Resolving Undefined Behaviour in Struct Array Memory Allocation in C

Discover how to fix undefined behaviour when using realloc with struct arrays in C, alongside practical tips for dynamic memory management. --- This video is based on the question https://stackoverflow.com/q/64772159/ asked by the user 'lowkeyhuman' ( https://stackoverflow.com/u/14434939/ ) and on the answer https://stackoverflow.com/a/64772336/ provided by the user 'Vlad from Moscow' ( https://stackoverflow.com/u/2877241/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Struct array realloc in function - produces undefined behaviour Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Understanding and Fixing Undefined Behaviour in Struct Arrays with realloc in C When working with dynamic memory allocation in C, particularly with structures that hold pointers to strings, it’s crucial to ensure that memory is managed correctly. A common issue many developers face is encountering undefined behaviour when reallocating memory for arrays of structs. In this post, we’ll delve into a specific case where this problem arises with a struct that stores student information, and we’ll provide effective solutions to fix it. The Problem In the scenario described, we have a struct named STUDENT that holds three char pointers for storing a student's name, phone number, and department. Here’s a simplified look at this struct definition: [[See Video to Reveal this Text or Code Snippet]] The issue arises when attempting to dynamically increase the size of an array of STUDENT structs using realloc. The original function, addNumber, successfully adds entries but fails to update the main array pointer correctly, leading to undefined behaviour. Occasionally, this manifests as segmentation faults, and sometimes it returns a malloc error indicating a pointer not allocated for reallocation. Examining the Code Let’s analyze the problematic portions of the initial implementation step by step. Here’s the initial way of creating the directory array and adding new entries: [[See Video to Reveal this Text or Code Snippet]] Issue: Memory Allocation for Zero Entries Notice that malloc is being called with nEntry set to 0. According to the C Standard, this behaviour is implementation defined—meaning it might return NULL or a valid pointer, leading to confusion. The proper way to handle this is to initialize directory to NULL instead: [[See Video to Reveal this Text or Code Snippet]] Issue: Passing the Struct Array Another significant problem is how the Array is passed to addNumber. In C, when you pass an argument to a function, it gets passed by value, meaning any changes to Array within addNumber don’t affect the original directory. To address this, the function signature should be modified to pass a pointer to the pointer: [[See Video to Reveal this Text or Code Snippet]] This allows you to change the original pointer from within the function. Correcting the Code Let’s rewrite the addNumber function ensuring we manage the addition of new entries in the struct array correctly: [[See Video to Reveal this Text or Code Snippet]] Allocation for Strings Previously, memory was allocated incorrectly as follows: [[See Video to Reveal this Text or Code Snippet]] Here, you’re only allocating enough space for a pointer, not the actual string. Instead, read input into a temporary buffer, and allocate memory based on the length of that string: [[See Video to Reveal this Text or Code Snippet]] Conclusion By correcting the memory allocation for the struct array and ensuring that we pass pointers to pointers when updating the directory, we can avoid undefined behaviour and control over memory more effectively. This way, our program can dynamically handle the inputs for multiple students without encountering segmentation faults or memory allocation errors. Implement these changes in your code, and you should see a significant improvement in the reliability of your student directory management! If you have any questions or further issues, feel free to seek advice or clarification. Happy coding!