2 semesters of Computer science failed to explain to me what this guy had done in just a video . thank you so much for your work sir , you're really saving some people out there .
Dear professor, with your videos I fell in love with C programming one more time when I was thinking that it is impossible for me to understand it. Just a huge thank you for all!
I would like to sincerely thank you for making this playlist, our operating systems professor zoomed through these topics and nothing was clear the first time I studied this. The detailed step by step explanation is a lifesaver : )
Man you're awesome. You should make more videos because you know how to explain complex things with such simplicity! And if you want to make a donation button, I will click on it for sure. Thank you very much!
Great content man. Thank you! One question though, isn't it redundant to check if we're in the parent process in order to execute wait()? Since it would be ignored in processes that do not have child processes (as if it didn't exist). I'd love to hear your take on this!
Yes, that's true. I just didn't want to make things more complex in the video. In this video I look at the wait function in more details: code-vault.net/course/46qpfr4tkz:1603732431896/lesson/sordz1xc2w:1603732432133
Outstanding, thank you! Is the stack copied so that each process has access to the original size or has the stack been divided into 2 or, something else?
First I want to thank you for your amazing lectures. I have a question, even though the variables of the parent process are copied to the child process, when you print the addresses of the variables you get the same addresses, why is this?
Usually you use processes when you want to launch an external program from your own program. You rarely need to execute your own code in the processes you're making. At least, for performance purposes, you can simply use threads instead of processes.
Thank you for these videos, they're so helpful. Just a question: why I can't call wait() without parameters? I have to use wait(NULL) or wait(&status), and I still don't get what the parameter stand for and how the function use it.
I talk about it in this lesson: code-vault.net/course/46qpfr4tkz:1603732431896/lesson/g0hfbb7k9e:1603732432483 You're right, I was using some default implementation of wait and that's why wait() without parameters was working but wait(NULL) does exactly the same thing, so no worries there.
First of all, thank you so much for the video. I just have a question. we do not know which process(parent process or child process) will execute first after the fork() is called. Right??
Pretty much. I'm sure if you dig deeper into how the OS does process scheduling you could find out more. In my case it was always the parent process that executed first
Usually, there's an internal buffer for any printf operation and it takes a fraction of a second for the text to actually show up on the terminal. With fflush(stdout); we're forcing that to output immediately.
how you can make multiple "forks" in order to have a sequence. How is it, for example, that the child inherits the PPID (from pid == 0) and, at the same time, changes the PID process? like this : process A : PID : 220 PPID:123 process B : PID : 221 PPID: 220 process C : PID : 222 PPID: 220 process D : PID : 223 PPID: 220 process E : PID : 224 PPID: 220
The return from the fork() function is not the same as the PID. You get the pid with the getpid() function. Explanation: code-vault.net/course/46qpfr4tkz:1603732431896/lesson/gcyivjh7m6:1603732432088
May i request you to pleaser post a Video explaining the memory access by parent and child process when fork is executed. because they become different processes means have different enteries in the process table. How the shared resources work ?
Some questions I asked ChatGPT that may be useful for someone else: Question - What happens with the variables when I call the C function fork()? I want to know about it's values and memory allocation, be it normal variables in stack or pointers to heap. Answer - When you call the C function fork(), a new process is created, which is a copy of the current process. The new process has its own memory space, including its own stack and heap. In the parent process, the values of all variables, including normal variables on the stack and pointers to heap memory, are preserved. The parent process continues executing from the point where the fork() call was made. However, in the child process, all variables are initialized to the same values as in the parent process at the time of the fork() call. Any modifications made to the variables in the child process will not affect the values of the same variables in the parent process, and vice versa. It's worth noting that in the child process, any changes to the memory allocated on the heap after the fork() call will not be reflected in the parent process. This is because the child process has its own copy of the heap memory, which is independent of the parent process's heap memory. In summary, when you call fork(), the values of all variables in the parent process are preserved, and the child process gets its own copy of the memory space, including its own stack and heap, initialized with the same values as in the parent process at the time of the fork() call. Question - If the child process get a different memory location from parent process, why does it have the same memory addresses? And why don't this same memory addresses point to the same place? Answer - When a child process is created using the fork() system call in a C program, it gets a copy of the parent process's address space, which includes the same virtual addresses as the parent process. This means that the child process has the same memory addresses as the parent process, but those addresses map to different physical memory locations. When a process accesses a memory address, the operating system's memory management unit (MMU) maps the virtual address to a physical address in main memory. In the case of a child process created by fork(), the MMU maps the virtual addresses of the child process to different physical memory locations than those used by the parent process. This is possible because each process has its own page table, which is used by the MMU to map virtual addresses to physical addresses. When a child process is created by fork(), the operating system creates a new page table for the child process that maps the virtual addresses to new physical addresses, but preserves the same virtual addresses used by the parent process. Therefore, while the virtual memory addresses are the same between the parent and child processes, they actually point to different physical memory locations, and any changes made to memory by the child process will not affect the parent process, and vice versa.
