Understanding char arrays Duplication in C After fork()

Описание: Explore whether `char arrays` are duplicated in C after a `fork()` call. Learn the implications of process memory management and address space behavior with practical code examples.
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This video is based on the question https://stackoverflow.com/q/74178862/ asked by the user 'Dogu Deniz Ugur' ( https://stackoverflow.com/u/8155505/ ) and on the answer https://stackoverflow.com/a/74179173/ provided by the user 'Fra93' ( https://stackoverflow.com/u/4952549/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Does char arrays get duplicated in C after fork()?

When programming in C, one common question that arises is regarding the behavior of memory allocation and duplication after a process is forked using the fork() system call. This particularly comes into play with structures like char arrays. In this post, we will explore if char arrays get duplicated after a call to fork() and the underlying principles of process memory management in C.

The Problem at Hand

Consider the following C code snippet:

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In this example, you might observe different outputs depending on how the fork() behaves with the msg array in the parent and child processes.

Expected Outputs

Output 1:

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Output 2:

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This example raises the question: Do char arrays get duplicated after fork()?

Understanding fork() and Duplicate Memory

What Happens During fork()?

When fork() is called, it creates a new child process by duplicating the current process (the parent). Both processes now have separate memory spaces. However, they initially share the same physical memory pages. Here are key points to note:

Each process has its own address space.

Changes made in one process’ address space do not reflect in the other’s.

This behavior is often referred to as copy-on-write. Initially, both processes can read the same physical memory, but if one writes to this memory, a separate copy is made for that process, ensuring isolation.

The Role of char arrays

In the context of char arrays, when the child process modifies the msg array, it is actually modifying its own copy after fork(). Therefore, the messages seen by the parent and child processes can differ.

To illustrate this concept, consider a simpler code example with an integer variable instead of a char array:

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Here, when the child modifies x, it creates a separate instance of x in its own memory space, confirming that each process has its own instance.

Why the Values Differ

Despite both processes reading from the same address, they see different values because they deal with separate address spaces:

Child Process: Gets the modified value (20).

Parent Process: Still has access to the original value (10).

Conclusion

So, do char arrays get duplicated in C after a fork()? The answer is both yes and no:

They do not get duplicated in the sense of creating multiple copies of the same data immediately.

They do become distinct when a modification is made, due to the process's own address space.

Understanding this memory management behavior is crucial for C programmers as they work with processes and multithreading.

If you have any further questions about fork(), memory management, or char arrays, feel free to reach out! Happy coding!