Difference between sizeof(name) and sizeof name

Question

I am following the C primer plus and encounter the following example:

#include <stdio.h>
#include <string.h> //provide strlen() prototype
#define PRAISE "You are an extraordinary being with a big big brain."
int main(void)
{
    char name[40];

    printf("What's your name? ");
    scanf("%s", name);
    printf("Hello, %s. %s\n", name, PRAISE);
    printf("Your name of %zd letters occupies %zd memory cells.\n", strlen(name), sizeof name);
    printf("Size of the name %zd", sizeof(name));

    return 0;
}

Compiled and run it:

$ ./a.out
What's your name? Trump
You are an extraordinary being with a big big brain."
Your name of 5 letters occupies 40 memory cells.
Size of the name 40

What puzzled me is that sizeof(name) is identical to sizeof name?

sizeof() is a library routine, but how abut sizeof, it seems as predicate and parameter as it was in command line.

So rude. My name is `abcdefghijklmnopqrstuvxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvxyzabcdefghijklmnopqrstuvwxyz` and I get a segfault from your program :( — hellow, Oct 15 '18 at 09:25
adding to what @Jabberwocky said, parenthesis **are** needed when evaluating the size of a type, for example `sizeof (char)` — Kovalainen, Oct 15 '18 at 09:26
I like to think of it as the parenthesis "belonging" to the type (pretty much like a cast). Some people overdo the parenthesis thing though (`sizeof (a)`, `return (42);`, ...) — pmg, Oct 15 '18 at 09:28

score 4 · Accepted Answer · answered Oct 15 '18 at 09:25

No sizeof isn't a library function, it's a special operator keyword.

It has two forms: One is with parentheses to get the size of a type. The other is not using parentheses and is for getting the size of the result of a generic expression.

With sizeof(name) you're really using the second variant, with the expression (name). I.e. the parentheses are part of the expression and not the sizeof operator.

P.W · Answer 2 · 2018-10-15T09:32:07.673

The sizeof operator (not function) yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type.

It can be used with or without parentheses if the operand is an expression.
It can only be used with parentheses if it is a type.

So in your case, sizeof(name) and sizeof name are both valid.
But something like sizeof char would be invalid, whereas sizeof(char) would be valid.

Difference between sizeof(name) and sizeof name

2 Answers2