What is a semaphore? How do they work? (Example in C)

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What is a semaphore? How do they work? (Example in C) // Semaphores cause a lot of confusion for students, largely because they are super flexible and a little weird. In this video, I try to provide some insight into how they work, and an example of where you could use semaphores to coordinate different processes.
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Welcome! I post videos that help you learn to program and become a more confident software developer. I cover beginner-to-advanced systems topics ranging from network programming, threads, processes, operating systems, embedded systems and others. My goal is to help you get under-the-hood and better understand how computers work and how you can use them to become stronger students and more capable professional developers.
About me: I'm a computer scientist, electrical engineer, researcher, and teacher. I specialize in embedded systems, mobile computing, sensor networks, and the Internet of Things. I teach systems and networking courses at Clemson University, where I also lead the PERSIST research lab.
More about me and what I do:
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people.cs.clemson.edu/~jsorber/
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[@vaggelismanousakis6147]

Bookmarked! If I had watched this video 6 months ago I would have passed the Operating Systems class. Thanks

[@playerguy2]

Oh, well. Better luck next time.

[@vaggelismanousakis6147]

@@playerguy2 thank you sir

[@JacobSorber]

Sorry I was late. Next time around will hopefully be better. Best of luck.

[@justinyork6006]

Literally just found this as I am trying to understand my OS assignment!

[@vaggelismanousakis6147]

@@justinyork6006 Keep working hard, you can do it! Same here

[@LDS-Hellgineer]

This video saved my sanity in a current project. Thank you very much, keep it up!! greetings from Austria

[@avineshwar]

Great resource. It would be great if you could shed some light on memory barriers (possibly with an code-level example) and why typically we should/not worry about.

[@af9466]

Thank you very much! This really helped me understand semaphores and apply them in my Unix lab work.

[@thangbui6383]

Thanks for the explanation, your video is much easier to understand than grinding at my college's book.

[@agustinlawtaro]

Muchas gracias, Jacob!!! Clarísimo.

[@mrunalsanghvi]

the whole thread playlist is awesome. great learning. but please add more videos to make the threadpool more robust whenever you get time. I am feeling that i kind of missing on something there. especially handling the slow reading client or a thread pool with EPOLL could also be a great thing to learn and apply, i feel. big thumbs up for all the effort. Cheers!!.

[@dusan7777]

"Getting fancy with semaphores is a great way to end up with software that doesn't work" I laughed so much at that :D Great explanation tho, thanks a bunch

[@rickarmbruster8788]

but fixing it is kind of easy because you know directly that you caused trouble with 01010 sequences that just doesnt seem to run in the right order ^^

[@kartikeykushwah3926]

Really needed to know!

[@peepypoopy8917]

This is what I needed. Would be cool, if you were to make a video on Mutexes also and some sort of concrete comparison between the binary semaphore and a mutex. Thank you.

[@kylek.3689]

It's a bit late to respond but I can share anyways, In a Mutex, only the thread that originally locked it can unlock it, but with a semaphore, any thread can wait() or post(), regardless of who has called what.

[@yonasmulissa5083]

Very helpful video. Thank you for making this. One little comment: To get all points across easily can you please slow down the pace at which you speak and take a brief pause in between important points?

[@Email1212xxpl]

This channel is really good mate :) Keep on doing great stuff.

[@chadj1797]

This is exactly what I needed for my OS class assignment. Thanks Jacob, cheers from India. You're doing God's work!

[@othmanekaramat1962]

Insane, Thank you !

[@abdelrhmanahmed1378]

should it be convenient to use cv(conditional variables) in consumer producer case ?

[@MazharKhan-jm7pj]

Hi Jacob, Thanks a lot! your videos are very informative and helpful. I have a request, can you make some videos on Operating system basic concepts like what are user and kernel modes? What are system calls and what is a Linux boot up process etc.

[@techtechlearn3438]

Great video !!!!!!!!!!

[@cambrown5777]

Using this to prep for Northwestern's OS midterm, didn't realize that Prof. Hester took a job at GT! Sad, but glad he's killing it

[@JacobSorber]

Yeah, it will be nice to have him closer to home. I'm sure he'll be missed up north. Good luck on your midterms.

