my question is somewhat conceptual, how is parent process' data shared with child process created by a fork() call or with a thread created by pthread_create()
for example, are global variables directly passed into child process and if so, does modification on that variable made by child process effect value of it in parent process?
i appreciate partial and complete answers in advance, if i'm missing any existing resource, i'm sorry, i've done some search on google but couldn't find good results
thanks again for your time and answers
The semantics of fork() and pthread_create() are a little different.
fork() will create a new process, where the global variables will be separate between the parent and children. Most OS implementations will use copy-on-write semantics, meaning that both the parent and child process will use the same physical memory pages for all global variables until one of the processes attempts to edit the physical memory, at which point a copy of that page is made, so that now each process gets its own copy and does not see the other process's, so that the processes are isolated.
pthread_create() on the other hand, creates a new thread within the same process. The new thread will have a separate stack space from the other running threads of the same process, however the global variables and heap space are shared between all threads of the same process. This is why you often need a mutex to coordinate access to a shared piece of memory between multiple threads of the same process.
TL;DR version: with fork(), you don't see the other guy's changes; with pthread_create() you do.
A fork creates an almost exact copy of the calling process, including memory and file descriptors. Global variables are copied along with everything else, but they are not in any way linked to the parent process. Since file descriptors are also copied, parent and child can interact via these (as long as they're setup properly, usually via pipe or socketpair).
There's a big difference between processes created by fork and between threads created with pthread_create. Processes don't share global variables and should communicate through pipes, sockets, or other tools provided by the OS. A good solution is MPI - which is a message-passing library for inter-process communication.
Threads are quite different. A thread created with pthread_create shares all the global variables with its caller. Moreover, the caller can pass an arbitrary structure into the thread, and this structure will also be shared. This means that one should be extremely careful when programming with threads - such amounts of sharing are dangerous and error prone. The pthread API provides mutexes and conditions for robust synchronization between threads (although it still requires practice and expertise to implement correctly).
Related
If I have a program running with threads and call fork() on a unix-based system, are the threads copied? I know that the virtual memory for the current process is copied 1:1 to the new process spawned. I know that threads have their own stack in the virtual memory of a process. Thus, at least the stack of threads should be copied too. However, I do not know if there is anything more to threads that does not reside in virtual memory and is thus NOT copied over. If there is not, do the two processes share the threads or are they independent copies?
No.
Threads are not copied on fork(). POSIX specification says (emphasize is mine):
fork - create a new process
A process shall be created with a single thread. If a multi-threaded process calls fork(), the new process shall contain a replica of the calling thread and its entire address space, possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of the exec functions is called.
To circumvent this problem, there exists a pthread_atfork() function to help.
man fork:
The child process is created with a single thread—the one that called fork(). The entire virtual address space of the parent is replicated in the child, including the states of mutexes, condition variables, and other pthreads objects; the use of pthread_atfork(3) may be helpful for dealing with problems that this can cause.
From The Open Group Base Specifications Issue 7, 2018 edition's fork:
A process shall be created with a single thread. If a multi-threaded process calls fork(), the new process shall contain a replica of the calling thread and its entire address space, possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of the exec functions is called.
When the application calls fork() from a signal handler and any of the fork handlers registered by pthread_atfork() calls a function that is not async-signal-safe, the behavior is undefined.
Originally, "fork" was achieved by writing the task to disk and then, rather than reading in a different thread (which would be done if swapping the task with a different one), modifying the task ID of the image still in memory and continuing with its execution (as the new task). This was a very simple modification to the basic task switching mechanism, where only one task would occupy RAM memory at a time.
Of course, as memory management got more elaborate this scheme was modified to suit the new environment.
I'm working with semaphores in C , especifically to control the access to a shared memory zone in linux. but there is one thing that I can't understand.
I am using a mutex to control the access to a specific zone because i have 2 processes that must read/write from that zone. the thing is, when we use the fork() to create a new child process, the whole program is "copied" to another program as if they were two seperate programs right ? so, when i do V(mutex) in one process, how does the other one know he can't access ?
I know its a noob question but nobody could explain this to me until now.
After the fork neither process is going to know about the memory actions of the other because they are separate copies. You have to put your shared variables in shared memory, including mutexes and semaphores. Then all the processes are operating on the same resource.
For unrelated (i.e. non-forked) process there are usually system facilities (e.g. named semaphores) that each process can open based on a path name or similar method that each can use to find and use the resource.
You synchronisation objects must be placed in process shared memory, for example created with mmap (... MAP_ANONYMOUS ...). In addition, they must have the PTHREAD_PROCESS_SHARED attribute set, for example, by using pthread_mutexattr_setpshared.
See here:
Semaphores and Mutex for Thread and Process Synchronization
So mutex in practice is often used in threads, which makes sharing trivial. For processes however, mutex could be stored as a part of the shared mem.
For semaphores however, linux has built in library, which identifies global semaphores by keys. See below.
http://beej.us/guide/bgipc/output/html/multipage/semaphores.html
Or you can use other IPC to sync. Signals, for example.
Hope this helps.
How is the implementation of threads done in a system?
I know that child processes are created using the fork() call
and a thread is a light weight. How does the creation of a thread differ from that of a child process?
Threads are created using the clone() system call that can make a new process that shares memory space and some of the kernel control structures with its parent. These processes are called LWPs (light-weight processes) and are also known as kernel-level threads.
fork() creates a new process that initially shares memory with its parent but pages are copy-on-write, which means that separate memory pages are created when the content of the original one is altered. Thus both parent and child processes can no longer change each other's memory and effectively they run as separate processes. Also the newely forked child is a full-blown processes with its separate kernel control structures.
