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Re: Two pthread libraries on Red Hat 9



Hi Thomas,

I don't really know what these two libraries are, but I would use some sort of sync to make sure that the main thread doesn't exit before the spawned one even starts. I do know that LinuxThreads doesn't necessarily immediately start the thread spawned by pthread_create(). Maybe the difference in behavior is just a race condition due to a subtle timing difference?

Oh, could the other thread library by NGPT or NPTL?

Don

Thomas Bailey writes:

Good morning,

I've already posted this to the general RedHat list - but it was
suggested it might be more appropriate to the development list.


I was wondering if anyone could explain to me the difference between the
pthread libraries in /lib and /lib/tls on RedHat 9. I ask because the
library in /lib/tls (which is linked by default when you supply
-lpthread to gcc) displays unexpected behavior, whereas the library in
/lib or /lib/i686, behaves correctly.


The code below demonstrates the difference:

#include <dlfcn.h>
#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <time.h>
#include <pthread.h>
#include <unistd.h>


#ifdef __cplusplus
extern "C" {
#endif


/// Our global key for accesing the TSS
static pthread_key_t gcKey;


/// Destructor
static void DestroyKey (void* pcValue)
{
printf("Freeing TSS\n");
abort();
free (pcValue);
}



/// Initialization code
static void* Init(void *arg)
{
pthread_key_create (&gcKey, DestroyKey);
pthread_setspecific(gcKey, malloc(42));
return (void *) 0;
}



int
main (int argc, char* argv[])
{
pthread_t myThread;
pthread_create(&myThread, NULL, Init, NULL);
return 0;
}
}


In this example program, we create a thread, and allocate some thread
specific storage, passing a pointer to the DestroyKey function to
pthread_key_create. We would expect this function to be called when the
thread's thread specific storage is destroyed, i.e. on thread exit. We
would then expect the program to print a message and abort (i.e. dump
core).


When linked against /lib/tls/libpthread-0.29.so, DestroyKey is not
called. If I change the permissions on /lib/tls/libpthread-0.29.so and
/lib/tls/libc-2.3.2.so to make them unreadable, so my program gets
linked against /lib/i686/libpthread.so.0 and /lib/i686/libc.so.6,
DestroyKey is called on thread exit.


I'm curious as to whether anyone knows a) if this is a bug in the
default threading library and b) if anyone knows a more elegant way to
force my application to link against the 'right' libraries other than
changing the permissions on the 'wrong' libraries. I have tried the
-nostdlib option to gcc.


Regards

Tom Bailey





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