Errors when making a dynamic C array of structs

Question

I have a struct called Collection:

typedef struct collection {
   char *type;
   char *arg;
} *Collection;

And I would like to have a dynamic array of this struct (or rather, of pointers to instances of this struct). This is what I tried:

Collection *rawCollections = malloc(0);
int colCounter = 0;
while (i < argc) {
    Collection col = malloc(sizeof(Collection));
    // code to fill in Collection
    rawCollections = realloc(rawCollections, sizeof(rawCollections) + sizeof(Collection));
    rawCollections[colCounter] = col;
    colCounter++;
}

My reasoning is that we will add sizeof(Collection) to the array each time I need to add another one. I am getting these errors, and I am not sure why:

realloc(): invalid next size
Aborted (core dumped)

You need to keep your own counter for the size to reallocate (for example colCounter). — Some programmer dude
– Some programmer dude, Commented Sep 11, 2020 at 21:31
Do not hide pointer nature behind a typedef. It confuses everyone, including you. — John Bollinger
– John Bollinger, Commented Sep 11, 2020 at 21:36
Collection col = malloc(sizeof(Collection)); is also wrong. — Christian Gibbons
– Christian Gibbons, Commented Sep 11, 2020 at 21:36
Collection is a typedef of struct collection *. So you are creating a variable of type struct collection * and pointing it to memory allocated to be the size of struct collection *. That is, you allocated the size of a pointer when you should have allocated the size of the object being pointed to. You can fix that by turning the malloc call into malloc(sizeof(*col)) — Christian Gibbons
– Christian Gibbons, Commented Sep 11, 2020 at 21:59

chqrlie · Accepted Answer · 2020-09-11 21:49:41Z

2

You must compute the new size for the array by multiplying the size of the array element (a pointer to a struct collection) by the new number of elements (colCounter + 1).

Note also how confusing it is to hide pointers behind typedefs: sizeof(Collection) is not the size of the structure.

Here is a modified version:

struct collection **rawCollections = NULL;  // no need for `malloc(0)`
int colCounter = 0;
while (i < argc) {
    struct collection *col = malloc(sizeof(*col));
    // code to fill in Collection
    rawCollections = realloc(rawCollections, sizeof(*rawCollections) * (colCounter + 1));
    rawCollections[colCounter] = col;
    colCounter++;
}

answered Sep 11, 2020 at 21:49

chqrlie

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2 Comments

shurup Over a year ago

sizeof(Collection) is the size of the pointer to the structure, correct?

chqrlie Over a year ago

@shurup: yes it is, but you write Collection col = malloc(sizeof(Collection)); which is incorrect: you should allocate the space for a structure, not the space for a pointer.

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