C Pointer becomes NULL in loop

Question

I have found many 'pointer becomes NULL' questions, but struggling to apply the answers to my problem

I have 3 nested structs. Struct A should contain an array of struct B, which contains a single struct C which itself should contain an array of int:

typedef struct {
    int32_t *data;
    uint16_t nData;
    uint16_t dataOffset;        
} Window;

typdef struct {
    Window *window;
    const char *datasetPath;
} Dataset

typedef struct {
    char *id;
    uint8_t count;
    Dataset *datasets;
} DatasetGroup;

I have a new function for the structs:

int8_t dataset_new(Dataset *ds, const char *datasetPath, uint16_t winSize){
    ds->datasetPath = datasetPath;
    ds->window = malloc(sizeof(*(ds->window));
    ds->window->data = malloc(sizeof(int32_t) * (winSize));
    return 0;

int8_t datasetgroup_new(DatasetGroup *dg, char *id){
    dg->id = id;
    dg->count = 0;
    dg->datasets = malloc(sizeof(*(dg->datasets)) * 255);
}

And I have an add function to add a Dataset to the DatasetGroup:

int8_t datasetgroup_add(DatasetGroup *dg, const char *filePath, uint16_t winSize){
       // Create the dataset
       Dataset ds; 
       dataset_new(&ds, filePath, winSize);

       // Add the dataset to the dataset array
       dg->datasets[dg->count] = ds;
       dg->count++;
       return 0;
       }

I then iterate through the datasets in order to populate the data, doing stuff like:

     for (i = 0 ; i < datasetCount ; i++){
         Dataset *ds = &(dg->datasets[i])

Always on the 2nd interation, the data array becomes a null pointer: data = ds->window->data

I understand that I have done something wrong passing pointers around(?) but I'm not sure what precisely I have done wrong....

Answer 1

This looks fine. So check another part of the code. You can debug code line by line so you can understand the segment which is creating this error.