[@mrcrackerist]

semaphore sound interesting, but thanks to this video I am going to lookup atomic types more. Thanks :)

[@unknown2.03]

Hi can you please tell me where this named semaphore has been stored after creation as mine. # cd /dev/shm directory isn't having my named semaphore.

[@DipietroGuido]

Wonderful explanation. Everything makes sense now!

[@miladgolfam49]

Matthew Mcconaughey is teaching C, I loved the this tutorial thanks.

[@kakerlakenmilch1155]

I love this stuff

[@00killerix]

Which sw are u using for your videos?

[@shamanthakrishnakg1978]

Man !!!!, Damn!!!! ,you are doing a great help to the humanity, your videos are very helpful,thank you very much. 🙏🙏🙏

[@CheesyAceGameplay]

I use semaphores for when I'm writing to a queue in one or multiple threads... and another thread is dequeing (removing) items from the queue.

[@yaroslavpanych2067]

Okay, 1st: in what world barries is anyhow part of IPC? Should I listen to the rest, or it is adequate?

[@DetectivePoofPoof]

Huh, that's cool, I always thought it sounded like some kind of civil engineering structure. "Oh we used to have _dreadful_ floods back in the day! But then the gu'vment came and they installed that 'ol semaphore up the creek and we've been as dry as a nun's gusset ever since!"

[@sd_1989]

Wonderful explanation. Thank you for your time.

[@mr.juniordev8091]

Thanks a lot.

[@dieTbshow]

Can somebody explain to me what exactly the parameters for sem_open(...) express?

[@miserablepile]

Thank you for your very clear explanation!

[@NGBigfield]

Such a great hand-on example and explanation . Thank you!

[@JacobSorber]

You're very welcome! Glad you enjoyed it.

[@arlenegrundy7671]

Very good info. Is it possible to enlarge the screen you use. It's somewhat difficult to actual read along. Just a friendly request...thanks.

[@friction5001]

this was very helpful and well explained cheers mate

[@gabrielkolletalves493]

2:20 strange... you're presenting the code as if it's a kind of busy waiting (thread locks in while loop untill the variable is incremented by another thread) - but semaphores are not a kind of busy waiting. I'm having a hard time trying to differenciate that from a simple "N" variable who counts the buffer size.

[@Sir.NathanDrake]

Nice video. Thanks for the clear explaination!

[@ReD4eva94]

Amazing. Best instructor on youtube.

[@tianrongwang5061]

Hi Jacob. You are a great teacher. Your videos are helping me a lot with my OS course project. Thank you!

[@JacobSorber]

You're welcome. Glad I could help.

[@aravinds123]

The best video on semaphores! Wonderful!

[@teeceetime2]

Liked and subscribed! This presentation format is superb and I hope more people come across your channel!

[@SteaM992]

Hello. Thank you for sharing your knowledge with us. Could you recommend a book or 2 that goes deeper into the subject of semaphores, memory allocation in unix like OS (mmap), file descriptors, etc. Thank you!

[@raymondshawn3864]

Check Linux programming interface

[@benjaminshinar9509]

can you do one about microsoft zune?

[@bartekburmistrz8679]

Amnazing Tutorial, well explained, a very good example, helped me a lot. Thanks!

[@JacobSorber]

You're welcome. Glad it helped.

[@tete1445]

Thanks Jacob! I've been following your videos recently and they're all informative and amazing. I gotta say I'm quite intrigued! I have a few questions though; I'm learning C++ and I've gotten to an "advanced beginner level" and I wonder if I get to a decent level in C++, will the transition to C be easier? And what about Java, C#, python etc? Second question, can these programming concepts (such as Semaphores and whatnot) also apply to C++? Thank you!

[@JacobSorber]

Yes, you can (in most cases) think of C++ as just C with a set of language extensions. So, anything you can do in C, you can generally also do in C++. So, learning C++ will strengthen your C, and vice versa. And, any of my C examples, can be used in C++. You just have to keep in mind, what is supported in each language. How this relates to java, python, c#, etc, really requires a longer answer than I can give in a comment. Maybe in a future video.

[@tete1445]

@@JacobSorber Thanks, Jacob. I intend to transition to other languages once I feel unstoppable in C++ haha.

