How to Properly Assign Values to a Global Struct in C from a Function

Explore how to pass values from a function to a global struct in C correctly, solve common pitfalls, and ensure your data persists across function calls.
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How to Properly Assign Values to a Global Struct in C from a Function

When programming in C, one common challenge may arise when trying to assign values to a global struct from a function. If you've found yourself confused about why your struct isn't being assigned the expected values, you're not alone. Let's dive into the issue and discover a straightforward solution.

The Problem

In a sample program, a C programmer experienced surprising results when trying to assign values to a struct. The intention was to capture the day, month, and year from user input and display these values. However, the output displayed whenever values were printed was unexpected:

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

This discrepancy occurs because:

The struct used in the function is local, meaning it only exists within the function scope.

The main function creates its own instance of the struct, which is not being updated by the receive_date function.

Let's Debug the Code

Initially, the code structure looks like this:

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

In this code:

The receive_date function defines its own local struct, which is not connected to the d struct in main.

As a result, when the program attempts to print values after calling receive_date, it shows uninitialized values.

The Solution

To ensure that the values entered by the user are correctly assigned to the global struct, you can use pass-by-reference. This involves altering both the function signature and the way we call the function.

Step-by-Step Fix

Change Function Prototype: Modify the prototype to accept a pointer to the struct.

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

Update the main function: Pass the address of the struct to the function.

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

Modify the receive_date function: Accept a pointer and dereference it to access the struct fields.

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

Conclusion

By passing your struct by reference, the data captured is retained and can be accessed globally where it's needed. This simple adjustment fixes the undefined behavior originally causing discrepancies in the output.

Feel free to incorporate this approach in your future C programming endeavors to avoid similar pitfalls!