Re: link as -lc -lpthread hangs

[Channa <channa.kad%gmail.com@localhost>]

Hi,

2009/9/11 Christos Zoulas <christos%astron.com@localhost>:
> In article 
> <515c64960909100612h29118e2ftf2af05f513aa103a%mail.gmail.com@localhost>,
> Channa  <channa.kad%gmail.com@localhost> wrote:
>>Hi,
>>I am using NetBSD rtld implementation in my library.
>>I am facing some issue when the application is built by linking
>>with libc before libpthread. i.e  '-lc -lpthread' as shown below
>>
>>/usr/local/${ARCH}-linux/gcc app.c -o app -lc -lpthread
>>
>>When i run the 'app' it hangs.
>
> This is not supposed to work. You need to link -lpthread before -lc
> since pthread overrides some libc functions. In fact the only portable
> way to compile threaded programs is using gcc -pthread app.c -o app
>
> christos

Yes true pthread overrides some of the symbols in libc.
But there is a way which we can avoid the hang condition.
When the application is compiled by the method by linking
'-lc -lpthread' in that case the needed section of the application
contains the libc.so first and then libpthread.so.

In the rtld implementation the needed libraries are loaded on the same
order as present in applications needed section, so we can add a fix by
not loading libc.so if its the first entry in needed objects.

This can be done in the following file

$ cat src/libexec/ld.elf_so/load.c
:
:
+                static int flag = 0;
               for (needed = obj->needed; needed != NULL;
                   needed = needed->next) {
+                       flag++;
                       const char *name = obj->strtab + needed->name;
                       /*
                         * Avoid loading of 'libc.so' if its the first object
                         * in the needed section
                         */
+                       if (flag == 1 &&
!(strncmp(name,"libc.so",strlen(name))))
+                               continue;
               for (needed = obj->needed; needed != NULL;
                   needed = needed->next) {
+                       flag++;
                       const char *name = obj->strtab + needed->name;

:
:

So by doing this we can avoid loading libc.so first.

This will avoid hang condition if the application is built by
linking '-lc -lpthread'

Please let me know your views on this.

Waiting for your reply

Thanks & Regards,