Hi, I have a c++ code with semaphores that works well if compiled in a Fedora virtual machine over Windows. The same code compiled in the last Ubuntu doesn't work. The sem_wait launch an EINVAL error and doesn't wait for any thread. I've upgraded and downgraded libraries but I get the same result.

example of the code:


//
#include <errno.h>
#include <pthread.h>

int nResult=sem_init(&sem,0,0);
printf("nResult=%d",nResult); //gives 0 (ok)


nResult=sem_wait();
printf("nResult=%d errno=%d\n",nResult,errno); //gives -1 22 (EINVAL)

//

Thx

I assume you've got an #include <semaphore.h>, and you're passing a pointer to your (successfully initialized) semaphore in to sem_wait ().

In general, "sem_wait()" and friends should work just as well on Ubuntu as any other Linux, and it shouldn't make any difference if you're running as a VM, or using a real, physical host.

semaphore.h has been removed from the kernel headers since a while back, so the headers versin used to compile glibc will make a difference.

So you think semaphores behavior may have changed, don't you?

I'm not sure that they are supported at all, or it may be that the declarations have been moved to mutex.h. In most places of the kernel, semaphores have been change to mutex's.

acedillo -

I assumed you're writing a user-space program that's using the POSIX "sem_wait()" functions. These are, by definition, completely different from kernel-space semaphores or mutexes.

I'm certain that it doesn't matter if you're using VMWare or a physical host. I'd be surprised if Fedora supported it but Ubuntu doesn't - but, frankly, I haven't checked:

http://www.softwareplug.com/UNIX+7/man/sem_wait/3p
Of course, it's also possible that Posix semaphores are an option on Ubuntu, and you simply need to enable them:
http://manpages.ubuntu.com/manpages/hardy/man4/sem.html

Personally, I've always preferred to use the SysV APIs for stuff like this.

STRONG SUGGESTION:
Check out Beej's Guide to IPC, and see if there are alternatives to Posix semaphores you might not have previously considered ... and might actually prefer:
http://beej.us/guide/bgipc/