While it used to be common practice to use integers and pointers interchangeably in C (although it was not considered good style), the C99 standard has mandated that pointer to integer and integer to pointer conversions are implementation defined. By definition of the standard, the only value which can be considered interchangeable between pointers and integers is the constant 0. This means that less the exception of 0, mixing integers and pointers may have undesired consequences depending on the implementation which is used.

Universal Integer/Pointer Storage

Non-Compliant Code

In this non-compliant code, the pointer ptr is used in an arithmetic operation that is eventually casted as an integer, as stated above, the actual result for both this assignment and following assignment to ptr2 are implementation defined.

unsigned int *ptr = 0xcfcfcfcf;
...
unsigned int number = ptr + 1;
unsigned int *ptr2 = ptr;

Compliant Solution

A union can be used to give raw memory access to both an integer and a pointer and will take up as much size as its largest element which will mitigate a loss of data.

union intpoint {
	unsigned int *pointer;
	unsigned int number;
} intpoint;
...
intpoint mydata = 0xcfcfcfcf;
...
unsigned int num = mydata.number + 1;
unsigned int *ptr = mydata.pointer;

Accessing Memory-Mapped Addresses*

Note: This is a bad idea. The following is a description of how to properly execute a bad idea.

Oftentimes in low level kernel or graphics code you will need to access memory at a specific location. Thus, you are going to have to make a literal integer to pointer to conversion.

Non-Compliant Code Example

In this non-compliant code, a pointer is set directly to an integer constant, where it is unknown whether the result will be as intended.

unsigned int *ptr = 0xcfcfcfcf;

Because integers are no longer pointers this could have drastic consequences.

Compliant Solution

The trick here is you are telling the compiler you really do want to do this bad thing. If on your system pointers are typecast to something other than an unsigned int, then you want to use the type they are defined to be instead of unsigned int.

Adding in the explicit cast may help the compiler make a decision that

unsigned int * ptr = (unsigned int *) 0xcfcfcfcf;