Question about decreasing malloc size by using realloc

Question

I am trying to decrease the size of a malloc array, but it is throwing -1073741819 (0xC0000005) when I realloc a smaller size for the malloc.

typedef struct {
    float reproduce_prob;
    int mature;
    int life;
    int age;
    char *direction;
    int X;
    int Y;
} slug;
slug *slugs = (slug *) malloc(sizeof(slug) * (slugCount + 1));
int slugCount = 0;
                        if (slugCheck(theGrid, frogs[i])) {
                            int pos = feedFrog(frogs[i], slugs, slugCount);
                            for (int z = pos; z < slugCount - 1; z++) {
                                slugs[z] = slugs[z + 1];
                            }
                            slugCount--;
                            slugs = realloc(slugs, sizeof(slugs) * (slugCount + 1));
                            frogs[i].initHung = 0;
                        }

the slugCount is not zero.

Answer 1

It is good practice to use obects not types in the sizeof s. Also it is good to have distinct and meaningful type and variable names to avoid this kind mistakes. I would call slug slugType or slug_type .

in this line you do not allocate enough space (assuming that the slug structure is larger than the pointer) as sizeof(slugs) is giving the size of the pointer to slug :

slugs = realloc(slugs, sizeof(slugs) * (slugCount + 1));

You also incorrectly use realloc as realloc may fail and you will have memory leak

slug *tmp;
tmp = realloc(slugs, sizeof(*tmp) * (slugCount + 1));
if(tmp)
{
    slugs = tmp;
}
else
{
    /* error handling */
}

As a side note: do not cast the result of malloc family functions. If Your code does not compile it means that you use C++ compiler to compile the C language code. It is not a good idea as C & C++ are different languages even if syntax looks similar.

Answer 2

How does this code compile?

slug *slugs = (slug *) malloc(sizeof(slug) * (slugCount + 1));
int slugCount = 0;

The compiler should be yelling at you for using slugCount before it has been defined.

If this code is building it means you've already defined slugCount in an enclosing scope, and its value may not be zero. That's one problem.

You should always check the result of malloc , calloc , and realloc . Are you certain that neither the malloc nor realloc call have returned NULL ?

This line is suspicious:

slugs = realloc(slugs, sizeof(slugs) * (slugCount + 1));

sizeof(slugs) gives you the size of the pointer , not the slug type - you haven't extended your array by one element, you've shrunk it by a whole helluva lot. You might want to change that to

slug *tmp = realloc( slugs, sizeof *slugs * (slugCount + 1) );

You should always assign the result of realloc to a temporary variable. realloc will return NULL if it can't satisfy the request and leave the original buffer in place . However, if you assign that NULL result back to your original pointer, you will lose your only reference to that memory.