指向const数组的指针

Question

I've created a menu structure in an embedded system as follows:

struct DisplayMenu
{
    short int MenuTitle;                                        // Menu title text offset - the title used in the calling menu
    char NoOfSubmenuItems;                                      // number of submenuItems in this menu ? static!
    char PositionInLastMenu;                                    // position in previous menu - helps restore previous menu screen
    struct DisplayMenu *PreviousMenu;                           // pointer to previous menu - need to be const?
    struct DisplayMenu *NextMenu[MAX_NO_OF_SUBMENU_ITEMS];      // array of pointers to submenus structs - Null if no submenu...function call instead
    void (*MenuFunctionsArray[MAX_NO_OF_SUBMENU_ITEMS])(void);  // array of function pointers - called if no submenu. Check if null
    const short int *SubMenuText;                               // pointer array of text offsets for submenu text - no set length variable length array allowed at end of struct in GCC at least
};

Which is instantiated thus for early prototyping:

const short int MainMenuTextStrings[]= { tSTATUS, tSETUP, tStartOfTextString, tANALYSER, tUNITS, tEndOfTextString, tUNITS, tALARM, tSCREEN, tREPORTS, tSERVICE, tStartOfTextString, tMANUAL, tAIR, tZERO, tEndOfTextString, tStartOfTextString, tMANUAL, tEndOfTextString, NULL };     // needs a new text entry

// main menu struct
struct DisplayMenu MainMenu =
{
    tMENU,                                                                                              // Menu title
    0,                                              // number of submenus
    0,                                                                                                  // no previous menu 
    NULL,                                                                                               // no previous menu to point to
    { NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL },                                     // to be set up later
    { NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL },                                     // function pointers
    MainMenuTextStrings
};

with a view to initialising function/menu pointers and the like later

It's compiling ok with GCC but when the code is running (there is a non-dynamic menu system up and running already) the IDE (MPLABX) is struggling to view the contents MainMenu and when I try and run the function DisplayMenuItems (as below) I can't step through it

Can anyone spot anything wrong with the way I've defined the struct and instantiated it?

I plan to use the following function to display items using the struct:

void DisplayMenuItems(struct DisplayMenu* Menu)
{

    while(*(Menu->SubMenuText) != NULL)
    {
        if(*(Menu->SubMenuText) == tStartOfTextString)       //if a few text strings need to be concatenated on one line
        {
            while(*(Menu->SubMenuText++) != tEndOfTextString)  //keep printing until you find the EndofTextString ident
                WriteStringToDisplay((char*) &pText[Config.ucLanguage][*(Menu->SubMenuText)], SmallFont);
        }
        else /*if(*(Menu->SubMenuText) != NULL) */
        {
            WriteStringToDisplay((char*) &pText[Config.ucLanguage][*(Menu->SubMenuText)++], SmallFont);
        }
        DisplayCarriageReturn(2,SmallFont);         // try and remove this eventually? Just increment Lcd.ucLine instead
    }
}

GCC might be letting me get away with something I'm not supposed to here so any tips appreciated!

Many thanks

Answer 1

The problem is probably that you are modifying the pointer in the loop. That means that if the DisplayMenuItems function is called a second time, the pointer will no longer point to the MainMenuTextStrings array, but beyond it and accessing that data leads to undefined behavior .

Instead of modifying the pointer, why not simply use array indexing syntax?

Like

for (unsigned int i = 0; Menu->SubMenuText[i] != NULL; ++i)
{
    ...
}