sizeof（）运算符的行为

Question

For the following piece of code:

char a[] = "Apple";
char *s[] = {"Apple"};
printf("%d %d\n", sizeof(a), sizeof(s[0]));

The output is:

6 4

Can someone tell me why sizeof() is giving different outputs?

EDIT: I did intend to type sizeof() originally, but typed strlen() instead. I apologize for the same. I have edited the statement now.

Answer 1

sizeof给你的数字char小号分配到a ，而strlen为您提供了可用的字符串的长度在a 。

Answer 2

sizeof(a) is the size of the array, which contains the terminator as well as the 5 printable characters.

strlen(s[0]) gives the length of the string, excluding the terminator, since that is what strlen is specified to do.

UPDATE: sizeof(s[0]) is the size of a pointer. There's no way to determine the size of an array given just a pointer to it.

Answer 3

\\0算作字符串内存大小的一部分，但是字符串本身的长度strlen()由strlen()给出strlen()仅由遇到\\0之前的字符给出。

Answer 4

A character array declared like this:

char a[] = "Apple";

has, according to the language specification, a null-terminator. Therefore, the length of the array is 6. There are 5 characters, and then the null terminator.

On the other hand, strlen() returns the number of characters that precede the null terminator. Which is 5.

Answer 5

The sizeof operator yields the size (in bytes) of its operand.

In this statement

char a[] = "Apple";

array a is initialized by characters of string literal "Apple" that includes the terminating zero. In fact this record is equivalent to

char a[] = { 'A', 'p', 'p', 'l', 'e', '\0'; };

So the size in bytes of a is equal to 6.

Standard C function strlen counts symbols in a string until it encounters the terminating zero. So

strlen( a )

will return 5 that is the number of characters in the array that are before the terminating zero.

Take into account that you could write for example

char a[100] = "Apple";

In this case sizeof( a ) will yield 100 because you explicitly specified the number of bytes that the array will occupy. However it was initialized only with 6 characters of the string literal. So how to find how many actual data are in the character array? For this purpose function strlen was introduced that to distinguish the size of a character array and the number of actual data in the character array.

Answer 6

Because in C there is no string type. String is a character array which is NULL terminated. strlen() counts the characters until the NULL character, whereas sizeof() actually returns the amount of memory used up by the charater array.

Answer 7

a is an array of char s, which contains 6 elements, hence sizeof returns 6 (the length of the string including zero termination).

s is an array of pointers to char . The pointer size is 4 bytes. sizeof(s[0]) returns the size of the first element, which is pointer, ie its size is 4.

Answer 8

When you define:

char a[] = "Apple";

It means an array of characters, which equals to the following definition:

char a[] = {'A', 'p', 'p', 'l', 'e', '\\0'}; // '\\0' is the string termination character which equals to 0

Since char type size is 1, the sizeof(a) returns 6 which is the size of the whole array.

Nevertheless, when you define:

char *s[] = {"Apple"};

It means an array of char pointer . Hence sizeof(s[0]) return the size of its first element which equals to sizeof(char*). For a 32-bit platform, sizeof(char*) = 4. If you do it on an 64-bit platform, 8 is the expected value.