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Standard Struct?

Craig Rohe

Having some problems working with a struct typedef. In RTX51, one of the system functions takes as an argument a pointer to a struct:

//function prototype
signed char os_check_mailboxes (t_rtx_allmbxtab xdata *table);
...
/* Type definition for system call os_check_mailboxes */
typedef struct {
            unsigned char message_cnt;
            unsigned char read_task_cnt;
            unsigned char write_task_cnt;
        } t_rtx_allmbxtab[8];

I've never seen an array-of-structs declared within the typedef before, and couldn't find any literature about it. It's declared like a single struct, and used just like an array of structs:

t_rtx_allmbxtab xdata mytable;

if(mytable[x].message.cnt < 2)...

however, I get a warning "C182: pointer to different objects" when trying to use it with the actual system function (exactly as shown in the RTX51 user manual):

os_check_mailboxes(&mytable); //<-- generates warning!

I'm not sure exactly how the array as part of the data type affects this, so i'm stumped. However, that didn't stop me from trying random things to see what DID work. Here's what I came up with; maybe somebody can explain to me why this works:

//declare as an array, effectively making a 2D array
//with one dimension being size 1 only
t_rtx_allmbxtab xdata mytable[1];

//use as a normal 2D array
if(mytable[0][x].message.cnt < 2)...

//system function now works fine. no warnings.
os_check_mailboxes(&mytable);

//using (&mytable[0]) or (&mytable[0][0]) instead will generate 'pointer to different objects' warnings.

Parents
  • 0 in reply to Dan Henry

    mytable is an array, the name of which produces its (implicit) address.

    Not in the programming language C it doesn't. The name of an array, just like with any other variable, specifies its value, i.e. a combination of type and address. It'll implicitly convert into into a pointer to the first element in most circumstances. Taking the address of the address is not one of those circumstances, so this:

    Specifying the address of an (implicit) address could reasonably generate a warning.

    doesn't even apply. &mytable is not taking the address of an address, it's taking the address of a variable. The type of the resulting term is "pointer to array[8] of struct {...}", and it's a perfectly allowed, correct operation. There's absolutely nothing to warn about.

Reply
  • 0 in reply to Dan Henry

    mytable is an array, the name of which produces its (implicit) address.

    Not in the programming language C it doesn't. The name of an array, just like with any other variable, specifies its value, i.e. a combination of type and address. It'll implicitly convert into into a pointer to the first element in most circumstances. Taking the address of the address is not one of those circumstances, so this:

    Specifying the address of an (implicit) address could reasonably generate a warning.

    doesn't even apply. &mytable is not taking the address of an address, it's taking the address of a variable. The type of the resulting term is "pointer to array[8] of struct {...}", and it's a perfectly allowed, correct operation. There's absolutely nothing to warn about.

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