Pointer Arithmetic for mmap

Question

recently I was attempting to adapt someone's mmap code and came across the following error. (I am actually a bit dubious about their code now as it looks like they are doing something unnecessary - attempting to use MAP_FIXED with their own page aligned memory. The manpage suggests calling mmap with NULL as the addr argument should do this on Linux. ) So I think I will at least test calling mmap with a NULL address. However I don't quite understand the error gcc is throwing up at me when I changed it. Their code works ok while I get lvalue required as left operand of assignment. Essentially they were doing the following: ``` uint8_t * ptr = (uint8_t *)mem; if ((uint32_t)ptr % PAGE_SIZE) { ptr += PAGE_SIZE - ((uint32_t)ptr % PAGE_SIZE); } ``` Where mem is a void * to some malloc'd memory. I am trying more or less the same with typecasts: ``` if ((uint32_t)mem % PAGE_SIZE) { (uint8_t *)mem += PAGE_SIZE - ((uint32_t)mem % PAGE_SIZE); /* ERROR */ } ``` So I thought I was being smart and r…

Accepted Answer

The mistake you're making is:

```
(uint8_t *)mem += /* Anything.  */
```

You simply cannot assign to the result of a cast. The result of a cast is not the same value as the original expression.

Think about how weird it would be to do:

```
(int) some_char_variable = 9999;
```

I had trouble for the same reason once.

Use a temp, and write back the result, OR, as R.. says in their comment:

```
mem = (void *) ( ( (uint8_t *) mem) + SOME_EXPRESSION );
```

Pointer Arithmetic for mmap

Solution

Problem