Get the length of dynamically allocated array in C [duplicate]

Question

Possible Duplicate:
length of array in function argument

How do I get the length of a dynamically allocated array in C?

I tried:

sizeof(ptr)
sizeof(ptr + 100)

and they didn't work.

Answer 1

You can't. You have to pass the length as a parameter to your function. The size of a pointer is the size of a variable containing an address, this is the reason of 4 ( 32 bit address space ) you found.

Answer 2

Since malloc just gives back a block of memory you could add extra information in the block telling how many elements are in the array, that is one way around if you cannot add an argument that tells the size of the array

e.g.

char* ptr = malloc( sizeof(double) * 10 + sizeof(char) );
*ptr++ = 10;
return (double*)ptr;

assuming you can read before the array in PHP, a language which I am not familiar with.