How to use snprintf instead of strcpy in this situation? - c

Question

void put(char* key, int value)
{
int i = 0;
// Iterate through elements of hashtable
while (array[i].flag == 1)
{
// If key already exists, update the value
// and return
if (strcmp(array[i].data->key, key) == 0)
{
array[i].data->value = value;
return;
}
i = i + 1;
// Error Handling, when end of hashtable is reached
if (i == max)
{
i = 0;
// p rintf("\n Hash table is full, cannot insert any more item \n");
// return;
}
}
// Insert new item into the hashtable
array[i].flag = 1;
array[i].data = (struct item*) malloc(sizeof(struct item));
array[i].data->key = (char *)malloc((strlen(key)+1)*sizeof(char));
// so here I have to use snprintf instead of strcy because it's
// forbidden, and I don't know how
**strcpy(array[i].data->key, key);**
//snprintf(array[i].data->key,sizeof(key),"%s",key);
array[i].data->value = value;
}

Answer 1

Firstly, never cast malloc, and do not forget verify the return value of malloc.
array[i].data = malloc(sizeof(struct item));
if (!array[i].data) {
// handle error
return; // for example
}
array[i].data->key = malloc((strlen(key)+1)*sizeof(char));
if (!array[i].data->key) {
// handle error
}
Using strlen instead of sizeof that returns the size of pointer, not the length of string:
snprintf(array[i].data->key, strlen(key) + 1,"%s", key);

Answer 2

here I have to use snprintf instead of strcy because it's forbidden, and I don't know how
strcpy() can overflow is the destination is insufficient. That really does not apply here as the needed space is allocated.
// array[i].data = (struct item*) malloc(sizeof(struct item));
// array[i].data->key = (char *)malloc((strlen(key)+1)*sizeof(char));
// strcpy(array[i].data->key, key);
array[i].data = malloc(sizeof *(array[i].data));
size_t sz = strlen(key) + 1;
array[i].data->key = malloc(sz);
snprintf(array[i].data->key, sz, "%s", key);
// or
strcpy(array[i].data->key, key);
// or
memcpy(array[i].data->key, key, sz);
Cast not needed
size of of referenced data better to use that sizeof type.
Avoid 2 trips finding the length.
Returns check omitted for brevity.
With checks
array[i].data = malloc(sizeof *(array[i].data));
if (array[i].data == NULL) return fail; // or do something to indicate error
size_t sz = strlen(key) + 1;
array[i].data->key = malloc(sz);
if (array[i].data->key == NULL) return fail;
int count = snprintf(array[i].data->key, sz, "%s", key);
if (count < 0 || (unsigned) count >= sz) return fail;
Primary mistake
OP's attempt fails as sizeof(key) is the size of a pointer, perhaps 2, 4 or 8. Instead, what is needed is the size of the allocation for array[i].data->key as in sz (see above) or strlen(key) + 1.
snprintf(array[i].data->key,sizeof(key),"%s",key); // bad