Array is and is not a pointer
Is array a pointer in C? seems we can treat it as a pointer when we access its element. But sometimes Array has different behavior from pointer.
Take this for example:
int main() {
int arr[10] = {0};
printf("size: %ld", sizeof(arr));
return 0;
}
If arr here is a pointer, then seems this should print out the size of a pointer (4 or 8, depends on the Operating System), but if we compile this code, it prints out 40, so clearly it is sizeof(int) * 10.
I like the quote here:
The first step to learning C is understanding that pointers and arrays are the same thing. The second step is understanding that pointers and arrays are different.
Pass array as parameter to function
Ok, from this sizeof at least we know they are different, but how do we explain this:
#include <stdio.h>
void func(int arr[]) {
size_t size = sizeof(arr);
printf("Inside func: %ld\n", size);
}
int main() {
int arr[10] = {0};
size_t size = sizeof(arr);
printf("Inside main: %ld\n", size);
func(arr);
return 0;
}
We passed arr as parameter to function func, if we compile and run this program, it prints out:
Inside main: 40
Inside func: 8
Well, although we set the parameter type to be arr[] in function func, but inside the function, arr behaves like a pointer now (it prints 8, instead of 40). So what's the magic here? we call the same sizeof function, why inside main function it prints out 40, and inside func, it prints 8?
This problem bothered me for a while, I decide to check the generate assembly code to see what's the difference for these 2 sizeof call.
$ gcc -S -fverbose-asm main.c
A file main.s is generated, open this file, first let's check the sizeof inside main function:
# main.c:10: int arr[10] = {0};
movq $0, -48(%rbp) #, arr
movq $0, -40(%rbp) #, arr
movq $0, -32(%rbp) #, arr
movq $0, -24(%rbp) #, arr
movq $0, -16(%rbp) #, arr
# main.c:11: size_t size = sizeof(arr);
movq $40, -56(%rbp) #, size
Here the movq is move 8 bytes command. We can see after we defined arr, it first moved 8 bytes for 5 times from the arr address, then for our sizeof call, the compiler immediately set the size to be 40. Ha, so this size would be already set during the compile time, not at run time! The compiler knows arr here is an array's base address, it knows the array's size.
But the sizeof call in func function gave us 8, its assembly code:
movq %rdi, -24(%rbp) # arr, arr
# main.c:5: size_t size = sizeof(arr);
movq $8, -8(%rbp) #, size
We can see here, compiler directly put 8 to the size variable. The reason is when we pass an array to a function, it will be downgraded to a pointer, so although we write the funcion to be func(int arr[]), when we pass the arr to this function, compiler will treat it as func(int* arr).
But it seems weird to me: the compiler should be definitely smart enough to track the passed parameter and can tell it is an array. The only reasonable explanation I found is the "historical" reason:
The only purpose of this rule is to maintain backwards compatibility with historical compilers that did not support passing aggregate values as function arguments.
But anyway, if the C standard says so, ok, at leaset we know when pass array as parameter, it is a pointer. So inside the function, if we want to know how many elements that array contains, we cann't use sizeof(arr)/sizeof(arr[0]) any more, because sizeof(arr) will just give us the size of a pointer.
So in the func function, how do we tell the array's size? An obvious way is we can pass another length paramter to tell this func the array length.
void func(int arr[], int len) {
// ...
}
Or we use the array's address as the paramter :
void func2(int (*arr)[10]) {
size_t size = sizeof(*arr);
printf("Inside func2: %ld\n", size);
}
If in our main function, we call this func2 as func2(&arr);, it will print Inside func2: 40.
For our func2 function, we write a fixed length 10 there. If we want the same trick works for variable length array, we could define the function like this:
void func3(int n, int (*arr)[n]) {
size_t size = sizeof(*arr);
printf("Inside func3: %ld\n", size);
}
Then call it like func3(10, &arr), it will also print out the size to be 40.
