using pointers in functions

[bhaltulpule]

Hi !
I am new to C. I am developing C for an Assembly and C combined code for a project. I need to display simple messages to the Hyper terminal using C.

To be specific, (See attached snippet of code which I am trying to run on Keil toolset) I am trying to declare a global array of characters and process it in one or more functions. Since the Char S[] statement is outside of Main () so it is global. However if I declare it as extern char s[] in the cDispMsg routine, I get an error --> " 's' : invalid class error" . On the other hand, if I comment out the extern char s[] declaration, I get an error -->"subscript on non-array or too many dimensions"
Given all this how can I access the elements of s[] as in the statements below (see attached file). send them to display or do some processing with them ?
for (i=0; s !='\0';i++)
cSend_Byte (s);
[By the way ignore the code for SBUF etc., it is for an 8051 embedded controller UART.]
So the bottom line is, can anyone tell me what I am doing wrong and how to fix it ?
if there is some standard way to declare a string or structure and have it accessible from a function ? Does this change if the function is in a different file ?

I tried the examples from K&R's classic book on C but they dont' do much with UARTs, just Printf so many examples are not useful.

[Narue]

First you need to decide if you want to use parameters to move s around, or access it globally. Right now you're mixing the two together and as a result you have way too many variables named s. This uses parameters rather than the global variable:

#include <stdio.h>

void cSend_Byte ( unsigned char );
void cGet_Byte ( void );
void cDispMsg ( const char * );

char s[] = "It is working";

/*
  JSW (2008-4-1): Added this for debugging without an emulator
*/
static unsigned char SBUF = 0;

void main ( void )
{
  unsigned char mybyte;

  do 
  {
    cGet_Byte();
    mybyte = SBUF;
    cSend_Byte ( mybyte );
    cSend_Byte ( mybyte + 1 );
    cSend_Byte ( 'Y' );
    mybyte = 'V';
    cSend_Byte ( mybyte );
    cDispMsg ( s );
  } while ( 1 );
}

void cDispMsg ( const char *s )
{
  int i;

  for ( i = 0; s[i] !='\0'; i++ )
    cSend_Byte ( s[i] );
  cSend_Byte ( '\n' );

  return;
}

void cSend_Byte ( unsigned char x )
{
  putchar ( x );
}

void cGet_Byte ( void )
{
  SBUF = (unsigned char)getchar();
}

And this uses the global directly:

#include <stdio.h>

void cSend_Byte ( unsigned char );
void cGet_Byte ( void );
void cDispMsg ( void );

char s[] = "It is working";

/*
  JSW (2008-4-1): Added this for debugging without an emulator
*/
static unsigned char SBUF = 0;

void main ( void )
{
  unsigned char mybyte;

  do 
  {
    cGet_Byte();
    mybyte = SBUF;
    cSend_Byte ( mybyte );
    cSend_Byte ( mybyte + 1 );
    cSend_Byte ( 'Y' );
    mybyte = 'V';
    cSend_Byte ( mybyte );
    cDispMsg ( s );
  } while ( 1 );
}

void cDispMsg ( void )
{
  int i;

  // Assuming cDispMsg is in a different
  // file from the definition of s
  extern char s[];

  for ( i = 0; s[i] !='\0'; i++ )
    cSend_Byte ( s[i] );
  cSend_Byte ( '\n' );

  return;
}

void cSend_Byte ( unsigned char x )
{
  putchar ( x );
}

void cGet_Byte ( void )
{
  SBUF = (unsigned char)getchar();
}

Notice that my examples use the "not useful" standard I/O functions. :icon_rolleyes:

[bhaltulpule]

Thanks for your help.
I tested solution 1 (passing paramenter s) and it worked !. As for 2nd suggestion, it kind of did not work at first because all routines were in the same file. However when I splt cDispMsg into a seperate file, it did not work as shown in your reply using globals directly. I kept getting error for the statement extern char s[]; in the cfunction. Is there a reason for that ? Anyway, when i tried it with passing paramenters technique (const char *) and it worked. (See attached files). Now I need to understand what all this means and how to use it next time..
So I thank you for your help.
One final question: C seems to be too much sensitive and dependent on how you declare variables and functions. Since I like to keep functions in one place and Main () clean, is there a standard cooky cutter approach that works (most) every time ?
Thanks.
Bhal

[Narue]

>C seems to be too much sensitive and dependent
>on how you declare variables and functions.
Yep, it's a very finicky language.

>is there a standard cooky cutter approach that works (most) every time ?
Ideally you would put your declarations in a separate header file and definitions in a separate implementation file that corresponds to the header file.