Memset In C Using Sizeof Operator :

Question

Here is a struct :

typedef struct tag_device_sys
{
    char device[STRING_SIZE];
    int id;
    char category;
} device_sys;

For memset, which one should be used from below ?

memset(dev_sys, 0, (size_t)NUM_DEVICES * sizeof(device_sys));

or

memset(dev_sys, 0, (size_t)NUM_DEVICES * sizeof(dev_sys));

What are the differences ?

Answer 1

I like to use objects as arguments to sizeof

struct whatever *ptr;
/* ... */
ptr = malloc(nelems * sizeof *ptr);
memset(ptr, 0, nelems * sizeof *ptr);

The advantage over using a parenthesized type is that the code only needs to be changed at one place should the type of the data change

struct before_change *ptr;
/* ... */
ptr = malloc(nelems * sizeof (struct before_change));
memset(ptr, 0, nelems * sizeof (struct before_change));

Answer 2

dev_sys is a pointer. sizeof(dev_sys) will just give you size of the pointer for your platform.

Answer 3

如果将dev_sys分配为(NUM_DEVICES * sizeof(device_sys)) memset(dev_sys, 0, (size_t)NUM_DEVICES * sizeof(device_sys))应该可以工作

Answer 4

Definitely

memset(dev_sys, 0, (size_t)NUM_DEVICES * sizeof(device_sys));

There are a few cases, if you have:

1) Static array, using sizeof in that context:

device_sys dev_sys[NUM_DEVICES] = { /* init data * };
...
sizeof(dev_sys);

In this case, sizeof operator will give the entire size of the array, so you will essentially end up with:

NUM_DEVICES * (NUM_DEVICES * sizeof(device_sys))

2) or you have a dynamically allocated piece of memory acccessed through a pointer, OR you are in the context of a function call where dev_sys is passed as a function argument. Either way:

sizeof(dev_sys) will give the size of a pointer (in the first, because it IS a pointer, in the second, due to pointer decay), whcih will most likely not be what you are after.

Answer 5

memset(dev_sys, 0, (size_t)NUM_DEVICES * sizeof(*dev_sys));

Always works as the way you've written it suggests dev_sys is either a pointer or an array. sizeof(*dev_sys) gives us the sizeof the first element.

In any case, I would write is as either

device_sys dev_sys[NUM_DEVICES];

memset(dev_sys, 0, (size_t) NUM_DEVICES * sizeof(*dev_sys));

or

device_sys *dev_sys = (device_sys *) calloc(NUM_DEVICES, sizeof(*dev_sys));

depending on if you want dev_sys on the stack or on the heap

Answer 6

Richard Stevens (Advanced Programming in the Unix Environment, etc) suggests bzero() or your own portable equivalent because it avoids the common bug of swapping memset() arguments. You could even make a single-arg macro ZERO(obj) that did bzero(obj, sizeof(*obj)) . Or ZERO(object) (memset (&(object), '\\0', sizeof ((object)))) perhaps is an alternative. Along those lines, nice way to make bugs less possible.