Thank you for the great video. Would you kindly explain how the memory is shared or copied between parent and child process? I wrote a simple program (but I cant trust my code) and seems like if you initiate a variable before calling fork, in both processes the address of that variable will be the same. Therefore, I would expect changing the value of one in one process change it in other one too, but thats not how it works.
int main(int argc, char* argv[]){ int a = 10; int pid = fork(); if (pid == 0){ int *p_a = &a; printf("Value in child: %d ", a); printf("Address in child: %p ", p_a); }else{ int *p_a = &a; printf("Value in main: %d ", a); printf("Address in main: %p ", p_a); wait(NULL); } if (pid == 0){ a = 20; printf("New value in child: %d ", a); }else{ printf("Value in main: %d ", a); } return 0; }
It has to do with virtual memory. The processes will have the same virtual memory addresses but the OS would then map those to different parts of the physical memory
what if the process have a grand child, does the the child's id (I mean the id of the parent of the grand child ) change from zero to a non zero value?
You're misunderstanding something: what fork() returns is NOT the process ID. If you want the process ID you can call getpid() Since fork() creates a new process (a new execution line) it has to return something in both the main line and the newly created one so: In the parent process: it returns the *child* process ID In the child process: it returns 0 (but that 0 is *not* the process ID, it just signifies that, after a fork() we are in the child process)
I usually use this website for documentation on C standard functions: cplusplus.com/reference/cstdio/fflush/ And for Linux related functions I just read the man. There are many online manuals too, like this one: man7.org/linux/man-pages/man2/read.2.html
fork returns process id of child. so int int id 0 will be stored. but when you print id in parent it is printing 4222 & when you print id in child it it printing 0. how diferent value is initilised ?
prog.c:5:1: warning: implicit declaration of function 'fork' [-Wimplicit-function-declaration] fork(); this error is coming. what to do ?? i have added unistd.h header file also
Considering the code from this video, If I don't assign values to n, it will stay uninitialized, so the value is whatever was before at that memory location. Since n is declared after the fork, and a child process is a copy with different memory addresses, when printing n, I get the same value. Why is that?
Looking at the assembly code, C does allocate memory for the local variables before ANY of the lines inside the function execute. So, when we execute fork(), n has already had its memory initialized so, when forking, it just copies whatever is at that memory. That's why you get the same values on both processes
Please, my question is this, when you have three processes running, which means the two child process are of the same level, do they both have the same process Id =0 ?
I explained about this question quite a few times in the comments. The idea is fork() does NOT return the current process id. fork() returns something different in both processes: In the parent process: it returns the process id of the child process In the child process: it returns 0
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I wish every student should get a professor like you to build their career
2 semesters of Computer science failed to explain to me what this guy had done in just a video .
thank you so much for your work sir , you're really saving some people out there .
this amount of hard work cannot go unsubscribed. It was really great. Keeping making more videos like this, you are gonna get many blessings.
Never an unclear moment, I hope you keep making videos
Dear professor, with your videos I fell in love with C programming one more time when I was thinking that it is impossible for me to understand it. Just a huge thank you for all!
