Understanding How to Pass a Value to a Function that Takes a Pointer in C

Learn how to effectively use pointers in C by understanding how to pass values to functions that require pointer arguments. This guide clarifies common mistakes and provides solutions to ensure your code functions correctly.
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This video is based on the question https://stackoverflow.com/q/69751428/ asked by the user 'DeckChain' ( https://stackoverflow.com/u/11595909/ ) and on the answer https://stackoverflow.com/a/69751549/ provided by the user 'babon' ( https://stackoverflow.com/u/452414/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Pointers in C: Passing Values to Functions

When venturing into the world of C programming, one of the essential concepts you'll encounter is the usage of pointers. They can be intimidating for beginners, leading to common mistakes. A frequent issue arises when attempting to pass a value to a function that takes a pointer as an argument. In this post, we'll explore a scenario where a programmer is seeking assistance with passing user input into a function via pointers, ultimately providing a comprehensive solution to the problem.

The Problem

A user encountered difficulties when trying to pass a value into the get_user_input(char *input) function. The goal was to use scanf within this function to capture user input. Here’s the relevant portion of the code they shared:

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The programmer received an error while trying to implement this functionality, partly due to misconceptions about pointers and variable declarations. So how can these issues be resolved?

The Solution

1. Correcting Variable Declaration

Firstly, note that the line void choice is not a valid way to declare a variable in C. Instead, it should be declared as a character variable that can hold the user’s input:

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2. Passing the Address of the Variable

The get_user_input function needs to be able to modify the value of choice directly. To achieve that, you must pass the address of the choice variable to the function. This is done using the address operator &:

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3. Adjusting the get_user_input Function

Since get_user_input does not need to return anything, you should modify its implementation to directly read input into the provided pointer. Here’s the adjusted code for get_user_input:

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4. Final Code Implementation

With all corrections made, your complete functions will look like this:

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Conclusion

Understanding pointers and their usage in function calls is a crucial part of becoming proficient in C programming. By explicitly passing the memory address of a variable, you can modify it within a function, ensuring your program operates as intended. Hopefully, this post provides clarity and guidance for anyone facing similar challenges with pointers. Happy coding!

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