Why Does `sizeof()` Return a Different Value for C Arrays Inside and Outside Functions?

C-Array's Misleading Sizeof() inside a Function

When dealing with C-arrays, understanding their behavior when passed to functions is crucial. This question sheds light on the surprising differences between sizeof() values of arrays in and outside functions.

Problem:

In the given C program, an array point of size 3 is declared and its size is correctly obtained using sizeof(point)/sizeof(point[0]). However, when the same array is passed to the test() function, sizeof(arr) returns an incorrect value of 2.

Reason:

When an array is passed to a function in C, it decays into a pointer to its first element. Consequently, sizeof() on the function parameter measures the size of the pointer, not the array itself. This leads to the erroneous result in the test() function.

Solution:

To address this issue, the size of the array should be passed as a separate parameter to the function:

void test(int arr[], size_t elems) {}

In the main() function, the array size can be passed using:

test(point, sizeof(point) / sizeof(point[0]));

Note:

It's important to note that the sizeof(point)/sizeof(point[0]) calculation is not applicable to dynamically allocated arrays, which are not allocated on the stack.

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