My 64-bit porting experiences - II

2. Assumptions on size of predefined datatypes

2.1 Explicit assumption

Ideally, any implementation should not place an assumption on the size of pre-defined datatypes, because these sizes are not standard, and are usually at the discretion of OS implementation provided some boundary conditions are satisfied (e.g., an int shall at least be 16 bits). Most of the code that I was porting, was based on the premise that int, long and pointer are all of the same size, and long long was twice this size. Assertions were placed in the code to ensure this:

assert( sizeof(long) == sizeof(int) );

assert( 2 * sizeof(char *) == sizeof(unsigned long long));

On 64-bit, these assertions were violated. In fact, I had to rearchitecture a significant part of the code.

2.2 Implicit assumption

Implicit assumption often takes the form of casting between different datatypes.

For a hash-table that hashes pointers, the key computation uses the pointer address:

unsigned int key = (unsigned)ptr >> 2;

This specific usage (casting pointer to integer) was treated in a different manner on different platforms. On AIX, there was no warning message; on Linux, the compiler issued a warning; on Solaris, the C compiler gave no warning, while C++ compiler issued an error.