So I had a phone interivew for an embedded software position with a
major communications company and was asked some questions that I didn't
seem to answer to the interviewer's satisfaction. enough, I got
called back for an on-site interview. Anyways, I want to run these
questions by others to see what the real answer is.
1) What is the difference between a mutex and a binary semaphore?
I couldn't really think of a difference but he seemed to be leading me
to an answer. He asked if you can use a mutex in an interrupt service
routine and then asked if you could use a semaphore within an interrupt
service routine. of the questions he asked during the interview was
whether I had heard the term "munching". Well, after the interview I
looked it up and it is some term specific to VxWorks. Not having worked
with VxWorks before I decided to read a VxWorks manual online. It seems
like almost all his questions were related to VxWorks even though he
didn't say so.
Is there really one "correct" answer to the mutex vs binary sempahore
question? There definitely does seem to be a difference in VxWorks as
mutexes handle priority inversion. It seems to be S / library
dependent and thus seems a bit unfair, but correct me if I'm wrong.
Thanks.

joshc wrote:
So I had a phone interivew for an embedded software position with a
major communications company and was asked some questions that I didn't
seem to answer to the interviewer's satisfaction. enough, I got
called back for an on-site interview. Anyways, I want to run these
questions by others to see what the real answer is.

1) What is the difference between a mutex and a binary semaphore?

The purpose differs. Expand the name MUTEX is MUTual EXclusion. You use
it to prevent two or more processes from executing the same section of
code at the same time. Whereas a semaphore is just a signalling
mechanism. Why he would specifically use a binary semaphore in the
questions if strange.

I couldn't really think of a difference but he seemed to be leading me
to an answer. He asked if you can use a mutex in an interrupt service
routine and then asked if you could use a semaphore within an interrupt
service routine. of the questions he asked during the interview was
whether I had heard the term "munching". Well, after the interview I
looked it up and it is some term specific to VxWorks. Not having worked
with VxWorks before I decided to read a VxWorks manual online. It seems
like almost all his questions were related to VxWorks even though he
didn't say so.

Is there really one "correct" answer to the mutex vs binary sempahore
question? There definitely does seem to be a difference in VxWorks as
mutexes handle priority inversion. It seems to be S / library
dependent and thus seems a bit unfair, but correct me if I'm wrong.

Thanks.

hth,
ed

Ed Prochak wrote:
joshc wrote:
So I had a phone interivew for an embedded software position with a
major communications company and was asked some questions that I didn't
seem to answer to the interviewer's satisfaction. enough, I got
called back for an on-site interview. Anyways, I want to run these
questions by others to see what the real answer is.

1) What is the difference between a mutex and a binary semaphore?

The purpose differs. Expand the name MUTEX is MUTual EXclusion. You use
it to prevent two or more processes from executing the same section of
code at the same time. Whereas a semaphore is just a signalling
mechanism. Why he would specifically use a binary semaphore in the
questions if strange.

hth,
ed

Yes, but a semaphore can be used for mutual exclusion too. The
differences I found in VxWorks' implementation of mutexes and
semaphores are very implementation specific. Any more ideas on common
differences between mutexas and semaphores?

Maybe the difference is in how the waitng processes are woken up. Maybe
relating to which process and structures are responsible for managing each.
I would think that semaphores are individually managed by each owning
process while mutexes are centralling managed. This difference may make a
big difference with respect to how interrupts impact each.

Probably not correct, but my 2 cents.

Hicks

"joshc" <josh.curtz@gmail.comwrote in message
news:1158292191.257574.114150@
So I had a phone interivew for an embedded software position with a
major communications company and was asked some questions that I didn't
seem to answer to the interviewer's satisfaction. enough, I got
called back for an on-site interview. Anyways, I want to run these
questions by others to see what the real answer is.

1) What is the difference between a mutex and a binary semaphore?
I couldn't really think of a difference but he seemed to be leading me
to an answer. He asked if you can use a mutex in an interrupt service
routine and then asked if you could use a semaphore within an interrupt
service routine. of the questions he asked during the interview was
whether I had heard the term "munching". Well, after the interview I
looked it up and it is some term specific to VxWorks. Not having worked
with VxWorks before I decided to read a VxWorks manual online. It seems
like almost all his questions were related to VxWorks even though he
didn't say so.

Is there really one "correct" answer to the mutex vs binary sempahore
question? There definitely does seem to be a difference in VxWorks as
mutexes handle priority inversion. It seems to be S / library
dependent and thus seems a bit unfair, but correct me if I'm wrong.

Thanks.

joshc wrote:
So I had a phone interivew for an embedded software position with a
major communications company and was asked some questions that I didn't
seem to answer to the interviewer's satisfaction. enough, I got
called back for an on-site interview. Anyways, I want to run these
questions by others to see what the real answer is.