I would like to sincerely thank you for making this playlist, our operating systems professor zoomed through these topics and nothing was clear the first time I studied this. The detailed step by step explanation is a lifesaver : )
man...you are a blessing!!!! Please dont stop making videos
Amazing videos, these are so much more helpful than how my professor explains things!
You can not imagine how much you helped me with this, great job!
Phenomenal explanation!
Absolutely a good video, I've gone through serval videos to learn this fork() function. This is the best one, so clear!
thank you so much for this playlist!
Thank you so much for your amazing explanation!!! I really appreciate
Well done dude! Great take on explaining this subject. Keep it up!
I have my final tomorrow and I wish i would've found your video wayy earlier! You explain things so well!!
Youre such a legend, I appreciate and respect you, sir.
Perfect explanation and amazing visualization. I wish my prof explained this to me like you!
This is the best explanation i've heard in any lecture ever !
Really useful! So clearly explained! Thank you very much!!
excellent explanation. thanks for your effort.
omg these videos are saving me! Thank you!
Great job! Thank you for the good explanation. There are many programmers, but not many can explain well. You have the talent, sir! Please, continue.
such a forking good video. kudos to the crystal clear explanations.
You are the best teacher ever!! I love you and your videos! Thanks to you i will not fail my class.
Love from Israel !
Clean explanation!
man, you are amazing!
Thank you. This are amazing videos.
Thank you, it's really helpful!
great work!
Thank you very much!
Thanks for explaining so nicely :-)
thank you so much professor
For real, man, you are my hero
perfect visualization. thanks
Thank you so much for the explanation
thank you
Wow, this kind of thing requires more work on your part but its absolutely worth it for the audience!!
Thanks a lot man!
Great work. Great social work
You are a great humble teacher
Thank you very much
It is really good content and excellent explanation . Thank you sir.
very helpful, thanks! :)
Man you're awesome. You should make more videos because you know how to explain complex things with such simplicity! And if you want to make a donation button, I will click on it for sure. Thank you very much!
Thanks! There's actually a place on our website where you can support the channel: code-vault.net/support-us
You just saved my exam, thank you very much for these videos
Amazing video!
amazing explanation 👏👏👏
Great job!
That illuminated me thanks
Amazing visualization of parent and child process.... very helpfull❤❤
Great videos by the way
Thanks alot sir
The type of explanation that should only exist
thank you, it 's helpful
Thank you
I'm your fan now!
Great work ❤❤❤🎉🎉
Thanks! Thanks.
good explanation
Great content man. Thank you!
One question though, isn't it redundant to check if we're in the parent process in order to execute wait()?
Since it would be ignored in processes that do not have child processes (as if it didn't exist). I'd love to hear your take on this!
Yes, that's true. I just didn't want to make things more complex in the video. In this video I look at the wait function in more details: code-vault.net/course/46qpfr4tkz:1603732431896/lesson/sordz1xc2w:1603732432133
u r amazing......sir
So helpful!
thank u re the best
omg... U save me T.T . Thank you very much
Thank u
Outstanding, thank you!
Is the stack copied so that each process has access to the original size or has the stack been divided into 2 or, something else?
The stack is copied and both processes have the original size (effectively doubling the memory usage).
First I want to thank you for your amazing lectures. I have a question, even though the variables of the parent process are copied to the child process, when you print the addresses of the variables you get the same addresses, why is this?
You are the best teacher in my life. With love from Russia
thanks
Thanks
thanks a lot
i love u so much for this video i hope u know that
Thank you for the third time.
Your lessons are perfect! A thousand thanks.
I have a question. Why should we use processes?
Usually you use processes when you want to launch an external program from your own program. You rarely need to execute your own code in the processes you're making. At least, for performance purposes, you can simply use threads instead of processes.
@@CodeVault so, the purpose is that threads or processes make my program more efficient?
Yep, exactly. Threads and processes are the methods to take advantage of multiple cores of your CPU, in turn, making your program faster
@@CodeVault Perfect! Now all is clear😍💪
Thank you for these videos, they're so helpful.
