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CS156 Sizeof

sizeof

sizeof() is a compile-time function that return the size, in bytes, of a variable or a type.

char c;
int i;
float f;
double d;
printf("%zu\n", sizeof(c));
printf("%zu\n", sizeof(i));
printf("%zu\n", sizeof(f));
printf("%zu\n", sizeof(d));
printf("%zu\n", sizeof(double));

1
4
4
8
8

The printf format for sizeof is %zu.

sizeof an array

• sizeof give you the size of an array, in bytes.
• It’s not a count.

int a[13];
printf("%zu\n", sizeof(a));

52

• If you want the number of elements, divide the total size by the size of each element:

int a[13];
printf("%zu\n", sizeof(a)/sizeof(a[0]));

13

Array Function Argument

• A function simply cannot tell how big an array given as an argument is.
• You must tell the function how big the array is via another argument.

void bar(int b[]) {
    printf("b: %zu\n", sizeof(b));
}

int main() {
    int a[1000];
    bar(a);
    printf("a: %zu\n", sizeof(a));
}

c.c: In function 'bar':
c.c:2: warning: 'sizeof' on array function parameter 'b' will return size of 'int *'
c.c:1: note: declared here
b: 8
a: 4000

Array Function Argument

• Remember how arrays are not passed by value?
• They’re passed by reference, like using & with scanf.
• Really, the address of the array is passed.
• These are exactly the same:
  • void bar(int b[]);
  • void bar(int *b);

Array Function Argument

• Here’s what really happens:

void bar(int *b) {
    printf("b: %zu\n", sizeof(b));
}

int main() {
    int a[1000];
    bar(a);
    printf("a: %zu\n", sizeof(a));
}

b: 8
a: 4000

• That makes more sense. b is a pointer. All pointers on this computer are that size.

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