If I put the following test program (test.c) into the unpacked libprelude tarball (libprelude-0.9.xx):

#include <prelude.h>
#include <prelude-list.h>
#include <prelude-timer.h>
#include <prelude-log.h>
#include <prelude-async.h>
#include <prelude-io.h>
#include <prelude-plugin.h>
#include <prelude-msg.h>
#include <prelude-message-id.h>
#include <prelude-msgbuf.h>
#include <prelude-client.h>
#include <prelude-string.h>
#include <prelude-timer.h>
#include <idmef-message-id.h>
#include <idmef-tree-wrap.h>
#include <idmef-data.h>

#include <stdio.h>
#include <string.h>

static prelude_client_t *client = NULL;

int main(int argc, char **argv) {
  int ret = 0;
  char *profile;
  prelude_client_flags_t flags;

  profile = strdup("test");
  ret = prelude_thread_init(NULL);
  printf("ret: %d\n", ret);
  ret = prelude_init(NULL, NULL);
  printf("ret: %d\n", ret);
  ret = prelude_client_new(&client, profile);
  printf("ret: %d\n", ret);

  flags = PRELUDE_CLIENT_FLAGS_ASYNC_SEND | PRELUDE_CLIENT_FLAGS_ASYNC_TIMER;
  ret = prelude_client_set_flags(client, prelude_client_get_flags(client) | flags);
  printf("ret: %d\n", ret);
}

compile it with

gcc -L src/.libs/ -I . -I src/include -I src/libprelude-error -lprelude test.c -o test

and run it via

LD_LIBRARY_PATH=src/.libs/ ./test

I get the following result with libprelude <= 0.9.19 (i.e. 0.9.16):

ret: 0
ret: 0
ret: 0
ret: 0

and with libprelude >= 0.9.20 (i.e. 0.9.21.3) I get the following
output:

ret: 0
ret: 0
ret: 0
18 Dec 14:07:16 (process:14773) ERROR: error creating asynchronous thread: Function not implemented. (prelude-async.c:243 do_init_async)
ret: 0

I'd expect that prelude_client_set_flags() should return != 0 if
something goes wrong, but maybe I just got the initialisation sequence
wrong.

This behaviour occurs on openSUSE 10.3, i586, gcc 4.2.1, glibc 2.6.1.

If you need any additional information for me or would like me to
perform some tests for you, feel free to ask, I'll be glad to help.