Variable Length Array
In the above example, we defined a fixed length array, and when compiler sees sizeof, it immidiately know the size. But what if the array's length is determined at the run time? Say if we have code:
int main() {}
int len = 0;
scanf("%d", &len);
int arr2[len];
size = sizeof(arr2);
printf("arr2 size: %ld\n", size);
return 0;
}
Here we take the length from user input, can compiler output the right array size in this case? or it will just treat the array as a pointer now?
If we try it: we type 10 as the length, it prints arr2 size: 40; we type 20 as the length, it prints arr2 size: 80. So although the size determined during run time, the compiler still has a way to tell use the correct size.
If we check the generated assembly code, this time the compiler generated much more than the fixed length:
# main.c:17: int arr2[len];
movl -124(%rbp), %ecx # len, len.0_19
# main.c:17: int arr2[len];
movslq %ecx, %rax # len.0_19, _1
subq $1, %rax #, _2
movq %rax, -112(%rbp) # _2, D.2335
movslq %ecx, %rax # len.0_19, _4
movq %rax, %r14 # _4, _5
movl $0, %r15d #, _5
movslq %ecx, %rax # len.0_19, _9
movq %rax, %r12 # _9, _10
movl $0, %r13d #, _10
movslq %ecx, %rax # len.0_19, _12
leaq 0(,%rax,4), %rdx #, _24
movl $16, %eax #, tmp122
subq $1, %rax #, tmp103
addq %rdx, %rax # _24, tmp104
movl $16, %edi #, tmp123
movl $0, %edx #, tmp107
divq %rdi # tmp123
imulq $16, %rax, %rax #, tmp106, tmp108
movq %rax, %rdx # tmp108, tmp110
andq $-4096, %rdx #, tmp110
movq %rsp, %rsi #, tmp111
subq %rdx, %rsi # tmp110, tmp111
movq %rsi, %rdx # tmp111, tmp111
.L3:
cmpq %rdx, %rsp # tmp111,
je .L4 #,
subq $4096, %rsp #,
orq $0, 4088(%rsp) #,
jmp .L3 #
.L4:
movq %rax, %rdx # tmp108, tmp112
andl $4095, %edx #, tmp112
subq %rdx, %rsp # tmp112,
movq %rax, %rdx # tmp108, tmp113
andl $4095, %edx #, tmp113
testq %rdx, %rdx # tmp113
je .L5 #,
andl $4095, %eax #, tmp114
subq $8, %rax #, tmp114
addq %rsp, %rax #, tmp115
orq $0, (%rax) #,
.L5:
movq %rsp, %rax #, tmp109
addq $3, %rax #, tmp116
shrq $2, %rax #, tmp117
salq $2, %rax #, tmp118
movq %rax, -104(%rbp) # tmp118, arr2.1
# main.c:18: size = sizeof(arr2);
movslq %ecx, %rax # len.0_19, _14
# main.c:18: size = sizeof(arr2);
salq $2, %rax #, tmp119
movq %rax, -120(%rbp) # tmp119, size
We won't dive into details for this code (to see the explanation, you can check it here), but we can see for VLA, sizeof can still correctly evaluate the array size in run time.
malloc and free
Another question is, we know if we allocate some memory using malloc, and when we free the memory with the free function, we never pass a second parameter to tell the size (how many memory needs to be free'ed), how does the compiler know that?
The magic is behind the malloc function, for example, when we call the p = malloc(10) function, although it returns us the address for 10 bytes, under the hood, it will allocate more to keep some meta data like the size. For example, a simplest method could be, it allocate 4 bytes + 10 bytes where the first 4 bytes is used to save the size 10, then it returns the memory address starts from the 10 bytes. Later when we call the free(p) function, free can first use p - 4 bytes to read what the size is, then do a proper delete.
Takeaways
- array and pointer are different
- when pass an array as parameter to function, it decays to pointer
- we can pass a array pointer as parameter to keep it behaves as an array instead of a pointer
- even array's length is determined during the run time, sizeof can still print out correct size
- malloc function will allocate more to save the size info such that later the free function can get the correct size to free
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