1) What is the difference between a mutex and a binary semaphore?
I couldn't really think of a difference but he seemed to be leading me
to an answer. He asked if you can use a mutex in an interrupt service
routine and then asked if you could use a semaphore within an interrupt
service routine. of the questions he asked during the interview was
whether I had heard the term "munching". Well, after the interview I
looked it up and it is some term specific to VxWorks. Not having worked
with VxWorks before I decided to read a VxWorks manual online. It seems
like almost all his questions were related to VxWorks even though he
didn't say so.

Is there really one "correct" answer to the mutex vs binary sempahore
question? There definitely does seem to be a difference in VxWorks as
mutexes handle priority inversion. It seems to be S / library
dependent and thus seems a bit unfair, but correct me if I'm wrong.

A mutex is used to ensure that only a single "thread" accesses
a resource. A (binary) semaphore is a mechanism that can be
used to implement a mutex. Not all semaphores are mutex'es
(just like not all counters are semaphores).

"joshc" <josh.curtz@gmail.comwrote in message
news:1158292191.257574.114150@
So I had a phone interivew for an embedded software position with a
major communications company and was asked some questions that I didn't
seem to answer to the interviewer's satisfaction. enough, I got
called back for an on-site interview. Anyways, I want to run these
questions by others to see what the real answer is.

1) What is the difference between a mutex and a binary semaphore?
I couldn't really think of a difference but he seemed to be leading me
to an answer. He asked if you can use a mutex in an interrupt service
routine and then asked if you could use a semaphore within an interrupt
service routine. of the questions he asked during the interview was
whether I had heard the term "munching". Well, after the interview I
looked it up and it is some term specific to VxWorks. Not having worked
with VxWorks before I decided to read a VxWorks manual online. It seems
like almost all his questions were related to VxWorks even though he
didn't say so

In VxWorks perspective, a mutex is roughly a semaphore with an initial value
of 1.
(With a few restrictions and variations).

Steve

Is there really one "correct" answer to the mutex vs binary sempahore
question? There definitely does seem to be a difference in VxWorks as
mutexes handle priority inversion. It seems to be S / library
dependent and thus seems a bit unfair, but correct me if I'm wrong.

Thanks.

joshc wrote:

1) What is the difference between a mutex and a binary semaphore?

Thanks.

Not sure for all RTS-es, but usually a mutex has priority inheritance,
while a semaphore has not.
Then a mutex is usually used for mutual exclusion of critical paths,
while a semaphore is used for inter-thread synchronization and event
notifications.

Adrian wrote:
joshc wrote:

1) What is the difference between a mutex and a binary semaphore?

Thanks.

Not sure for all RTS-es, but usually a mutex has priority inheritance,
while a semaphore has not.
Then a mutex is usually used for mutual exclusion of critical paths,
while a semaphore is used for inter-thread synchronization and event
notifications.

To be more specific, if you have two concurrent threads, with mutexes
you will always have a pair of get/put mutex in each thread.
With semaphores, you may wait (get) for the semaphore in one thread and
put the semaphore in the other thread. Never use this with mutexes

joshc schrieb:

routine and then asked if you could use a semaphore within an interrupt
service routine. of the questions he asked during the interview was
whether I had heard the term "munching". Well, after the interview I
looked it up and it is some term specific to VxWorks. Not having worked
with VxWorks before I decided to read a VxWorks manual online. It seems
like almost all his questions were related to VxWorks even though he
didn't say so.

In the interrupt you should not wait a resource so you should only set
the semaphore and never wait

Adib.
Is there really one "correct" answer to the mutex vs binary sempahore
question? There definitely does seem to be a difference in VxWorks as
mutexes handle priority inversion. It seems to be S / library
dependent and thus seems a bit unfair, but correct me if I'm wrong.

Thanks.

It's really this simple. The task that owns mutex semaphore can take it again, while if it attempts to take again the binary semaphore that it owns, it'll fail. In other words, vxworks mutex sema are recursive (re-entrant), while binary are not.

Yes, I have access to vxworks source code, so I looked it up.

This is the most dumbest question ever asked in interviews.

"What is the diff between a mutex and a semaphore".

Well the answer is:

"which context are ya referring to ? Linux, VxWorks, Symbian, pThreads, Windows ? "

Caus these words have diff connotations in diff Oses.

Mutexes and binary semaphores are the same in Linux.
Binary semaphores don't exist in pThreads, hence mutexes and semaphores (which the interviewer means to say counting semaphores) are different in pThreads.
As already mentioned in this thread there are subtle differences in vxWorks as well.

Thanks,
Timothy Noel