 |
 |
| View previous topic :: View next topic |
| Author |
Message |
Jim Hearne
Joined: 22 Dec 2003
Posts: 109
Location: West Sussex, UK
|
| Void pointer to different variable types. |
 Posted: Fri Oct 20, 2006 10:00 am |
 |
|
Hi All,
I'm having trouble using a pointer to change different variable types, the code below (cut down from 30k's worth) shows the problem.
It works with 8 bit variables but not 16 (not shown) or 32 bit.
I'm probably doing something very silly, please put me out of my misery.
Thanks,
Jim Hearne.
| Code: |
#include <18f458.h>
#device adc=10 *=16
#device ICD=true
#include <STDLIB>
#use delay(clock=20000000,RESTART_WDT)
#fuses NOWDT, WDT128, HS, PROTECT, NOOSCSEN, BROWNOUT, BORV20, PUT, STVREN, NODEBUG
#fuses NOLVP, NOWRT, NOWRTD, NOWRTB, NOWRTC, NOCPD, NOCPB, NOEBTR, NOEBTRB
struct
{
int8 thingy1;
int32 thingy2;
} stuff;
typedef struct data_entry_struct
{
int8 message[16]; // Message on display
int8 entry_method; // data_entry method
int8 var_size; // var size in bytes(1=int8,2=16,4=32 etc)
void *var_ptr; // pointer to number location, cast to correct type later using var type
int8 dp; // position of DP on display
int8 units; // units to display after value, ms etc
int32 min; // minimum value
int32 max; // maximum value
};
const struct data_entry_struct const data_entry[2]=
{
// message entry method, var size var dp units min max
{"Enter thingy1", 1 , sizeof(stuff.thingy1), &stuff.thingy1, 2, 0, 0, 100},
{"Enter thingy2", 2 , sizeof(stuff.thingy2), &stuff.thingy2, 0, 0, 0, 10000},
};
void main()
{
int32 temp_data;
int8 data_index;
int8 data_byte_size;
stuff.thingy1=123;
stuff.thingy2=12345;
// loads of code here
data_index=1; // this is normally set in the code
temp_data=(int32)*data_entry[data_index].var_ptr; // get variable value, this works fine
// code to change temp_data here
data_byte_size=data_entry[data_index].var_size;
// now put value back in variable according to size
if(data_byte_size==1)
*(int8)data_entry[data_index].var_ptr=(int8)temp_data; // this works
else if(data_byte_size==4)
*(int32)data_entry[data_index].var_ptr=temp_data; // this doesn't work, only writes low byte
// more code here
while(1);
}
|
|
|
 |
PCM programmer
Joined: 06 Sep 2003
Posts: 21708
|
|
| Posted: Fri Oct 20, 2006 11:31 am |
|
|
You need to tell the compiler that 'var_ptr' is a pointer to a 32-bit value.
You have this:
| Code: |
*(int32)data_entry[data_index].var_ptr = temp_data; |
Change it to this:
| Code: |
*(int32 *)data_entry[data_index].var_ptr = temp_data; |
The line for the 8-bit one should also be changed. It should be:
| Code: | | *(int8 *)data_entry[data_index].var_ptr = (int8)temp_data; |
|
|
|
Jim Hearne
Joined: 22 Dec 2003
Posts: 109
Location: West Sussex, UK
|
|
| Posted: Fri Oct 20, 2006 1:31 pm |
|
|
Many thanks PCM for that quick fix, i knew it was something silly, i was close.
Thinking about it on the way home from work i think i need to cast the pointer for the variable read as well as it's probably reading 32 bits at the moment and only seems to be working because most of the variables are 0.
Despite programming with CCS for the last 4 years and Borland C for many years before that i still have trouble getting my head more complex pointer usage.
Has anybody got any pointers  to some good reading on the subject.
Most stuff now seems to be more related to c++ and windows programming and that confuses me even more.
Many thanks,
Jim Hearne |
|
|
PCM programmer
Joined: 06 Sep 2003
Posts: 21708
|
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
FAQ 
Search
Register
Profile 
Log in to check your private messages 
Log in
