Question

I’m trying to write a simple C program on Ubuntu using Eclipse CDT (yes, I’m more comfortable with an IDE and I’m used to Eclipse from Java development), and I’m stuck with something weird. On one part of my code, I initialize a char array in a function, and it is by default pointing to the same location with one of the inputs, which has nothing to do with that char array. Here is my code:

char* subdir(const char input[], const char dir[]){
[*] int totallen = strlen(input) + strlen(dir) + 2;
    char retval[totallen];
    strcpy(retval, input);
    strcat(retval, dir);
...}

Ok at the part I’ve marked with [*], there is a checkpoint. Even at that breakpoint, when I check y locals, I see that retval is pointing to the same address with my argument input. It not even possible as input comes from another function and retval is created in this function. Is is me being unexperienced with C and missing something, or is there a bug somewhere with the C compiler?

It seems so obvious to me that they should’t point to the same (and a valid, of course, they aren’t NULL) location. When the code goes on, it literally messes up everything; I get random characters and shapes in console and the program crashes.

Answer 1

I don’t think it makes sense to check the address of retval BEFORE it appears, it being a VLA and all (by definition the compiler and the debugger don’t know much about it, it’s generated at runtime on the stack).

Try checking its address after its point of definition.

EDIT

I just read the "I get random characters and shapes in console". It’s obvious now that you are returning the VLA and expecting things to work.

A VLA is only valid inside the block where it was defined. Using it outside is undefined behavior and thus very dangerous. Even if the size were constant, it still wouldn’t be valid to return it from the function. In this case you most definitely want to malloc the memory.