Скачать или смотреть Understanding C - Why You Can't Change a String Inside a Function

Discover why modifying strings in C functions doesn't work as expected and learn how to return updated values correctly.
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This video is based on the question https://stackoverflow.com/q/75054195/ asked by the user 'iocode' ( https://stackoverflow.com/u/19604630/ ) and on the answer https://stackoverflow.com/a/75054260/ provided by the user 'user28731' ( https://stackoverflow.com/u/20296810/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why You Can't Change a String Given to a Function in C

C programming can be tricky, particularly when it comes to manipulating strings within functions. If you've found yourself frustrated because a string doesn't seem to change when modified inside a function, you're not alone. This post will help clarify why this happens and how you can properly return modified strings from functions in C.

The Issue: Strings in Functions

To illustrate the problem, let’s look at a piece of code:

[[See Video to Reveal this Text or Code Snippet]]

Program Output

This code will produce the following output:

[[See Video to Reveal this Text or Code Snippet]]

As you can see, although the changeStr() function does print the modified string, the main function still shows String=(null). Why does this happen? Let's break it down.

Understanding C Function Parameters

1. Copying of Parameters:

In C, when you pass a variable to a function, a copy of that variable is made. This means that changes made to that variable inside the function do not affect the original variable in the calling function. In the case of pointers, the pointer itself is copied, but what it points to is not.

2. Reallocation Behavior:

The realloc() function reallocates memory for the pointer passed to it. When you call realloc(), it may create a new memory block and return a pointer to that block. However, inside the changeStr() function, when you assign the result of realloc() to the local variable string, you are just changing what the local pointer points to. The original pointer in the main() function remains unchanged.

The Solution: Return the Updated Pointer

To effectively modify the string in the caller function, you need to return the newly allocated pointer back to the caller. Here's how you can do it:

Updated Function Code

Change the changeStr() function to return the updated pointer:

[[See Video to Reveal this Text or Code Snippet]]

Main Function Code

In the main() function, capture the returned value:

[[See Video to Reveal this Text or Code Snippet]]

When you do this, the pointer str in main() will now correctly point to the modified string returned from changeStr().

Conclusion

C programming requires a good understanding of memory management and variable scope. When passing pointers into functions, remember that the local copy won't affect the original in the calling function unless explicitly returned. By adjusting your function to return the new pointer, you can overcome this limitation and successfully modify the string in C. Happy coding!