Working with void pointers

**john7** · 05-12-2019

I have a struct

Code:

typedef struct 
{  
    void *l_var;      
    void *r_var;    
}EXPR;

EXPR expr;

I initialize it

Code:

expr.l_var = &motor_rt_params[0].position;
expr.r_var = &motor_rt_params[1].position;

Now I want to operate with the values

Code:

result = *(expr.l_var) – *( expr.r_var);

I get an exception
Error[Pe041]: expression must have arithmetic or pointer type

I have to cast it to a known type

Code:

result = *((int32_t *) expr.l_var) - *((int32_t *)expr.r_var);

But not all variables is 32 bit? there is 16, 8 bit.
How can cast it to an appropriate type in run time?

**laserlight** · 05-12-2019

Originally Posted by john7

But not all variables is 32 bit? there is 16, 8 bit.
How can cast it to an appropriate type in run time?

You would cast to a pointer to whatever is the underlying type, in this case it would be the type of &motor_rt_params[0].position. It is also always safe to cast to a pointer to a character type to compute the difference in bytes, but is that what you're trying to do here?

By the way, I don't recommend EXPR as a struct alias. It is common convention in C for fully uppercase names to be used for macros or constants. A capitalised name like Expr might be a suitable alternative.

**GReaper** · 05-12-2019

You cast it to whatever pointer type it should be. I don't understand your problem, did you think you could only cast it to a single type?

**john7** · 05-12-2019

Originally Posted by laserlight

You would cast to a pointer to whatever is the underlying type, in this case it would be the type of &motor_rt_params[0].position. It is also always safe to cast to a pointer to a character type to compute the difference in bytes, but is that what you're trying to do here?

By the way, I don't recommend EXPR as a struct alias. It is common convention in C for fully uppercase names to be used for macros or constants. A capitalised name like Expr might be a suitable alternative.

But how I decide what type to cast? It is generic function

Code:

void SCRIPT_Process(void *l_var, void *r_var, uint32_t oper)
{
    int32_t res;
    
    switch (oper)
    {
        case OP_PLUS: 
            res = *((??? *) l_var) + *((??? *)r_var);
        break;
        case OP_MINUS: 
            res = *((??? *) l_var) - *((??? *)r_var);
        break; 
    }
}

How can I know what type the argument points and what type to cast?

Code:

SCRIPT_Process(expr.l_var , expr.r_var , OP_PLUS);

**laserlight** · 05-12-2019

One approach is to do something similiar to the standard qsort function: have a function pointer parameter, and hence offload the casting and relevant operation to a function specified by the caller. Another approach is to introduce a macro such that the type for the cast can be a macro argument, hence when the macro is used, the appropriate type can be provided.

**john7** · 05-12-2019

Originally Posted by laserlight

One approach is to do something similiar to the standard qsort function: have a function pointer parameter, and hence offload the casting and relevant operation to a function specified by the caller. Another approach is to introduce a macro such that the type for the cast can be a macro argument, hence when the macro is used, the appropriate type can be provided.

sorry. I missed to understand. how can I do it practically? the only way I see - to add some variable
type_size = sizeof(my_current_var);
and according to a type_size do some cast in SCRIPT_Process.

**flp1969** · 05-12-2019

Originally Posted by GReaper

You cast it to whatever pointer type it should be. I don't understand your problem, did you think you could only cast it to a single type?

If I undestand correctly your question, it is why you have to cast a void pointer to a specific type pointer, isn't it?

If you think about the pointer variable as a container of a memory address, when you use the indirection operator * you are reading/writing a bunch of bytes in/from memory - void pointers are generic pointers (they can point to any type), when using * to read/write data pointed, the compiler doesn't know how many bytes it should deal with:

Code:

 int x;
void *p = &x;

// How many bytes to write to the address
// given by p? 1, 2, 4, 8?
*p = 0;

*(int *)p = 0; // ahh... write 4 bytes (sizeof(int) bytes)!

**john7** · 05-12-2019

Originally Posted by flp1969

If I undestand correctly your question, it is why you have to cast a void pointer to a specific type pointer, isn't it?

No! Let me simplify for better understanding.

Code:

switch (type)
 {
        case TYPE_u8:
            *(uint8_t*)res = *(uint8_t*)l + *(uint8_t*)r;
            break;
        case TYPE_u16:
            *(uint16_t*)res = *(uint16_t*)l + *(uint16_t*)r;
            break;
        case TYPE_u32:
            *(uint32_t*)res = *(uint32_t*)l + *(uint32_t*)r;
            break;
        default:
            assert(0);
 }

but I pass a void* (generic) argument. How can I know it's type (TYPE_u8, TYPE_u16, TYPE_u32) ?

**flp1969** · 05-12-2019

Originally Posted by john7

No! Let me simplify for better understanding.

Code:

switch (type)
 {
        case TYPE_u8:
            *(uint8_t*)res = *(uint8_t*)l + *(uint8_t*)r;
            break;
        case TYPE_u16:
            *(uint16_t*)res = *(uint16_t*)l + *(uint16_t*)r;
            break;
        case TYPE_u32:
            *(uint32_t*)res = *(uint32_t*)l + *(uint32_t*)r;
            break;
        default:
            assert(0);
 }

but I pass a void* (generic) argument. How can I know it's type (TYPE_u8, TYPE_u16, TYPE_u32) ?

If you have a void pointer, you can't... (void *) is a wildcard, but it doesn't carry any type information with it...

**christop** · 05-13-2019

You could add a "type" field to your "EXPR" structure and then pass that to the "SCRIPT_Process" function to determine the type.

**john7** · 05-13-2019

Originally Posted by christop

You could add a "type" field to your "EXPR" structure and then pass that to the "SCRIPT_Process" function to determine the type.

Yes. The only way. I was trying to avoid it cause it inflates my struct (it's actually an array of struct) and it consumes a lot of space, so I'm struggling for every byte.

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