[@alexandersteinbach3555]

Finally I understand it xD Thank you!

[@dduartez99]

you literally saved me, I have a project due on the 2nd of sept on System V shared mem and semaphores and didn't know a thing

[@JacobSorber]

Glad I could help.

[@thepickicool97]

thank you for saving my semester

[@eddaly4160]

Great video. As usual. Seems like there could easily be a case were I can post repeatedly with out a wait happening because my "consumer" is busy or unresposive...do I need to guard against an overflow on a semaphore...if so, is there a proper way to deal with this scenario?

[@artemiocabrillosjr.244]

Well explained. Thanks !

[@JacobSorber]

Welcome.

[@luizfrancois7472]

Very good lesson.Thanks!

[@bhaveshvarma4313]

I wasted 2 hours to exactly understand but don't get difference b/w mutex and semaphore , But only one line makes clear that any thread can call post(Signal) where in mutex owned by thread only that thread can release it.

[@georgioziozas9031]

great video , bookmarked. waiting for a video which uses processes and semaphores , a full example ! gj

[@olegkuzenko]

Thank you. Great video!

[@m4l490n]

Be careful with semaphores, can turn things very confusing and bugs would be hard to find.

[@JacobSorber]

Amen.

[@u263a3]

Just because it’s there doesn’t mean you need to use it

[@sumaiyatasnim102]

Very helpful. Is there any way to manipulate times (like arrival time or wait time in FCFS order of something) using semaphore and multiple threads.

[@cricket9282]

hey! im not entirely sure where to ask you questions like this and i apologize for the off topic-ness, but, how do you feel about nim?

[@thinktubers6581]

Hello sir, Thanks you for the video , I tried the same but when I try to run the program am getting error like sem_open/producer: no such file or directory Am I doing anything wrong , if anyone know how to solve this issue , please help me to solve this problem Thank you !

[@thinktubers6581]

Hello??

[@jesusalanlopezaldaran1663]

Hello! I had the following problem: "shm_open no such file or directory" I solved it by changing "IPC_CREAT" to "O_CREAT". To make this change you will need to import the library "#include " "O_CREAT" creates the semaphore if it does not exist. Here is the code snippet for the "readshmem.c" file ... //setup some semaphores sem_unlink(SEM_CONSUMER_FNAME); sem_unlink(SEM_PRODUCER_FNAME); sem_t *sem_prod = sem_open(SEM_PRODUCER_FNAME, O_CREAT, 0660, 0); if(sem_prod == SEM_FAILED){ dogr("sem_open/producer"); exit(EXIT_FAILURE); } sem_t *sem_cons = sem_open(SEM_CONSUMER_FNAME, O_CREAT, 0660, 1); if(sem_cons == SEM_FAILED){ dogr("sem_open/consumer"); exit(EXIT_FAILURE); } ...

[@anelemadonda6191]

fanks a lot Mr Sorber

[@abhisheksa6635]

Why not discuss all other stuffs like spin lock, barriers, monitors and usage case basis, will be a great add.

[@mohammedzaid6634]

hey jacob thanks to you. your videos and teaching methods are very clear and its so helpful to understand how network programming works and what are thread process etc. so much help from you to community. and i think most of us(students) don't know what do to with network programming so please make video about some of YOUR ideas or project ideas or useful theories about network programming in C.

[@JacobSorber]

I have some network programming videos. And, I'm sure there will be more in the future. Let me know what you think I'm missing.

[@mohammedzaid6634]

@@JacobSorber i already watched your whole network programming videos and i wrote all of codes on my own its really helped me. but i am still confused what are the core usefulness of network programming? and how does peer-to-peer network works? how can i develop onion routed chat program using C with a central server? how real world encrypted communication looks like? what encryption use to? these are the questions that confuse me a LOT!!!! i hope, you will do something about it thanks for your kind reply!!! :)

[@quocvnguyen7117]

Thank you so much, This is very helpful.

