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Rusty programmer needs reality check...

 
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neil



Joined: 08 Sep 2003
Posts: 128

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Rusty programmer needs reality check...
PostPosted: Mon Aug 07, 2006 9:51 am     Reply with quote
Hi, I haven't done any programming for about a year and have just had to get back into the swing of it. It has taken 3 weeks just to start feeling like I know my way around again!

I now want to write a menu for a 40x2 LCD, which I have acheived before with some ugly code (state-machine based). I started looking for more elegant ways of doing it and came across Mark's example of using function pointers.
I couldn't understand what was being done with the pointers, so I got out K&R's book and looked up "pointers to functions" (5.11) but just can't get my head around the concept at all! I know this is the part of C that most people find difficult, so I have gone back to basics and started at the beginning of the 'pointers' section!

As a practise, I have re-written one of my serial RX buffers to use pointer arithmetic, instead of array subscripting. Can someone please check the following code snips for me? I believe the two are identical, at least according to K&R.

Regards,
Neil.

Code:
// Version using array subscripts
#byte RX_reg = 0x01A
byte RX_Buff[8];  // Using a binary number for efficiency
int Buff_index=0;

#int_RDA
Data_RX_ISR(){
     RX_Buff[Buff_index++] = RX_reg;
     Buff_index &=7;  // Roll around 0-7...0 (8 Bytes long)
     }

Code:
// Version using pointer arithmetic
#byte RX_reg = 0x01A
byte RX_Buff[8];  // Using a binary number for efficiency
int *Buff_ptr;  // Create a pointer to int (or should it be byte?)
Buff_ptr = RX_Buff;  // Left out the '&' because RX_Buff is an array! Buff_ptr points to the 1st element of RX_Buff.

#int_RDA
Data_RX_ISR(){
     *Buff_ptr = RX_reg;  // RX_reg is stuffed into location the pointer currently points to.
     (Buff_ptr++) &=7;  // Roll around 0-7...0 (8 Bytes long)
     }


Am I 'overloading' on that last line?
Ttelmah
Guest
PostPosted: Mon Aug 07, 2006 10:13 am     Reply with quote
You cannot have the line:

Buff_ptr = RX_Buff; // Left out the '&' because RX_Buff is an array! Buff_ptr points to the 1st element of RX_Buff.

Outside of the program flow.

Do it as an initialisation, with:

byte RX_Buff[8]; // Using a binary number for efficiency
int *Buff_ptr=RxBuff; // Create a pointer to int (or should it be byte?)

Int is fine. Bytes, and integers are identical size in CCS C.

Best Wishes
neil



Joined: 08 Sep 2003
Posts: 128

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PostPosted: Tue Aug 08, 2006 2:32 am     Reply with quote
Thanks, for the help.

Where you said the initialisation can't be done outside program flow, does this differ from standard C? K&R's book seems to allow the declaration of a pointer, then the assignment to a variable.
All on one line does look neater though Smile

Cheers,
Neil.
Ttelmah
Guest
PostPosted: Tue Aug 08, 2006 4:51 am     Reply with quote
It's not allowed in K&R either.
It is fine to do this:
Code:

void main(void) {
    char buff[8];
    char * buff_addr;
    buff_addr=buff;
.......


}

Since here you are declaring a _local_ variable inside a function, and the instruction is inside the program, and will get executed.
The problem is that the line:
Buff_ptr = RX_Buff; // Left out the '&' because RX_Buff is an array!

Is actually a line of _program_ code, and if put outside the program itself, will never actually be executed.

Best Wishes
neil



Joined: 08 Sep 2003
Posts: 128

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PostPosted: Thu Aug 10, 2006 5:19 am     Reply with quote
Thanks Ttelmah,
I see that now!

I have just written another test prog, before I think about putting it into my real code. I am using Typemod to create some easily accessible variables in the internal data EEprom. The typemod is per the CCS example and works, but I am using a 2D array to store some related variables. This also works, until I come to printf it to a terminal....

Here is my complete code....
Code:
#include <16F877.h>
#device *=16
#fuses NOWDT,HS, PUT, NOPROTECT, BROWNOUT, NOLVP, NOCPD, NOWRT, NODEBUG
#use delay(clock=8000000)
#use rs232(baud=9600,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8)

#define AMPS 0
#define VOLTS 1
#define WATTS 2
#define EMISS 0
#define OPER 1
#define TRIP 2

void dataEE_read(int32 addr,int8 * ram,int bytes); // Prototypes
void dataEE_write(int32 addr,int8 * ram,int bytes);

typemod <dataEE_read> EEPROM;

int loop1,loop2;
long test_2[3][3],temp;

long EEPROM op_vars[3][3], test[3]={1024,1000,500};

void main() {
   setup_adc_ports(NO_ANALOGS);
   setup_adc(ADC_OFF);
   setup_psp(PSP_DISABLED);
   setup_spi(FALSE);
   setup_timer_0(RTCC_INTERNAL|RTCC_DIV_1);
   setup_timer_1(T1_DISABLED);
   setup_timer_2(T2_DISABLED,0,1);

   op_vars[AMPS][EMISS] = 1400;  // fill up EEPROM locations in array with
   op_vars[AMPS][OPER] = 1650;   // some meaningful test values.
   op_vars[AMPS][TRIP] = 1800;
   op_vars[VOLTS][EMISS] = 300;
   op_vars[VOLTS][OPER] = 380;
   op_vars[VOLTS][TRIP] = 400;
   op_vars[WATTS][EMISS] = 4000;
   op_vars[WATTS][OPER] = 6000;
   op_vars[WATTS][TRIP] = 7000;
   
   for(loop1=0;loop1<3;loop1++){    // Dump EEPROM values back to terminal
      for(loop2=0;loop2<3;loop2++){
         temp=op_vars[loop1][loop2];
         printf("%lu\t",temp);    // This works
      }
   }
   while(TRUE);
}



void dataEE_read(int32 addr,int8 * ram,int bytes){
   int i;
   for(i=0;i<=bytes;i++,ram++,addr++) *ram = read_eeprom(addr);
}

void dataEE_write(int32 addr,int8 * ram,int bytes){
   int i;
   for(i=0;i<=bytes;i++,ram++,addr++) write_eeprom(addr, *ram);
}

I have found that prinf won't accept a 2D array from the typemod. It will accept a 2D array in normal RAM, or it will accept a 1D array from the typemod. Is this something I am doing wrong, or is it a limitation of the typemod?
Code:
   for(loop1=0;loop1<3;loop1++){    // Dump EEPROM values back to terminal
      for(loop2=0;loop2<3;loop2++){
         printf("%lu\t",op_vars[loop1][loop2]); // This tells me the printf format is invalid and will not compile
      }
   }


Code:
   for(loop1=0;loop1<3;loop1++){    // Dump EEPROM values back to terminal
      for(loop2=0;loop2<3;loop2++){
         printf("%lu\t",test_2[loop1][loop2]); // This works (test_2 is in RAM)
      }
   }

Code:
   for(loop1=0;loop1<3;loop1++){    // Dump EEPROM values back to terminal
      for(loop2=0;loop2<3;loop2++){
         printf("%lu\t",test[loop2]); // this 1D array from EE works!
      }
   }



I can't see why myself... can you explain?
Even stranger, when I 'preview this post, the typemod line in code comes out different from what I typed in! it should read "typemod <dataEE_read> EEPROM;" Is it being seen as HTML?
... nope, still won't read correctly, the write function and address parameters are still being missed out. I really did type them in!

Thanks,
Neil.[/code]
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