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Wipster
Joined: 09 Oct 2008
Posts: 16
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| Datatypes |
 Posted: Fri Dec 05, 2008 5:39 am |
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Hi
I'm trying to make a data type ITEM which has a variable type, name and pointer to something else but my code errors with;
'Expression must evaluate to a constant' and 'Expecting a declaration' on the first line of initializing.
How can I get around this problem? I'm not experienced with making data types.
I am using PCH Compiler v4.080
| Code: |
typedef struct ITEM_STRUCT
{
int8 Type;
char *Text;
char *Pntr;
} ITEM;
ITEM Group[] = {
{0, "TEST", 0},
{3, 0, 0},
}; |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Dec 05, 2008 11:47 am |
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I changed the structure element to an array and it compiled.
| Code: | #include <18F452.H>
#fuses HS,NOWDT,NOPROTECT, PUT, BROWNOUT, NOLVP
#use delay(clock=20000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)
typedef struct ITEM_STRUCT
{
int8 Type;
char Text[5];
char *Pntr;
} ITEM;
ITEM Group[2] =
{
{0, "TEST", 0},
{3, "", 0}
};
//=========================
void main()
{
while(1);
} |
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andrewg
Joined: 17 Aug 2005
Posts: 316
Location: Perth, Western Australia
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| Posted: Sat Dec 06, 2008 12:13 am |
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I've done this sort of thing, but I'm pretty sure you need a "const": | Code: | const ITEM Group[] = {
//...
}; |
_________________
Andrew |
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Wipster
Joined: 09 Oct 2008
Posts: 16
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| Posted: Mon Dec 08, 2008 6:32 am |
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Well I did a bit of thinking and with the error 'Expression must evaluate to a constant' I decided to try:
| Code: | typedef struct ITEM_STRUCT
{
int8 Type;
const char *Text;
const char *Pntr;
} ITEM; |
But then it says 'Unknown type' on the first line with the const, could this be a compiler bug? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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Wipster
Joined: 09 Oct 2008
Posts: 16
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| Posted: Wed Dec 10, 2008 7:09 am |
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Thanks for linking those threads unfortunately I have already read them, it doesn't look like I can have a pointer to "TEST" because it says it needs to be a constant but you cant have a const pointer because it complains unknown type.
The reason I need pointers is that Pntr should point to another instance of ITEM.
Is there any way around it? |
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Wed Dec 10, 2008 9:31 am |
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I think, the confusion comes from the fact, that C generally allows to use a string constant in assignments in place of a char* pointer. But it's important to understand, that the compiler has to insert a pointer to an implicitely generated string constant.
With CPU's that have different pointer types for data and program memory, as a PIC, it cant' work without copying a rom string constant to data memory before.
The above defined struct ITEM can't hold the string itself, only a pointer, which must be a pointer to data memory (at least in default #device CONST = ROMI mode).
Other pointer options are mentioned in CCS C readme.txt, you can check if they are suitable for your application. A pointer to data memory can only be initialized with the address of a data memory object, or NULL. The below construct compiles without errors.
| Code: | char ctext1[] = "TEST";
ITEM Group[] = {
{0, ctext1, 0},
{3, 0, 0},
}; |
Last edited by FvM on Wed Dec 10, 2008 9:36 am; edited 1 time in total |
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Ttelmah
Guest
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| Posted: Wed Dec 10, 2008 9:33 am |
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A whole 'suite' of different solutions, depending on the likely sizes and numbers of the strings involved.
First, store the text message into RAM. Then pointers to these are fine.
Second, don't store a pointer, but store an array index into a larger 'message' array. If (for instance), you make a 20 line constant array, containing the messages, then all that needs to be stored in the structure, is the row numbr containing the required message for the particular item.
Third, if you have a late model compiler, you can have _ROM_ declarations (rather than 'CONST' declarations), that support pointers. You can switch the behaviour of the compiler so const uses this form, but these declarations take more space, and are slower than const declarations, so to my mind it is better to use the separate 'ROM' keyword, and avoid this where it is not needed.
Best Wishes |
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