Just a question: why I can't call wait() without parameters? I have to use wait(NULL) or wait(&status), and I still don't get what the parameter stand for and how the function use it.
I talk about it in this lesson: code-vault.net/course/46qpfr4tkz:1603732431896/lesson/g0hfbb7k9e:1603732432483
You're right, I was using some default implementation of wait and that's why wait() without parameters was working but wait(NULL) does exactly the same thing, so no worries there.
oh perfect, and thank you again
Sir, it was really an awesome explanation !, Sir please try to make videos on dup and creat as well.
There's one on dup2, dup works mostly the same way. Here's the link: code-vault.net/lesson/43zvcsz6o1:1603732432539
And I'll look into creat
@@CodeVault Ok sir , thank you for your efforts !
you are the best
You’re saving my ass, thank you so much.
better than my professor lol thank you
First of all, thank you so much for the video. I just have a question. we do not know which process(parent process or child process) will execute first after the fork() is called. Right??
Yea, the order of execution is undefined
Amazing. Thanks from Pakistan!
Briljant!!
In a real case scenario after calling `fork()` we cannot know whether the parent or the child process will execute first, right?
Pretty much. I'm sure if you dig deeper into how the OS does process scheduling you could find out more. In my case it was always the parent process that executed first
Amazing video, one question though at 2:47 you said that 4300 is the child process id..isn't that the opposite?
No, what I said is correct. fork() returns the process id of the CHILD process in the parent process and 0 in the child process
well explained
sir why this fflush(stdout) is used?
Usually, there's an internal buffer for any printf operation and it takes a fraction of a second for the text to actually show up on the terminal. With fflush(stdout); we're forcing that to output immediately.
how you can make multiple "forks" in order to have a sequence. How is it, for example, that the child inherits the PPID (from pid == 0) and, at the same time, changes the PID process?
like this :
process A : PID : 220 PPID:123
process B : PID : 221 PPID: 220
process C : PID : 222 PPID: 220
process D : PID : 223 PPID: 220
process E : PID : 224 PPID: 220
The return from the fork() function is not the same as the PID. You get the pid with the getpid() function. Explanation: code-vault.net/course/46qpfr4tkz:1603732431896/lesson/gcyivjh7m6:1603732432088
I just thought about the wait() in the if statement: is it necessary?
Yea, of course. You always should wait for the child processes you're creating. Otherwise they remain without a parent process
May i request you to pleaser post a Video explaining the memory access by parent and child process when fork is executed. because they become different processes means have different enteries in the process table. How the shared resources work ?
I will look into it
Молодец! Спасибо за видео, приятно слушать хороший английский
hands
Some questions I asked ChatGPT that may be useful for someone else:
Question -
What happens with the variables when I call the C function fork()? I want to know about it's values and memory allocation, be it normal variables in stack or pointers to heap.
Answer -
When you call the C function fork(), a new process is created, which is a copy of the current process. The new process has its own memory space, including its own stack and heap.
In the parent process, the values of all variables, including normal variables on the stack and pointers to heap memory, are preserved. The parent process continues executing from the point where the fork() call was made. However, in the child process, all variables are initialized to the same values as in the parent process at the time of the fork() call. Any modifications made to the variables in the child process will not affect the values of the same variables in the parent process, and vice versa.
It's worth noting that in the child process, any changes to the memory allocated on the heap after the fork() call will not be reflected in the parent process. This is because the child process has its own copy of the heap memory, which is independent of the parent process's heap memory.
In summary, when you call fork(), the values of all variables in the parent process are preserved, and the child process gets its own copy of the memory space, including its own stack and heap, initialized with the same values as in the parent process at the time of the fork() call.
Question -
If the child process get a different memory location from parent process, why does it have the same memory addresses? And why don't this same memory addresses point to the same place?
Answer -
When a child process is created using the fork() system call in a C program, it gets a copy of the parent process's address space, which includes the same virtual addresses as the parent process. This means that the child process has the same memory addresses as the parent process, but those addresses map to different physical memory locations.
