Sizeof with variables in C

Question

I'm trying to learn how to use sizeof() in C by determining the sizes of the following variables. Can someone tell me if i'm wrong or right? I'm unsure if I'm understanding the concept or not.

int x[] = {5, 6, 7};
float y[5];
double d;
char c[5] = {1, 2};

• sizeof(x): 3 --- because the array has 3 elements

• sizeof(y): 5 --- the array has 5 elements

• sizeof(d): 8 --- always the size of a double

• sizeof(c): 5 --- the array has 5 elements

Answer 1

The sizeof operator gives you the number of bytes⁽¹⁾ an item needs to store its data, not the number of elements in an array⁽²⁾.

Hence, for a 32-bit integer, 8-bit character, "normal" IEEE-854 implementation (32-bit single-precision float and 64-bit double-precision double), you'll see your sizes as 12 (three int elements), 20 (five float elements), 8 (a double), and 5 (five char elements).

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⁽¹⁾ Keep in mind that the ISO C definition of byte is not necessarily an 8-bit value. Instead it's the smallest addressable item (such as a char). You'll find many standards documents prefer to use the term "octet" for a guaranteed-8-bit value.

Or, to put it another way, sizeof(char) is always one.

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⁽²⁾ To work out array sizes, you can divide the size of the whole array by the size of the first element:

int xyzzy[] = {1, 2, 3};
int num_elems = sizeof(xyzzy) / sizeof(*xyzzy);

Answer 2

The sizeof operator tells you the size of its operand in bytes.

So assuming an int is 4 bytes, a float is 4 bytes, and by definition a char is 1 byte, then:

• sizeof(x) = 12
• sizeof(y) = 20
• sizeof(c) = 5

Answer 3

others have pointed out that your understanding is wrong. But since you menton arrays in the question its worth showing you a very common c idiom

taking your

int x[] = {5, 6, 7};

you will see this

int xLen = sizeof(x)/sizeof(x[0]);

ie divide the length of the array by the size of each element in order to determine how many elements in a given array

In your case (assuming 4 bytes for an int)

int xLen = 12/4;

ie 3

Answer 4

Just as an addendum: when you begin working with structs, padding may make sizeof not work the way you might expect.

struct A {
    char b[3];
};

For the above, sizeof(struct A) (on my machine anyway) returns 3.

But if I add a four byte int to the struct:

struct A {
    char b[3];
    int c;
};

Now sizeof(struct A) does not return 7 but rather 8.