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shl_load fails
01-18-2012
shl_load fails
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pthread_stubs(5) File Formats Manual pthread_stubs(5) NAME
pthread_stubs - list of pthread calls for which the stubs are provided in the C library DESCRIPTION
The libc shared libraries in libc cumulative patches, and onwards, contain stubs for the pthread functions in and The stubs allow non- threaded applications to dynamically load thread-safe libraries successfully, so that the pthread symbols are resolved. Applications that resolve pthread/cma calls to the stub must be built without or on the link line. Stubs provided in do not have any functionality, these are dummy functions returning zero, except the pthread_getspecific(3T) family of APIs, which have full functionality implemented in the stubs. The pthread calls to any of the stub functions below return zero. pthread_atfork(3T) pthread_attr_destroy(3T) pthread_attr_getdetachstate(3T) pthread_attr_getguardsize(3T) pthread_attr_getinheritsched(3T) pthread_attr_getschedparam(3T) pthread_attr_getschedpolicy(3T) pthread_attr_getscope(3T) pthread_attr_getstackaddr(3T) pthread_attr_getstacksize(3T) pthread_attr_setdetachstate(3T) pthread_attr_setguardsize(3T) pthread_attr_setinheritsched(3T) pthread_attr_setschedparam(3T) pthread_attr_setschedpolicy(3T) pthread_attr_setscope(3T) pthread_attr_setstackaddr(3T) pthread_attr_setstacksize(3T) pthread_cancel(3T) pthread_cond_broadcast(3T) pthread_cond_destroy(3T) pthread_cond_init(3T) pthread_cond_signal(3T) pthread_cond_timedwait(3T) pthread_cond_wait(3T) pthread_condattr_destroy(3T) pthread_condattr_getpshared(3T) pthread_condattr_init(3T) pthread_condattr_setpshared(3T) pthread_continue(3T) pthread_detach(3T) pthread_getconcurrency(3T) pthread_getschedparam(3T) pthread_join(3T) pthread_kill(3T) pthread_mutex_destroy(3T) pthread_mutex_getprioceiling(3T) pthread_mutex_init(3T) pthread_mutex_lock(3T) pthread_mutex_setprioceiling(3T) pthread_mutex_trylock(3T) pthread_mutex_unlock(3T) pthread_mutexattr_destroy(3T) pthread_mutexattr_getprioceiling(3T) pthread_mutexattr_getprotocol(3T) pthread_mutexattr_getpshared(3T) pthread_mutexattr_gettype(3T) pthread_mutexattr_init(3T) pthread_mutexattr_setprioceiling(3T) pthread_mutexattr_setprotocol(3T) pthread_mutexattr_setpshared(3T) pthread_mutexattr_settype(3T) pthread_once(3T) pthread_rwlock_destroy(3T) pthread_rwlock_init(3T) pthread_rwlock_rdlock(3T) pthread_rwlock_tryrdlock(3T) pthread_rwlock_trywrlock(3T) pthread_rwlock_unlock(3T) pthread_rwlock_wrlock(3T) pthread_rwlockattr_destroy(3T) pthread_rwlockattr_getpshared(3T) pthread_rwlockattr_init(3T) pthread_rwlockattr_setpshared(3T) pthread_self(3T) pthread_setcancelstate(3T) pthread_setcanceltype(3T) pthread_setconcurrency(3T) pthread_setschedparam(3T) pthread_sigmask(3T) pthread_suspend(3T) pthread_testcancel(3T) The stubs for the following pthread calls have full functionality. Refer to pthread(3T) for more details. pthread_key_create(3T) pthread_getspecific(3T) pthread_setspecific(3T) pthread_key_delete(3T) pthread_exit(3T) Calls to the stubs listed below, pthread_self(3T) always returns 1. returns (arg1==arg2). pthread_create(3T) and pthread_attr_init(3T) return The above mentioned stubs are provided in libc because on HP-UX if a non-threaded application links to a thread-safe library, calls to thread-safe routines from the application fail at run time due to unresolved symbols of the form of To resolve these symbols it is neces- sary to link the non-threaded application to a threads library or However, linking to a threads library forces the application to use thread-safe features even if it creates no threads, resulting in a subsequent loss of performance. To overcome the above problem, stubs for APIs have been provided in the C library. Providing stubs for API's in the HP-UX C language library have two direct effects for non-threaded applications: o thread symbols are resolved if a non-threaded application links to a thread-safe library. o Avoids the overhead of a real thread library. Especially the overhead associated with mutexes when