为什么sizeof运算符给出不同的输出

Question

I am learning the basics of C for a course I am taking soon, and right now I am learning about pointers and arrays. I have been using a number of references to learn it but in this "Head First into C" book I have, there is the following:

char s[] = "How big is it?"
char *t = s;
printf("%i\n", sizeof(s));  /* returns 15 */
printf("%i\n", sizeof(t));  /* returns 4 or 8 */

Thats all fine, but if i change the last line to:

printf("%i\n", sizeof(*s));  /* returns 1 (on my compy anyway) */

So my question is why is this different? Apologizes if it's a really basic thing, but I have yet to wrap my brain around the concept of pointers.

Answer 1

One is the size of an array and the other is the size of a pointer.

Arrays and pointer are different types in C although there are implicit conversion rules between arrays and pointers..

The size of an array initialized by a character string literal is the number of characters of the string + 1 for the null terminator.

The size of a pointer to char (ie, char * ) is an implementation defined size of an object large enough to hold the pointer value.

Answer 2

This statement

printf("%i\n", sizeof(s));  /* returns 15 */

outputs the number of bytes that that array s occupies.

This statement

printf("%i\n", sizeof(t));  /* returns 4 or 8 */

displays the number of byte occupied by pointer t. Usually pointers have size either 4 or 8 bytes depending of using system.

This statement

printf("%i\n", sizeof(*s));  /* returns 1 (on my compy anyway) */

displays sizeof char because expression *s has type char. In all system size of an object of type char is equal to 1.

Answer 3

sizeof(s) returns the size of the char array, s , which is 14 char s long, plus one for the null terminator; ergo 15 bytes.

sizeof(t) returns the size of the char pointer t . t isn't aware of s 's length, because when you assign its value from s , s decays from an array type to a pointer type, losing the length information in the process. As you have discovered, pointers can vary in size depending on the target platform.

sizeof(*s) and sizeof(*t) both dereference the address of the first char in the array, yielding a single char . sizeof(char) == 1, because a char is per definitionem a single byte.