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Questions tagged [copy-on-write]

215 questions
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In place modification of matrices in R

I there any way to avoid copy-on-modify for in-place modifications of matrices in R? I am trying to copy a smaller matrix to a slice of larger matrix as follows. library(data.table) y <- matrix(c(11,21,31,12,22,32),nrow=3,ncol=2) address(y) [1]…
Crops
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Will copy-on-write prevent data duplication on arrays?

I am programming a web API client in PHP that parses CSV data into associative arrays and I want to protect my users from data-duplication when using these arrays. My users will never be writing to these arrays (theoretically they could but it makes…
thwd
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Get the copy-on-write behaviour of fork()ing, without fork()

I have a large buffer: char *buf = malloc(1000000000); // 1GB If I forked a new process, it would have a buf which shared memory with the parent's buf until one or the other wrote to it. Even then, only one new 4KiB block would need to be…
fadedbee
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How to prove "copy-on-write" on String type in Swift

As the title said, I tried to prove myself that COW(copy on write) is supported for String in Swift. But I cannot find a proof. I proved the COW on Array and Dictionary after trying the following codes: func address(of object: UnsafeRawPointer) ->…
Wu_
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Copy-on-write support in STL

I was just reading a Wikipedia article on Copy-on-write (curious if there are any filesystems that support it), and was surprised by the following passage: COW is also used outside the kernel, in library, application and system code. The string…
Septagram
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Why the address of variable of child process and parent process is same

Here is my code int main() { pid_t pid; int y = 3; if ( (pid = fork()) <0 ) return -1;; if( pid == 0 ) /* child */ { printf(" before: %d %p\n", y, &y ); y *= 10; printf("after: %d %p\n", y, &y ); } else /* father */ …
Janus.Le
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Garbage collector in Ruby 2.2 provokes unexpected CoW

How do I prevent the GC from provoking copy-on-write, when I fork my process ? I have recently been analyzing the garbage collector's behavior in Ruby, due to some memory issues that I encountered in my program (I run out of memory on my 60core…
Thomas Kalmus
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How does copy-on-write work in fork()?

I want to know how copy-on-write happens in fork(). Assuming we have a process A that has a dynamical int array: int *array = malloc(1000000*sizeof(int)); Elements in array are initialized to some meaningful values. Then, we use fork() to create a…
Min Fu
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How does copy-on-write in fork() handle multiple fork?

According to wikipedia (which could be wrong) When a fork() system call is issued, a copy of all the pages corresponding to the parent process is created, loaded into a separate memory location by the OS for the child process. But this is not…
ssgao
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macOS with APFS: Copy-On-Write in Terminal

I am writing some little script that assembles backup data into one directory. The directory content will then be uploaded to a cloud service and after that we can remove it. I was wondering how one could utilize APFS' copy-on-write feature with a…
Anticro
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In Java can I depend on reference assignment being atomic to implement copy on write?

If I have an unsynchronized java collection in a multithreaded environment, and I don't want to force readers of the collection to synchronize[1], is a solution where I synchronize the writers and use the atomicity of reference assignment feasible?…
MilesHampson
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Confusion about Copy-On-Write and shared_ptr

I have searched the web and read through the Boost documentation about shared_ptr. There is a response on SO that says that shared_ptr for Copy-On-Write (COW) sucks and that TR! has removed it from the string libraries. Most advice on SO says to…
Thomas Matthews
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Which value types in Swift supports copy-on-write?

I read about copy-on-write implementation for Array in Swift here. Arrays, like all variable-size collections in the standard library, use copy-on-write optimization. Multiple copies of an array share the same storage until you modify one of the…
SwiftyFinch
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How to know whether a copy-on-write page is an actual copy?

When I create a copy-on-write mapping (a MAP_PRIVATE) using mmap, then some pages of this mapping will be copied as soon as I write to specific addresses. At a certain point in my program I would like to figure out which pages have actually been…
user458577
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Qt undocumented method setSharable

I stumbled about a method which seems to be present in all data objects like QList, QQueue, QHash... I even investigated so far I can see the source code of it, which is inline void setSharable(bool sharable) { if (!sharable) detach();…
drahnr
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