Error:attempt to create a pointer to a constant

[gokul.ftpl]

This code generates an error but that works fine in an ansi c complaint compiler

Code:

void Test(char *str){     char Next_Line='\n';     char Prev_Line='\p';     char Tab_Space='\t';     unsigned int i;     while(*str!='\0'){             if(*str == Next_Line){            str++;            printf("\n Found Next_line");         }         else if (*str == Prev_Line){            printf("\n Found prev_Line");            str++;         }         else if (*str == Tab_Space){              printf("\n Found Tab Space");            str++;         }                 printf("%c",*str);         str++;     }     return; } int main() {        Test("Hello \n \p \t");    return 0; }

but this one works fine

Code:

int main() {    char str[] ="Hello \n \p \t";    Test(str);    return 0; }

can any one tell me why this happens
[/quote]

[Ttelmah]

Worth reading the manual...
A search under 'constant', or 'pointer to constant', will find a description of this.

The key is that the PIC, has a Hoffman memory architecture, with separate memory 'spaces' for the code memory, and the variable memory. Now on the older PICs, the code memory is not even accessible for data storage by the program, so if you define a 'constant' string, it is implemented as a program to return the data, rather than as addressable storage locations, and cannot be directly accessed. CCS, elected on this basis, to have no provision to access this memory, and 'pointers' only access the variable memory. Latter chips do allow the memory to be accessed, but as yet the old restriction remains in place for CCS code.
When you declare:
"This is a test"

This is constant data. The declaration:
char fred[]="This is a test";

Actually creates program to return the data string, and the code to call this, and copy it into the RAM storage addressed by 'fred'. Once the data is in this RAM, it can then be accessed by a pointer.

You can also use the 'strcpy' function to access such constants, so once one RAM string area is defined, you can use:
strcpy(fred,"Another");

To put a different message into the same area, and save RAM.

As an 'aside', CCS, have an extra 'shortcut', that can be used to avoid the need to copy the data into RAM for such accesses. If you have an output function like 'putc', which accepts single characters, you can use:

putc("This is a test");

and the compiler will automatically call the program to retrieve the bytes, and then send these sequentially to the output program.

The decision not to allow pointer access to the ROM, reduces code size, and pointer sizes. Allowing this is a feature that is due to be 'added' to the latest compilers (with an option to turn it on/off), but does not yet work properly.

Best Wishes

[gokul.ftpl]

Thank you for your reply i tried with the alternative shortcut which you have specified and it was working fine

[jimbo]

I like your tip on turning a char routine into a string.
I used it as follows:

serPutByte("Hello World");

Q1: But my problem is: The NULL comes out. Is there a way to avoid that?


Q2: Is strcpy() written in assembly? what I need is a strcat() that works like strcpy(). Eg takes a constant...


strcpy(buf,"Hello ");
strcat(buf,"World"); //Attempt to make a pointer to a constant


Any ideas? I tried the manual (ccsc.exe/? If that is what you mean?).

Rgds,
Jim Pruett

[PCM programmer]

[carterson2]

Thanks for that. I still think an extra char comes out, but your snippet says no. hmmm.

Anywho, what I learned from this thread is:

printf() has a very very small footprint. I just replaced all strcat's with it!!!!
I love it!

Historically I never even considered using stdio.h (until now!) Its always a big clunky libc.a or such. NOW I WILL! Thanks CCS!

(I approve this message ;-)...