[@jaminyah]

Hi Professor! I noticed that in the following code using a named semaphore: ```code int main(int argc, char* argv[]) { sem_t *sem = sem_open("/ex_sem", O_CREAT, O_RDWR, 1); if (sem == SEM_FAILED) { printf("Failed to acquire semaphore "); printf("%d ", errno); return -1; } ... return 0; ``` On the second run of this code segment sem is assigned SEM_FAILED. Is there a way to check for an error and still have the code run any number of times after it is compiled? Without error checking there are no issues. Thanks.

[@erkanturan7215]

Another awesome video... thanks a lot

[@JacobSorber]

My pleasure!

[@cubedude76]

Thanks for the good video, but please get a filter to smooth out the s's they are very loud

[@JacobSorber]

I've added one since this video was recorded. Has your listening experience improved?

[@khoaang5712]

Thank you, your video is very helpful

[@dibbyabarua9499]

Very good description of semaphores... very clear and concise! Could you suggest me a good book for Operating Systems? I am using two books right now: (1) OS by William Stallings and (2) OS Principles and Practice by Thomas Anderson and Mike Dahlin. I like the one by Anderson more than the Stallings one so far but I was wondering if there is any better suggestion?

[@JacobSorber]

I use Anderson & Dahlin in my class. It's probably my favorite, at the moment.

[@dibbyabarua9499]

@@JacobSorber ya. I like that one too haha

[@user-cf5uf9sw1u]

Awesome, as always

[@JacobSorber]

Thanks again!

[@eanerickson8915]

Best description ever!

[@JacobSorber]

Thanks. Glad you enjoyed it.

[@sanketgandhi3139]

Thank you for such nice video. One small request if possible please make videos on mutex locks and condition variables

[@n0kodoko143]

now I'm understanding where go channels come from. (You also caught me immediately thinking about named pipes... guess I'll be using that in my own time). -- love the shirt!

[@konulaliyevaa]

Thanks for the awesome video.

[@robinjohannsen5029]

great video !

[@user-is2uw7ee3u]

Thanks, it's very helpful,

[@carl-johanlandin1419]

Not sure why but I fail to run this code on my machine. It seems it exits here at this segment: ``` sem_t *sem_prod = sem_open(SEM_PRODUCER_FNAME, IPC_CREAT, 0660, 0); if (sem_prod == SEM_FAILED) { perror("sem_open/producer"); exit(EXIT_FAILURE); } ``` And the terminal output looks like this: ``` ❯ ./readshmem.elf sem_open/producer: No such file or directory ``` Any ideas why? I'm running Linux Debian and have tried to compile with both clang and gcc. I noticed you didn't need to link to pthreads library in your makefile which I had to do for a build to work.

[@carl-johanlandin1419]

It seems the directory /dev/sem did not exist on my OS, creating that folder solved the issue. Really like your videos btw! The best C content on youtube.

[@caesar104]

Could you make videos about library and its functions and also Annex K functions ? Are these functions necessary? Btw your videos are awesome and very informative. I learned more than my college lessons :) Thank you very much for your great effort sir ,please keep going :)

[@cyberceel]

Very well made 👍

[@lcsmuller]

Thank you this helps me a lot! Could you perhaps make a video on the topic of C11 atomics?

[@glumfish6862]

Yeah there is very little info on them

[@thanakornpasangthien5590]

Please do OS videos

[@honey-py9pj]

when we write in the producer function : ```sem_wait(sem_cons) ```, is it because we don't want to write anything , until a reader is avaible to read it (consume it?) ?

[@l0_0l45]

Please make a video on lock free programming! Cpp con has a talk on it but it was way too difficult to understand!

[@lewiscole5193]

Well, if he does do a vide on lock free programming, I would suggest that he might as well deal with wait free programming either in the same video or in it's own.

[@michaelkotthaus7120]

Again a very interesting topic, Jacob. (Also very important in embedded real-time applications.) I wonder, if a simple mutex (or a binary semaphore) was more simple for the example. With only a single mutex, the writer (producer) has to check by the first byte (block[0]==0) if the reader (consumer) has already processed the data.

Hello! I was able to make this code work in my mac, but when I try to run in a ubuntu machine, I cant. I added the flag -lpthread which allowed me to compile, but after that, I get the error "sem/open/producer: No such file or directory". What could it be?

[@vapula87]

Hey Éverton! Not sure if you ever figured it out, but semaphores on Linux are implemented slightly differently.

