CODEWITHSUNDEEP
cmemset
ant2009
ant2009

Reputation: 22486

initializing a structure array using memset

gcc 4.4.4 c89

I have the following structure.

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

int main(void)
{
    struct device_sys dev_sys[NUM_DEVICES];

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

    return 0; 
}

I get a stack dump when I call memset. Is this not the correct way to initialize an structure array?

Upvotes: 24

Views: 99381

Answers (4)

Thom Smith
Thom Smith

Reputation: 14086

For an array, sizeof gets you the entire size of the array, not the size of an individual element. The sizeof operator is one of the few places where an array is not treated as a pointer to its first element.

Upvotes: 2

AnT stands with Russia
AnT stands with Russia

Reputation: 320361

Either

memset(&dev_sys, 0, sizeof dev_sys);

or 

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

Or, if you prefer

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

but not what you have in your original variant.

Note, that in your specific case in all variants you can use either &dev_sys or dev_sys as the first argument. The effect will be the same. However, &dev_sys is more appropriate in the first variant, since if follows the memset(ptr-to-object, object-size) idiom. In the second and third variants it is more appropriate to use dev_sys (or &dev_sys[0]), since it follows the memset(ptr-to-first-element, number-of-elements * element-size) idiom.

P.S. Of course, instead of using all that hackish memset trickery, in your particular case you should have just declared your array with an initializer

struct device_sys dev_sys[NUM_DEVICES] = { 0 };

No memset necessary.

Upvotes: 48

Hans Passant
Hans Passant

Reputation: 941208

There's a typo in your code. Fix:

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

Picking good names avoid half the bugs. I'd recommend "devices".

Upvotes: 11

Lucas
Lucas

Reputation: 14129

You have to pass the sizeof operator the type and not the variable. 

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

I prefer to use typedef for the struct.

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

The you can use memset as follows:

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

Upvotes: 1

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