Each process has its own address space aka range of virtual addresses that the process can access. When a new process is forked a duplicate copy of all the resources involved has to be made. After the forking is complete the child and the parent have their own distinct address space and all the resources involved within it.Naturally, this is an performance intensive operation.
While all threads in the same process share the same address space, So when a new thread is spawned each thread only needs its own stack and there is no duplication of all resources as in case of processes.Hence spawning of an thread is considerably less performance intensive.
Ofcourse the two operations cannot and should not be compared because both provide essentially different features for different requirements.
Well it differs very much, first of all child process is in some way copy of parent program and have all variables duplicated, and you differ child from parent by its PID. Threads are like new programs , they run at the same time as main program (it looks like at the same time, due to slicing time of cpu by os ). Threads could use global variables in program, but they don't make duplicate as processes. So it`s much cheaper to use threads then new processes.
Well you've read the important parts, now here's something behind the curtains:
In current implementations(where current means the last few decades), the process memory isn't technically copied immediately upon forking. Read-only sections are just shared between the two processes (as they can't change anyway), as well as the read-only parts of shared libraries, of course. But most importantly, everything writeable is initially also just shared. However, it is shared in a write-protected manner, and as soon as you write to the child process memory (e.g. by incrementing a variable), a page fault is generated in the kernel, which only then causes the kernel to actually copy the respective page (where the modification then occurs).
This great optimization, which is called "copy on write", results in child processes usually not really consuming exactly as much (physical) memory as their parent processes. To the program developer (and user), however, it's completely transparent.
I am new to threads and processes.
I have code that works fine right now with forking the code into multiple processes. However each process needs to add to a global variable, but from what I read, each time the process forks, it takes a copy of the global, and adds them independently. Is there a way to join them, like you can with threads?
Different processes can communicate and exchange data via shared memory.
On linux, you can look:
man shm_overview
for attaching a memory segment on several processes
and
man sem_overview
for the semaphore library for controlling parallel access.
You should define a struct with two fields, one for your global and one for a semaphore. Then, before any forking occurs, create some shared memory in the parent process big enough to hold this struct and initialize one there. In the children, map in the shared memory so they can access the global. All processes, parent and children, should obey the rules of the semaphore when accessing the global.
To avoid unnecessary blocking which can hurt performance, try not to hold the semaphore too long. When reading the global, make a quick copy of it in a process and use that, rather than holding the semaphore for the entire time you are using its value. Likewise, when changing the global, prepare your changes ahead of time (before you grab the semaphore) and, once you have the semaphore, copy them in all at once. Sometimes your work depends on reading and writing the global without it changing in between being read and written. In this case, some blocking may be inevitable.
It is not clear what platform you are on, but all major PC and server platforms (Windows, Linux/Unix/Mac OS) have support for shared memory and semaphores. The APIs may be different, but the functionality you need is there.
I've tried to look this up, but I'm struggling a bit to understand the relation between the Parent Process and the Child Process immediately after I call fork().
Are they completely separate processes, only associated by the id/parent id? Or do they share memory? For example the 'code' section of each process - is that duplicated so that each process has it's own identical copy, or is that 'shared' in some way so that only one exists?
I hope that makes sense.
In the name of full disclosure this is 'homework related'; while not a direct question from the book, I have a feeling it's mostly academic and, in practice, I probably don't need to know.
As it appears to the process, the entire memory is duplicated.
In reality, it uses "copy on write" system. The first time either process changes its memory after fork(), a separate copy is made of the modified page (usually 4kB).
Usually the code segment of a process is not modified, in which case it remains shared.
Logically, a fork creates an identical copy of the original process that is largely independent of the original. For performance reasons, memory is shared with copy-on-write semantics, which means that unmodified memory (such as code) remains shared.
File descriptors are duplicated, so that the forked process could, in principle, take over a database connection on behalf of the parent (or they could even jointly communicate with the database if the programmer is a bit twisted). More commonly, this is used to set up pipes between processes so you can write find -name '*.c' | xargs grep fork.
A bunch of other stuff is shared. See here for details.
One important omission is threads — the child process only inherits the thread that called fork(). This causes no end of trouble in multithreaded programs, since the status of mutexes, etc., that were locked in the parent is implementation-specific (and don't forget that malloc() and printf() use locks internally). The only safe thing to do in the child after fork() returns is to call execve() as soon as possible, and even then you have to be cautious with file descriptors. See here for the full horror story.
They are separate processes i.e. the Child and the Parent will have separate PIDs
The child will inherit all of the open descriptors from the Parent
Internally the pages i.e. the stack/heap regions which can be modified unlike the .text region, will be shared b/w the Parent and the Child until one of them tries to modify the contents. In such cases a new page is created and data specific to the page being modified is copied to this freshly allocated page and mapped to the region corresponding to the one who caused the change - could be either the Parent or Child. This is called COW (mentioned by other members in this forum above in their answers).
The Child can finish execution and until reclaimed by the parent using the wait() or waitpid() calls will be in ZOMBIE state. This will help clear the child's process entry from the process table and allow the child pid to be reused. Usually when a child dies, the SIGCHLD signal is sent out to the parent which would ideally result in a handler being called subsequent to which the wait() call is executed in that handler.
In case the Parent exits without cleaning up the already running or zombie child (via the wait() waitpid calls), the init() process (PID 1) becomes the parent to these now orphan children. This init() process executes wait() or waitpid() calls at regular intervals.
EDIT: typos
HTH
Yes, they are separate processes, but with some special "properties". One of them is the child-parent relation.
But more important is the sharing of memory pages in a copy-on-write (COW) manner: until the one of them performs a write (to a global variable or whatever) on a page, the memory pages are shared. When a write is performed, a copy of that page is created by the kernel and mapped at the right address.
The COW magic is done by in the kernel by marking the pages as read-only and using the fault mechanism.