the non-threaded application uses thread stubs rather than the real thread library procedures. Link Order Problems An application may inadvertently pick up the stubs present in when it intended to use the real pthread APIs, or cma APIs, due to link order issues. An application that needs cma behavior must link to and must do so in the supported link order, i.e. the link line should only be shared and should not contain before As long as this condition is met, the correct cma functions will be referenced. Similarly, a multi- threaded application that needs pthread library behavior must link to libpthread and must do so in a supported link order, and only use shared and EXAMPLES
Below are examples of potential link order problems. Example 1 An application or any library linked, that requires pthread/cma calls to resolve to the pthread stubs in must be built without or on the link line. If is specified before or on the link line, pthread/cma calls resolve to pthread stubs in This may lead to problems as given in the exam- ples below: $ cat thread.c #include <pthread.h> #include <stdio.h> void *thread_nothing(void *p) { printf("Success "); } int main() { int err; pthread_t thrid; err = pthread_create(&thrid, (pthread_attr_t *) NULL, thread_nothing, (void *) NULL); sleep(1); if (err) { printf("Error "); return err; } } $ cc thread.c -lc -lpthread $ a.out Error $ chatr a.out a.out: shared executable shared library dynamic path search: SHLIB_PATH disabled second embedded path disabled first Not Defined shared library list: dynamic /usr/lib/libc.2 <- libc before libpthread dynamic /usr/lib/libpthread.1 shared library binding: deferred global hash table disabled ... Solution for Example 1 For threaded applications, run the executable with environment variable set to the library or link the executable with $ LD_PRELOAD="/usr/lib/libpthread.1" a.out Success $ cc thread.c -lpthread $ a.out Success $ chatr a.out a.out: shared executable shared library dynamic path search: SHLIB_PATH disabled second embedded path disabled first Not Defined shared library list: dynamic /usr/lib/libpthread.1 dynamic /usr/lib/libc.2 shared library binding: deferred global hash table disabled ... Example 2 Specifying before in threaded applications can cause run-time problems like the following because the pthread calls get resolved to stubs in rather than the functions in pthread library. o Calls to pthread functions fail, due to uninitialized internal structures. o Calls to gethostbyname(3N) fail and return null. o Apache webmin and perl DBI applications fail with the following error message: o Calls to shl_load(3X) fail with the following error: because the stub returns zero. $ cat a.c #include <stdio.h> #include <dl.h> extern int errno; main() { shl_load("lib_not_found", BIND_DEFERRED, 0); printf("Error %d, %s ", errno, strerror(errno)); } $ cc a.c -lc -lpthread $ a.out Error 22, Invalid argument $ LD_PRELOAD=/usr/lib/libpthread.1 ./a.out Error 2, No such file or directory $ cat b.c #include <stdio.h> #include <dlfcn.h> void* handle; extern int errno; main() { handle = dlopen("lib_not_found", RTLD_LAZY); printf("Error %d, %s ", errno, strerror(errno)); if (handle == NULL) { printf("Error: %s ",dlerror()); } } $ cc b.c -lc -lpthread $ a.out Error 22, Invalid argument Error: $ ./a.out $ LD_PRELOAD=/usr/lib/libpthread.1 Error 0, Error 0 Error: Can't open shared library: lib_not_found Due to the problems mentioned above, should never be specified in the build command of an executable or shared library. By default, the compiler drivers automatically pass to the linker at the end of the link line of the executables. To see if a shared library was built with look at the shared library list in the output (see chatr(1)), or list the dependent libraries with (see ldd(1)): $ cc +z -c lib1.c $ ld -b -o lib1.sl lib1.o -lc $ ldd lib1.sl /usr/lib/libc.2 => /usr/lib/libc.2 /usr/lib/libdld.2 => /usr/lib/libdld.2 /usr/lib/libc.2 => /usr/lib/libc.2 $ cc +DA2.0W +z -c lib1.c $ ld -b -o lib1.sl lib1.o -lc $ ldd lib1.sl libc.2 => /lib/pa20_64/libc.2 libdl.1 => /usr/lib/pa20_64/libdl.1 To see the order in which dependent shared