Length of array in function argument

Question

This is well known code to compute array length in C:

sizeof(array)/sizeof(type)

But I can\'t seem to find out the length of the array pass

Answer 1

The array decays to a pointer when passed.

Section 6.4 of the C FAQ covers this very well and provides the K&R references etc.

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That aside, imagine it were possible for the function to know the size of the memory allocated in a pointer. You could call the function two or more times, each time with different input arrays that were potentially different lengths; the length would therefore have to be passed in as a secret hidden variable somehow. And then consider if you passed in an offset into another array, or an array allocated on the heap (malloc and all being library functions - something the compiler links to, rather than sees and reasons about the body of).

Its getting difficult to imagine how this might work without some behind-the-scenes slice objects and such right?

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Symbian did have a AllocSize() function that returned the size of an allocation with malloc(); this only worked for the literal pointer returned by the malloc, and you'd get gobbledygook or a crash if you asked it to know the size of an invalid pointer or a pointer offset from one.

You don't want to believe its not possible, but it genuinely isn't. The only way to know the length of something passed into a function is to track the length yourself and pass it in yourself as a separate explicit parameter.