[@abdesamiesalama9408]

Hi .. _check(my_input,(int)_Env); Could you tell me what the second param means ?

[@marcel151]

2 months earlier and I would have a better degree in my exam about operating systems. Thanks for that video.

[@socat9311]

Bit irrelevant q: what specs and software you have for the screen capture? Quality is pretty good. Trying to do it from my laptop and ends up rubbish...

[@migtvill]

Can someone here link non-youtube studies of atomic studies of semaphores outside of programming, as in physical or physics applications?🎉

[@jonathanluo0]

In my ubuntu 18.04.2 system, in semaphore reader, "sem_t *sem_prod = sem_open(SEM_PRODUCER_FNAME, IPC_CREAT, 0660, 0)" failed, but "sem_t *sem_prod = sem_open(SEM_PRODUCER_FNAME, O_CREAT, 0660, 0)" succeeded

[@sidneysherrill4870]

I'm experiencing the same thing

[@maryjonson9692]

Yeah, changing IPC_CREAT with O_CREAT solved the issues for me !

[@SleepyChoco]

👍👍👍

[@joseville]

v is the value of the semaphore, right? At 2:59, thread A calls wait(), v is > 0, so v-- and then returns, right? (I'm following the code for wait() at 2:20)

[@lewiscole5193]

Correct. "v" is the value of the semaphore. The important thing to keep in mind is that the pseudo-code that Professor Sorber gave for the semaphore primitives is for show and have the theoretical potential to allow a process to get stuck indefinitely waiting on the semaphore. I'm sure that Professor Sorber is well aware of this fact and only gave the pseudo-code examples to show what conceptually goes on which is why he says don't compile it. On the off-chance that someone thinks that the code looks close enough and tries to use it anyway, one should be aware that while it'll probably always work, there is a very remote chance that it won't, so stick with a library implementation which will no doubt call an OS routine to actually get there from here.

[@bastawa]

great stuff! would love to see a video about lock-free queues! thanks🐼🐼🐼🐼🐼🐼

[@lewiscole5193]

I like lock-free and wait-free programming too, but when it's appropriate, namely when there's a reasonably possibility that the thread that wants to lock a structure (a list, queue, or stack) can die before it unlocks the structure thereby leaving other threads hanging in the wind ... something that is really *BAD NEWS* in an OS say. If that's not something that can happen or if you choose to ignore its possibility, the traditional locking methods work better (i.e. lower overhead) and are easier to understand. So while I wouldn't mind seeing Dr. Sorber cook up a video or two on lock free and/or wait free programming, I'm curious what the motivation is for others. Do you have something in particular in mind or are you just interested in the ideas behind it or something else entirely?

[@greatbullet7372]

I love you so much :D

[@shivaprasad4379]

Please post more videos on other IPC mechanisms and Synchronization concepts, bottlenecks & downsides around that code in production environment

[@AyooItsMars]

Hello Sir Jacob. Is there any link for the source codes?

[@JacobSorber]

Yes. Code is available through Patreon. See the description for links.

[@shalip]

@@JacobSorber link returns an error

[@JacobSorber]

@@shalip What error are you seeing? The site authenticates with Patreon, and only active patrons can access it.

[@justusmzb7441]

I have to wonder: Our professor presented semaphores as if they did not contain a busy wait. But your presentation clearly suggests that they do. Would it be possible eg. for the wait() function to put the process into a ready state so that it can try again when it is next dispatched, and the core it would elsewise be used for a busy wait is free for use?

[@intheshell35ify]

I have to google ever other word and splice the definitions together to try to understand what the hell he just said. Might need training wheels. And a beer.

[@mustafakarakas1116]

I was struggling with my midterm project. I did not even know what is wrong. I was feeling useless. Then I wrote those unlink lines and everything works just perfect. Thanks.

[@anon_y_mousse]

I like the messages you send to the consumer. Used to hear that all the time as a kid.

[@tktktktktk826]

Wouldn't it be easier to use 1 mutex in the example shown? Or am I missing something?

[@olliohls7927]

Is it good practice to use a while(1) in wait()? Isn't for better so there is no chance to get stuck.