C- Syntax to allocate Global variables to consecutive memory locations

[techie82]

Hello all

I'm a fresher to Microcontroller programming. I have defined certain global variables that I need to store at consecutive memory locations, say starting from 0x8000h . Will appreciate if anyone can give me the syntax / instructions to go about the same.

thanks

techie

[Infarction]

If you know where you want the data, just create a global variable pointing to it. This of course assumes that you point it to a valid address, and you'll have to make sure they line up (and don't overlap) yourself.

[techie82]

hi

can you please give me an example... assuming I have three interger gloabal variables a, b, c and an array test[10] of type double, how do i go about allocating memory for them from say, 0x8000h ??

thanks

techie

[Infarction]

This isn't tested, but it should give you the idea:

int* a = 0x8000;
int* b = 0x8000 + sizeof(int); // account for a
int* c = 0x8000 + 2*sizeof(int); // account for a and b
double* test = 0x8000 + 3*sizeof(int); // account for a,b,c
int* yetAnother = 0x8000 + 3*sizeof(int) + 10*sizeof(double); // etc...

Alternatively, you could hardcode direct addresses (0x8000, 0x8004, etc...), but you'd have to know the size of each type.

[Salem]

Placing things in memory is down to the linker, not the compiler.
Which tool chain (compiler, linker etc) are you using?

[thekashyap]

Seems like you're trying to do something wrong. :)
If you want contiguous chunk of memory you can just declare an array of appropriate size and it's guaranteed that it's continuous. But I suppose your problem is variables and not just one variable.
Well I donno whether you can do this or not, my guess is it's not possible to force allocation of variables on continous locations. But irrespective of that I somehow don't like the idea of writing some code that depends on how the variable are allocated in memory.

Now if you are hell bent on it and have a very good reason, you can misuse typecasting like this.

//say we want to have 3 global variables,
//one double, one char array[10] and one int.
//say on the machine where we run, sizeof() double is 8,
//char is 1 and int is 4.
//So total = 8 + (1*10 + 1) + 4 = 23 bytes

//declare/define a global char array of this size
char global_allocated_memory[23] ;

//declare 3 variables. (have to be pointers in C, in C++
//you have the option of using reference instead)
double* g_double ;
int* g_int ;
char* g_charArr;

int main()
{
    g_charArr = (char*) &(global_allocated_memory[0]) ;
    g_double = (double*) &(global_allocated_memory[11]) ;
    g_int = (int*) &(global_allocated_memory[19]) ;

    *g_int = 11 ;
    *g_double = 22 ;
    strcpy( g_charArr, "Char Arr" ) ;

    printf("%f, %d, \"%s\"\n", *g_double, *g_int, g_charArr ) ;

    return 0 ;
}

Remember you still have the problem that you are NOT controlling the allocation of 3 pointers ( g_double, g_int and g_charArr), they may not be continuous.

[Infarction]

Seems like you're trying to do something wrong. :)

No, I'm pretty sure his demands are legit. There are times in a low-level application where you need to specify a certain address (MMIO is an example of this, albeit a poor one). He'll be just fine if he sets the addresses as he needs, so long as he double checks his memoy layout.