C (std=c99) pointer to structs memory allocation [closed] - c

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So, this is working fine... that means, no compiler errors, it seems that there is no memory leak and it is doing what I wanted it to do. That said should it be working? When I go to books_init I send a local variable to collection, doesn't it mean that when I go back to main it shouldn't be working? (or undefined behavior?). Also, in case you say that I have to malloc it, do I have to free it after? (commented on cleanup)
/* pseudo struct Collection{
size_t size, capacity;
Volume *volumes;
} */
void collection_init(Collection *col, size_t capacity){
col->size = 0;
col->capacity = capacity;
col->volumes = malloc(sizeof(Volume) * capacity);
}
void collection_resize(Collection *col, size_t capacity){
Volume *v = realloc(col->volumes, capacity * sizeof(Volume));
if(!v) return;
col->capacity = capacity;
col->volumes = v;
}
void collection_add(Collection *col, Volume *volume){
if(col->size >= col->capacity)
collection_resize(col, col->capacity * 2);
col->volumes[col->size++] = *volume;
}
void collection_clean(Collection *col){
//for(vol : col->vol) free(vol);
//should I free every element or just volumes?
free(col->volumes);
}
void books_init(Collection *col){
for(int i = 0; i < 25; ++i){
Volume v = {.swag = i};
collection_add(col, &v);
}
}
int main(){
Collection col;
collection_init(&col, 10);
books_init(&col);
for(int i = 0; i < col.size; ++i){
printf("\tVol[%d].id = %d\n", i, col.volumes[i].swag);
}
collection_clean(&col);
return 0;
}
Thanks for your time

Answer 1

This line in books_init
Volume v = {.swag = i};
creates a local variable called v with member swag initialized to i. The address of that variable is then passed to collection_add. That is allowed because v is still in scope.
This line in collection_add
col->volumes[col->size++] = *volume;
makes a copy of the contents of the Volume structure, and stores that copy in the memory that was allocated in collection_init.
After collection_add returns, the variable v in books_init goes out of scope, but that's OK because the contents of v were copied and saved in the memory that col->volumes points to.
When the program ends, collection_clean only needs
free(col->volumes);
to remove all of the Volume copies from memory.
The only flaw I see in your program occurs if realloc fails. In that case, you still write to the Volume array. This will cause a buffer overrun and memory corruption. To avoid this, the collection_add function should verify that the collection_resize function succeeded before performing the copy. For example, you could check again that col->capacity > col->size before doing the copy.
TL;DR your code is fine as long as the realloc always succeeds.