Thread: Pointers in a function, sizeof doesn't get array length

When a function takes a pointer to an array as a parameter, how do I determine the size of the array and not the pointer? This is what I mean, for a brief example:

Code:

unsigned char RandomArray[16384];

void TestFunction(unsigned char *ArrayPointer)
{
   const int ArrayLength = sizeof(ArrayPointer); // * or & in front doesn't work
   const int ObviousRoute = sizeof(RandomArray); // this works as expected
}

I need the top case to work, not the bottom case. The function processes multiple arrays and I need to make sure I don't exceed the array's limit. Whether I put * or & in the front, I don't get the 16,384 I'm expecting, just 1 in the case of the * and 4 in the case of the & (the latter I'd expect since it's getting the address instead, a 32-bit value).

You're just going to have to pass the size as an additional parameter.

There is NO WAY to discover how much memory a pointer points to from just looking at the pointer.

Originally Posted by ulillillia

When a function takes a pointer to an array as a parameter, how do I determine the size of the array and not the pointer? This is what I mean, for a brief example:

Code:

unsigned char RandomArray[16384];

void TestFunction(unsigned char *ArrayPointer)
{
   const int ArrayLength = sizeof(ArrayPointer); // * or & in front doesn't work
   const int ObviousRoute = sizeof(RandomArray); // this works as expected
}

I need the top case to work, not the bottom case. The function processes multiple arrays and I need to make sure I don't exceed the array's limit. Whether I put * or & in the front, I don't get the 16,384 I'm expecting, just 1 in the case of the * and 4 in the case of the & (the latter I'd expect since it's getting the address instead, a 32-bit value).

You could use a structure, rather than passing the array itself to functions:

Code:

#define spanof( data )\
( sizeof( data ) / sizeof( data[ 0 ] ) )

#define define_array( type )\
struct array_##type  {\
type* data;\
size_t size;\
};\

// Example

#include "stdio.h"

define_array( int );

void display( array_int values ) {
    for( size_t idx = 0; idx < values.size; ++idx ) {
        printf( "%d ", values.data[ idx ] );
    }
}

int main( ) {
    int loc[ 7 ] = { 8, 6, 7, 5, 3, 0, 9 };
    array_int vls = { loc, spanof( loc ) };    
    display( vls );
}