symbol is  linked  and
    |              the  symbol  is  not  defined, the value of the weak
    |              symbol becomes zero with no error.
    
    The last sentence applies to our situation: A weak undefined symbol
    is replaced with zero and no error occurs.
    
        regards   Christian Ehrhardt

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=4372