open ("/dev/my_dev",O_RDWR): Operation not permitted

I'm running Vanilla 2.6.30.1 with busybox.
/dev contains a device file called my_dev. It was created using:
mknod my_dev c 248 0
I have a multithreaded application. When I try to open /dev/my_dev using:
rc=open ("/dev/my_dev",O_RDWR)
in the main thread (just after main), I get rc=6 which is OK.
But when I try to open my_dev withing a thread I get:
rc=-1, errno=1 (Operation not permitted).
How can I debug this problem ?
What can cause: Operation not permitted ?

Thanks.

This is the EPERM error on the open, which would indicate that the thread does not have permission to open the file. It would be worth checking how you have created the thread (because that is where the permissions are first set).

Originally Posted by neonsignal This is the EPERM error on the open, which would indicate that the thread does not have permission to open the file. It would be worth checking how you have created the thread (because that is where the permissions are first set).

Thank you for your reply. Here is pthread_attr_t:
typedef struct __pthread_attr_s
{
int __detachstate;
int __schedpolicy;
struct __sched_param __schedparam;
int __inheritsched;
int __scope;
size_t __guardsize;
int __stackaddr_set;
void *__stackaddr;
size_t __stacksize;
} pthread_attr_t;

Which of the fields in the structure can cause the open error ?

Thanks.

here is some sample code (the open succeeds in both the main thread and the secondary thread).

#include <pthread.h>
#include <fcntl.h>
#include <iostream>
void *threadFn(void *p)
{
// open device in second thread
	int handle = open("/dev/dsp",O_RDWR);
	std::cout << "secondary thread " << handle << std::endl;
	if (handle > 0) close(handle);
}
int main()
{
// open device in main thread
	int handle = open("/dev/dsp",O_RDWR);
	std::cout << "main thread " << handle << std::endl;
	if (handle > 0) close(handle);
// start second thread
	pthread_t thread;
	pthread_create( &thread, 0, threadFn, 0);
	pthread_join(thread, 0);
}

Note that you cannot open a handle for writing from both threads at the same time (and you don't need to, since they share open handles, though you do need to arbitrate access to the handle).

The example assumes that the user is in the dsp group (audio).

Originally Posted by neonsignal here is some sample code (the open succeeds in both the main thread and the secondary thread). #include <pthread.h> #include <fcntl.h> #include <iostream> void *threadFn(void *p) { // open device in second thread int handle = open("/dev/dsp",O_RDWR); std::cout << "secondary thread " << handle << std::endl; if (handle > 0) close(handle); } int main() { // open device in main thread int handle = open("/dev/dsp",O_RDWR); std::cout << "main thread " << handle << std::endl; if (handle > 0) close(handle); // start second thread pthread_t thread; pthread_create( &thread, 0, threadFn, 0); pthread_join(thread, 0); } Note that you cannot open a handle for writing from both threads at the same time (and you don't need to, since they share open handles, though you do need to arbitrate access to the handle). The example assumes that the user is in the dsp group (audio).

It was a silliy mistake. I tried to open the file when it was already opened. Thank you for your time and effort.