threads and printf problem

[Ferenc Engard]

Hi all,

I am learning linuxthreads, and have the following problem:

I start a hundred of threads with the following code:

void *task(void *arg) {
	int i=rand()%100000;
	printf("%4d start (sleep: %5d)\n",(int)arg,i);
	usleep(i);
	printf("%4d stop\n",(int)arg);
	return 0;
}

The problem is, that in the output there are several duplicate rows:

[...]
  63 start (sleep: 36327)
  64 start (sleep: 60336)
  64 start (sleep: 60336)
   3 stop
   5 stop
[...]

The 64th thread's start row is duplicated! The duplication is occures
when a 'start' row is followed by a 'stop' row. It is quite frequent,
but if I redirect the output to a file (./test >filename) then it does
not occure.

Moreover, after many-many tries here is one more:

[...]
  95 start (sleep: 10012)
  96   7 stop                <-- 96 start... 1st half
  12 stop
[...]
  72 stop
 start (sleep: 36226)        <-- 96 start... 2nd half
  97 start (sleep: 98586)
[...]

I thought that glibc (including printf) is thread-safe, but it
seems is not. My question is, how can I determine which functions are
thread-safe and which are not?

glibc 2.3.1, linux 2.4.20

Thanks:
Circum