string question

[electroRF]

Hi,
I have a question please.

When one performs

char *s1 = "abcd";

or

char *s2;

s2 = "123456";

a const (i.e. read-only) memory is created in the data segment - (i.e. not the heap/stack? is it created already in compilation?) - 5 bytes for "abcd" and 7 bytes for "123456" ?

so one cannot write new characters into s1 or s2, right? as they point on "read-only" memory?


what about:

strcpy(s3, "hello");

also here a 'read-only' memory is created in the data segment?


Thank you very much.

 

[Ian Rogers]

Are you still in C.... The initialization in the first two examples is not allowed in C...

The third is correct but you will have a string "Hello" in code space ( ROM ) and a copy in data space RAM.. The string S3 can be modified..

To create a const rom string you need to do this

char string[] = {"Hello!"}

 

[electroRF]

Hi Ian,
Thank you very much.

yes, i'm writing in C.

Following your comment, perhaps my compiler approved it as its a c++ compiler, i'm installing now Visual Studio Express to use C compiler.

The third is correct but you will have a string "Hello" in code space ( ROM ) and a copy in data space RAM.. The string S3 can be modified..

So I could actually have s3 pointing on the address in the Code Space in order to save space in the data memory?

Would it be legal?

 

[Ian Rogers]

Yes!

In C18 or XC8 on the pic... you just create a pointer to point at the relevant text..

const char text[] = {"Hello world!};
const char* romptr = &text[0];

// as opposed to

char text[] = {"Hello world!};
char* ramptr = &text[0];

Some compilers create the second array as a constant as the text is already a constant... So you would declare it volatile so it will create it in RAM..

 

[electroRF]

Thank you Ian!

If I got it correctly, then the following code would have s pointing on an address in the code space, where the compiler created "help"?

char* mystrcpy(char* dest, char* source)
{
    dest = source;
    return dest;
}


void main(void)
{
    char *s = NULL;

    s = mystrcpy(s, "help");

}

 

[Ian Rogers]

Yes precisely... Now you only have one copy of "Help"...