CODEWITHSUNDEEP
cpointersmallocdynamic-memory-allocationrealloc
shurup
shurup

Reputation: 871

Errors when making a dynamic C array of structs

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)

Upvotes: 1

Views: 50

Answers (1)

chqrlie
chqrlie

Reputation: 144695

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++;
}

Upvotes: 2

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