Question

Lets say I have an array of structs and I want to delete an entry that has a struct with an entry matching some criteria.

This array is dynamically allocated with malloc, I keep the element count in a separate variable.

How do I go about deleting the entry?

I’m thinking of

for (i = pos; i < arr_len; i++) {
  arr[i] = arr[i+1];
}
arr_len--;

But this leaves the same amount of memory for the array while I actually need less and an orphan (sort of) last entry.

Is issuing a realloc in such situation an accepted practice? Would realloc do memcpy in this case? (shortening the allocated memory by one block).

Answer 1

realloc is ok … but keep reading 🙂

realloc will not move parts of the memory; it may move the whole block. So you need to copy the data before changing the allocated size.

To move the data, memmove (not memcpy) is a good option: it works for memory areas that belong to the same object. Pay attention to not go over your array limits, though; like you do in your code.

for (i = pos; i < arr_len; i++) {
  arr[i] = arr[i+1];
}

The arr[i] = arr[i + 1]; will try to access one past the allowed size. You need

for (i = pos + 1; i < arr_len; i++) {
  arr[i - 1] = arr[i];
}

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There is somewhat of an overhead when calling realloc. If your structs are not large and/or they live only for a short while, consider keeping both an element count and allocated count and only realloc to enlarge (when (element_count + 1) > (allocated_count)).

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If the struct is large, also consider a different data structure (linked list perhaps).