Pointer vs Array, or char * vs char[]

I ran into a strange thing when compiling code for my DSP recently.  I 
would like to know what is going on, if this is something I don't 
understand in a difference of C vs C++, or something really strange in 
the Code Composer Studio Compiler / Linker.

My understanding was that a a char array was simply dealt with as a 
pointer to the chunk of memory that was declared.  The following code 
snippet that I was using, I would have expected to print the same line 
twice. It did not.  The second line was some other location from ram.  I 
have never run into a problem with something like this using MS Visual 
C, so I'm wondering if I'm doing something that is really wrong, or if

this is something that is strange happening in the linker, and if 
there's some way to tell the linker to fix what it is doing.  I'm not 
getting any linker errors, it is just not producing the proper output, 
and I'm wondering if this may be related to some other problems I have 
been having.

File1:

char ReallyBigString[] = "really big string\r\n";

main()
{
    printf(ReallyBigString);
    File2Subroutine();
}

File2:

extern char * ReallyBigString;
File2Subroutine()
{
    printf(ReallyBigString);
}
	

William,

if you declare the string this way:

    File1: 
    
    char str[] = "yeah\r\n"

    File2:

    extern char str[];

//or in this way:

    File1: 
    
    char *str = "yeah\r\n"

    File2:

    extern char *str;
	both printf() gives the correct result. It seems to be that both declarations,
local and extern must be identical so that the compiler understands it well.

Charles
	

I think I was able to fix the problem by building my example without 
sizing the string, but declaring it as an array in both places.  But I'm 
wondering if there is something fundamental that I'm not understanding.  
I would not be concerned if the problem generated an error while 
linking.  I'm concerned because it appeared to link properly, but 
accessed the wrong memory.  Since I've not got any memory protection in 
the DSP, tracing down a related problem is very hard. 

My concern is also there because I'm dealing with large amounts of code 
that are not my own, and The way I learned C first and C++ Second is 
fairly common among the people I'm working with.
	This example works as expected:
File1:

char ReallyBigString[] = "really big string\r\n";

main()
{
    printf(ReallyBigString);
    File2Subroutine();
}

File2:

extern char ReallyBigString[];
File2Subroutine()
{
    printf(ReallyBigString);
}

I would expect to be able to change where a pointer was pointing, but 
expect an array name to not want to be changed.

char BigString[] = "foo";        // Normal Array Declaration
char * ptrBigString = BigString; // Normal Pointer Declaration
char ArrBigString[] = BigString; // I would expect this to fail in the 
same file
	Andrew Oyaide wrote:
> I have had similar problem with this in the past. 
It looks like 
> it's a bug with the compiler/linker.  I got around this by declaring 
> the variable with the size e.g.
>  
> File1:
>
> char ReallyBigString[30] = "really big string\r\n";
>
> main()
> {
>     printf(ReallyBigString);
>     File2Subroutine();
> }
>
> File2:
>
> extern char ReallyBigString[30];
> File2Subroutine()
> {
>     printf(ReallyBigString);
> }
>
>
> >>> William C Bonner <wbonner@wbon...> 20/03/2006 19:58:16
>>>
> I ran into a strange thing when compiling code for my DSP recently.  I
> would like to know what is going on, if this is something I don't
> understand in a difference of C vs C++, or something really strange in
> the Code Composer Studio Compiler / Linker.
>
> My understanding was that a a char array was simply dealt with as a
> pointer to the chunk of memory that was declared.  The following code
> snippet that I was using, I would have expected to print the same line
> twice. It did not.  The second line was some other location from ram.  I
> have never run into a problem with something like this using MS Visual
> C, so I'm wondering if I'm doing something that is really wrong,
or if
> this is something that is strange happening in the linker, and if
> there's some way to tell the linker to fix what it is doing.  I'm
not
> getting any linker errors, it is just not producing the proper output,
> and I'm wondering if this may be related to some other problems I have
> been having.
>
> File1:
>
> char ReallyBigString[] = "really big string\r\n";
>
> main()
> {
>     printf(ReallyBigString);
>     File2Subroutine();
> }
>
> File2:
>
> extern char * ReallyBigString;
> File2Subroutine()
> {
>     printf(ReallyBigString);
> }
	

The original code definitely does not work. What I've changed was
the printf operators, where a format was added, and the most
important change was in the extern declaration in the File2.

A good excersise in handling strings :)

Regards,

Andrew

File1:

#include <stdio.h>

char ReallyBigString[] = "really big string\r\n";

void File2Subroutine (void);

int main (void)
{
    printf ("%s", ReallyBigString);
    File2Subroutine ();
}

File2:

#include <stdio.h>

extern char ReallyBigString[];

void File2Subroutine (void)
{
    printf ("%s", ReallyBigString);
}
	Regards,

Andrew
	> Date: Mon, 20 Mar 2006 11:58:16 -0800
> From: William C Bonner <wbonner@wbon...>
> Subject: Pointer vs Array, or char * vs char[]
>
> I ran into a strange thing when compiling code for my DSP recently.  I
> would like to know what is going on, if this is something I don't
> understand in a difference of C vs C++, or something really strange in
> the Code Composer Studio Compiler / Linker.
>
> My understanding was that a a char array was simply dealt with as a
> pointer to the chunk of memory that was declared.  The following code
> snippet that I was using, I would have expected to print the same line
> twice. It did not.  The second line was some other location from ram.  I
> have never run into a problem with something like this using MS Visual
> C, so I'm wondering if I'm doing something that is really wrong,
or if
> this is something that is strange happening in the linker, and if
> there's some way to tell the linker to fix what it is doing.  I'm
not
> getting any linker errors, it is just not producing the proper output,
> and I'm wondering if this may be related to some other problems I have
> been having.
>
> File1:
>
> char ReallyBigString[] = "really big string\r\n";
>
> main()
> {
>    printf(ReallyBigString);
>    File2Subroutine();
> }
>
> File2:
>
> extern char * ReallyBigString;
> File2Subroutine()
> {
>    printf(ReallyBigString);
> }
>