libraries will be loaded at run-time (order is only valid in 64-bit mode), use the command on the executable in 32-bit mode displays the order in which libraries are loaded in reverse order): $ cc +DA2.0W thread.c -lpthread $ ldd a.out libpthread.1 => /usr/lib/pa20_64/libpthread.1 libc.2 => /usr/lib/pa20_64/libc.2 libdl.1 => /usr/lib/pa20_64/libdl.1 $ cc +DA2.0W thread.c -lc -lpthread $ ldd a.out libc.2 => /usr/lib/pa20_64/libc.2 libpthread.1 => /usr/lib/pa20_64/libpthread.1 libdl.1 => /usr/lib/pa20_64/libdl.1 $ cc +DA2.0W thread.c -lpthread -lc $ ldd a.out libpthread.1 => /usr/lib/pa20_64/libpthread.1 libc.2 => /usr/lib/pa20_64/libc.2 libdl.1 => /usr/lib/pa20_64/libdl.1 Recommendations: o Remove from the build command of all shared libraries o Remove from the build command of all executables o Use the environment variable set to the full pathname for or which will cause the library to be loaded at program startup before other dependent libraries. functionality is available in and later Linker patches. See the dld.sl(5) man page. o If you link directly with the ld(1) command instead of with a compiler driver, add as the last component on the link line. Example 3 (64-bit) If a 64-bit shared library is built with but the executable is not, is loaded before (due to breadth-first searching), and the pthread calls are resolved to the pthread stubs in libc. At run-time, after the is loaded, the dependencies of are loaded in breadth-first order: is loaded as a dependent of before is loaded as a dependent of The dependency list of the first case is: a.out / / lib1 lib2 libc | | libc libpthread Therefore the load graph is constructed as: This is the desired behavior for non-threaded applications, but causes threaded applications (that use either or to fail. specifies specifies and no on $ cc -c +z +DA2.0W lib1.c lib2.c lib1.c: lib2.c: $ ld -b -o lib1.sl -lc lib1.o $ ld -b -o lib2.sl -lpthread lib2.o $ cc +DA2.0W thread.c -L. -l1 -l2 $ a.out Error $ ldd a.out lib1.sl => ./lib1.sl lib2.sl => ./lib2.sl libc.2 => /usr/lib/pa20_64/libc.2 libc.2 => /lib/pa20_64/libc.2 libpthread.1 => /lib/pa20_64/libpthread.1 libdl.1 => /usr/lib/pa20_64/libdl.1 specifies and no on $ ld -b -o lib1.sl lib1.o $ ld -b -o lib2.sl -lpthread lib2.o $ cc +DA2.0W thread.c -L. -l1 -l2 $ a.out Error $ ldd a.out lib1.sl => ./lib1.sl lib2.sl => ./lib2.sl libc.2 => /usr/lib/pa20_64/libc.2 libpthread.1 => /lib/pa20_64/libpthread.1 libdl.1 => /usr/lib/pa20_64/libdl.1 The same problem will occur if is listed as a dependent library of a shared library, and you would need to link the executable with Recommendation for Example 3 For threaded applications, run the executable with set to the library or link the executable with Use to load first $ ld -b -o lib1.sl lib1.o $ ld -b -o lib2.sl -lpthread lib2.o $ cc +DA2.0W thread.c -L. -l1 -l2 $ a.out Error $ ldd a.out lib1.sl => ./lib1.sl lib2.sl => ./lib2.sl libc.2 => /usr/lib/pa20_64/libc.2 libpthread.1 => /lib/pa20_64/libpthread.1 libdl.1 => /usr/lib/pa20_64/libdl.1 $ LD_PRELOAD="/lib/pa20_64/libpthread.1" a.out Success correctly lists for a threaded application. $ ld -b -o lib1.sl lib1.o $ ld -b -o lib2.sl -lpthread lib2.o $ cc +DA2.0W thread.c -L. -l1 -l2 -lpthread $ a.out Success $ ldd a.out lib1.sl => ./lib1.sl lib2.sl => ./lib2.sl libpthread.1 => /usr/lib/pa20_64/libpthread.1 libc.2 => /usr/lib/pa20_64/libc.2 libpthread.1 => /lib/pa20_64/libpthread.1 libdl.1 => /usr/lib/pa20_64/libdl.1 Example 4 (archived libc) If the link line of your shared library contains to explicitly link in remove Otherwise, shared libraries may be referencing while the may refer to older (archived) version. Thus the application will actually be using two different versions of and possibly mixing the code. This may cause compatibility problems. Basically, an application or library should never directly link against All programs need to be linked against (which the compiler does automatically), so a shared library will always have the interfaces it needs to execute properly without needing to specify on the link line. SEE ALSO
chatr(1), ld(1), ldd(1), pthread(3T), shl_load(3X), dld.sl(5). pthread_stubs(5)