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)

`sizeof(rawCollections)` [does not change](https://stackoverflow.com/q/2259890/11683). — GSerg, Sep 11 '20 at 21:28
You need to keep your own counter for the size to reallocate (for example `colCounter`). — Some programmer dude, Sep 11 '20 at 21:31
Do not hide pointer nature behind a typedef. It confuses everyone, including you. — John Bollinger, Sep 11 '20 at 21:36
`Collection col = malloc(sizeof(Collection));` is also wrong. — Christian Gibbons, Sep 11 '20 at 21:36
@user3121023 that's supposed to say `rawCollections` -> fixed now — shurup, Sep 11 '20 at 21:41
@JohnBollinger That's what our prof wants us to use unfortunately — shurup, Sep 11 '20 at 21:42
@ChristianGibbons Can you elaborate? That part seems to work fine. — shurup, Sep 11 '20 at 21:54
`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, Sep 11 '20 at 21:59

score 2 · Accepted Answer · answered Sep 11 '20 at 21:49

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 '20 at 21:49

chqrlie

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`sizeof(Collection)` is the size of the pointer to the structure, correct? – shurup Sep 11 '20 at 21:53
1

@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. – chqrlie Sep 11 '20 at 22:01

Errors when making a dynamic C array of structs

1 Answers1