👀 Views: 77 💬 Answers: 1 📅 Created: 2025-06-05

c variable-length-array memory-management functions debugging C

I'm wondering if anyone has experience with I'm working on a project and hit a roadblock. I'm encountering an issue with variable-length arrays (VLAs) in C when using them inside nested functions. My code compiles fine, but I see unexpected behavior during execution. Specifically, I have a function that initializes a VLA based on a parameter passed from another function, and it seems to not retain the expected values after the function exits. Here's a simplified version of the code: ```c #include <stdio.h> #include <stdlib.h> typedef struct { int *arr; size_t size; } ArrayHolder; void fill_array(ArrayHolder *holder, int n) { holder->size = n; holder->arr = (int *)malloc(n * sizeof(int)); for (size_t i = 0; i < n; i++) { holder->arr[i] = i * 2; } } void process_data(int count) { int vla[count]; // Variable-length array ArrayHolder holder; fill_array(&holder, count); for (size_t i = 0; i < count; i++) { vla[i] = holder.arr[i]; // Attempt to fill VLA with values } free(holder.arr); // Free the allocated memory printf("VLA contents: "); for (size_t i = 0; i < count; i++) { printf("%d ", vla[i]); } printf("\n"); } int main() { process_data(5); return 0; } ``` When I run this code, it compiles without warnings or errors, but the output is not what I expect. I get an output like "VLA contents: 0 0 0 0 0" instead of "VLA contents: 0 2 4 6 8". I've verified that the `fill_array` function correctly populates the `holder.arr` array before accessing it in `process_data`. I suspect this might be related to the scope and lifetime of the VLA, but I'm not entirely sure. I've also tried printing values inside the `fill_array` function to ensure it's being filled correctly, and the output is as expected. Additionally, I've ensured that `count` is positive and remains within a reasonable range. Is there an issue with how I’m using VLAs here, or am I missing something fundamental about variable lifetimes in C? Would appreciate any insights or suggestions for troubleshooting this issue further. This is part of a larger API I'm building. My team is using C for this REST API. What's the best practice here? This is for a web app running on Windows 10. Could someone point me to the right documentation?