On Thu, Mar 17, 2011 at 9:59 PM, Pan ruochen <panruochen@xxxxxxxxx> wrote:
> Hi Ian,
>
> Yes, setting the environment LD_DYNAMIC_WEAK did work.
> Now I get another question about weak symbols of glibc.
> I find there are some symbols with the same names existing both in libc
> and in libpthread. All these symbols are weak. I guess the libpthread
> provides multi-threading safe functions rather than libc provides
> single-threading versions. But since all these symbols are weak, how the
> dynamic loader choose the symbol in libpthread if libpthread is linked
> against the application?
>
> $nm -D libpthread.so.0 | grep '\<sendto\>'
> 00010d54 W sendto
> $nm -D libc-2.8.so | grep '\<sendto\>'
> 000e0704 W sendto
>
>From gABI:
NOTE: The behavior of weak symbols in areas not specified by this
document is implementation defined. Weak symbols are intended
primarily for use in system software. Applications using weak symbols
are unreliable since changes in the runtime environment might cause
the execution to fail.
In another word, you should use weak symbols only in ways specified in
gABI, which are
Global and weak symbols differ in two major ways.
* When the link editor combines several relocatable object files,
it does not allow multiple definitions of STB_GLOBAL symbols with the
same name. On the other hand, if a defined global symbol exists, the
appearance of a weak symbol with the same name will not cause an
error. The link editor honors the global definition and ignores the
weak ones. Similarly, if a common symbol exists (that is, a symbol
whose st_shndx field holds SHN_COMMON), the appearance of a weak
symbol with the same name will not cause an error. The link editor
honors the common definition and ignores the weak ones.
* When the link editor searches archive libraries [see ``Archive
File'' in Chapter 7], it extracts archive members that contain
definitions of undefined global symbols. The member's definition may
be either a global or a weak symbol. The link editor does not extract
archive members to resolve undefined weak symbols. Unresolved weak
symbols have a zero value.
Any other usage is implementation defined.
--
H.J.
