c - Accessing struct array of unsigned ints -

i have struct has space unsigned ints:

typedef struct {     unsigned int *arr; } contents; 

when allocate memory:

contents *allocator() {     contents *cnt = malloc(sizeof(contents));     cnt->arr = calloc(1, sizeof(unsigned int)); } 

i later retrieve dereferencing passing in pointer contents , doing:

void somefunction(contents *cnt) {    unsigned int * arr = cnt->arr;    arr[0] >>= 1; // in future 0 replaced loop on array items    cnt->arr = arr;  } 

once exit out of function, cnt->arr becomes empty. have memcpy? not understanding how struct laid out? understand

cnt->arr = (*cnt).arr 

thanks!

the problem you're doing unsigned int *arr = cnt->arr, declares unsigned int pointer , makes point cnt->arr. once modify array, attempt re-set array - re-assigning pointers, haven't changed contents of array; you've changed pointers. thus, cnt->arr = arr line doesn't change anything. then, "unsigned int *arr" runs out of scope, , pointer destroyed, leaving unrecoverable data.

you'll need copy array somewhere temporary instead, , operations on array instead, , copy back, or (the easier method) use arr pointer , don't try cnt->arr = arr - effect have been achieved anyway