When a process accesses a memory address, the operating system's memory management unit (MMU) maps the virtual address to a physical address in main memory. In the case of a child process created by fork(), the MMU maps the virtual addresses of the child process to different physical memory locations than those used by the parent process.
This is possible because each process has its own page table, which is used by the MMU to map virtual addresses to physical addresses. When a child process is created by fork(), the operating system creates a new page table for the child process that maps the virtual addresses to new physical addresses, but preserves the same virtual addresses used by the parent process.
Therefore, while the virtual memory addresses are the same between the parent and child processes, they actually point to different physical memory locations, and any changes made to memory by the child process will not affect the parent process, and vice versa.
Thank you for the great video.
Would you kindly explain how the memory is shared or copied between parent and child process?
I wrote a simple program (but I cant trust my code) and seems like if you initiate a variable before calling fork, in both processes the address of that variable will be the same. Therefore, I would expect changing the value of one in one process change it in other one too, but thats not how it works.
int main(int argc, char* argv[]){
int a = 10;
int pid = fork();
if (pid == 0){
int *p_a = &a;
printf("Value in child: %d
", a);
printf("Address in child: %p
", p_a);
}else{
int *p_a = &a;
printf("Value in main: %d
", a);
printf("Address in main: %p
", p_a);
wait(NULL);
}
if (pid == 0){
a = 20;
printf("New value in child: %d
", a);
}else{
printf("Value in main: %d
", a);
}
return 0;
}
It has to do with virtual memory. The processes will have the same virtual memory addresses but the OS would then map those to different parts of the physical memory
Very interesting. Thank you.
can you explain what the difference is between wait(0) and wait()
wait(0) ( or wait(NULL) ) is the correct version of the function call. wait() without paramenters might work but is not standard
what if the process have a grand child, does the the child's id (I mean the id of the parent of the grand child ) change from zero to a non zero value?
You're misunderstanding something: what fork() returns is NOT the process ID. If you want the process ID you can call getpid()
Since fork() creates a new process (a new execution line) it has to return something in both the main line and the newly created one so:
In the parent process: it returns the *child* process ID
In the child process: it returns 0 (but that 0 is *not* the process ID, it just signifies that, after a fork() we are in the child process)
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I'm not Russain, but thanks! :)
Where do you get those function not published on common C tutorial wepages, like fflush(), etc?
😮
I usually use this website for documentation on C standard functions: cplusplus.com/reference/cstdio/fflush/
And for Linux related functions I just read the man. There are many online manuals too, like this one: man7.org/linux/man-pages/man2/read.2.html
@@CodeVault Thanks for your cooperation.
UEPH
fork returns process id of child. so int int id 0 will be stored. but when you print id in parent it is printing 4222 & when you print id in child it it printing 0. how diferent value is initilised ?
I made a video to clarify this subject. You can check it out here: code-vault.net/lesson/c5746b15bc917ba13615a0ec8c0011c1
prog.c:5:1: warning: implicit declaration of function 'fork' [-Wimplicit-function-declaration]
fork();
this error is coming. what to do ?? i have added unistd.h header file also
It should work. Are you running this on a Linux system?
What does the argc and argv means? Why do we have to place them there ?
It's just a standard. I explain them in this video: code-vault.net/lesson/dbijqbwu2a:1603733526118
@@CodeVault I'll watch it, thanks a lot
Considering the code from this video, If I don't assign values to n, it will stay uninitialized, so the value is whatever was before at that memory location. Since n is declared after the fork, and a child process is a copy with different memory addresses, when printing n, I get the same value. Why is that?
Looking at the assembly code, C does allocate memory for the local variables before ANY of the lines inside the function execute. So, when we execute fork(), n has already had its memory initialized so, when forking, it just copies whatever is at that memory. That's why you get the same values on both processes
@@CodeVault Thank you for your answer.
Please, my question is this, when you have three processes running, which means the two child process are of the same level, do they both have the same process Id =0 ?
I explained about this question quite a few times in the comments. The idea is fork() does NOT return the current process id.
fork() returns something different in both processes:
In the parent process: it returns the process id of the child process
In the child process: it returns 0
Thank you