How to get an array size in a function without counting or passing the length as a parameter?

I tried use sizeof(array)/sizeof(int), but it seems this doesn't work if I passed the array into another function. So is there any other way to know the size of the array?

By the way: I know a char[] ends with '\0'; which 开发者_运维百科character ends the int[]?

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In C, there is a difference between an array and a pointer to data. When "passing an array to a function", you in effect pass a pointer to the function. Unfortunately, you can't know if a pointer points to one element, or to a sequence of elements, and it's not possible to tell how many elements the sequence consist of.

Also, unlike a string, an int array is not ended with a specific value.

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When an expression of array type appears in most contexts¹, it is implicitly converted ("decays") to an expression of pointer type and its value is the address of the first element in the array. Similarly, in the context of a function parameter declaration (but only in that context), declarations like T a[] and T a[N] are interpreted as T *a; despite the array notation, the parameter a is a pointer, not an array type.

So given code like

int arr[10];
...
foo(arr);

The type of the expression arr in the call to foo is "10-element array of int". In this context, the type of the expression is converted to "pointer to int", so foo will receive a pointer value.

Similarly, given a function definition

void foo(int a[10])
{
  ...
}

the type of the parameter a is not "10-element array of int", but rather "pointer to int".

In short, you cannot pass an array argument to a function and have it received as an array type; all you get is a pointer value. As a result, you must either pass the array size as a separate parameter, or terminate the array with a sentinel value (such as using a 0 in an array of char to terminate strings).

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¹ The exceptions to this rule are when the array expression appears as an operand to either the sizeof or unary & operators, or if it is a string literal being used to initialize another array in a declaration.

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A char array only ends with a \0 if it's being explicitly treated as a null-terminated string. There's no terminating character on arrays in general and no way to know the size of an array without passing the size around with it.