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slandry
Joined: 12 Mar 2007
Posts: 7
Location: New Hampshire
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| Passing a string to a function |
 Posted: Thu May 31, 2007 1:30 pm |
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Does anyone have a good way to pass a string to a function? In my application, there are many places where I want to display a message and then get a value from the operator. I would like to create a function that displays the text, inputs a response in ASCII and then returns the int. The call would look something like "i = GetInt("Enter Var 1");". I have no problem creating the function itself, but not being able to create a pointer to a constant (to pass the string) is making this difficult. There must be a clean way around it. BTW In section 7.4 of the PIC C intro book there is a perfect example of what I want to do, but it violates the pointer to constant limitation.
Thanks for any ideas! - Steve |
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kevcon
Joined: 21 Feb 2007
Posts: 142
Location: Michigan, USA
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| Posted: Thu May 31, 2007 1:59 pm |
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What version are you using?
4.035 and up allow pointers to constants. |
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slandry
Joined: 12 Mar 2007
Posts: 7
Location: New Hampshire
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| Posted: Thu May 31, 2007 2:25 pm |
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Thanks kevcon! you hit it on the head. I'm running 4.020 and I remember reading about the newer versions allowing pointers too. Guess I'm working too hard.
I'll give it a try and thanks again for the quick reply.
Best Regards,
- Steve |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu May 31, 2007 2:46 pm |
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| Quote: | | 4.035 and up allow pointers to constants. |
Can you post a demo program to show how this works ?
Can you make some modifications to the test program
that I posted in this thread:
http://www.ccsinfo.com/forum/viewtopic.php?t=27714&start=103
What do I have to do to make this test program work with
PCH vs. 4.039 ? |
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slandry
Joined: 12 Mar 2007
Posts: 7
Location: New Hampshire
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| Posted: Thu May 31, 2007 3:01 pm |
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| FYI, I'm now running 4.035 and I still get the same error message on the following line: "i = GetInt(" Enter Gain Value - ");" Things are never as simple as they seem ;-) - Steve |
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kevcon
Joined: 21 Feb 2007
Posts: 142
Location: Michigan, USA
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| Posted: Thu May 31, 2007 3:06 pm |
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Go higher.
this works in 4.038
| Code: |
#include "18F65J10.h"
#include "string.h"
#case
#fuses DEBUG //Enable Dedicated Debugger Pins
#fuses NOWDT //Disable Watchdog Timer
#fuses STVREN //Enable Stack Over/Underflow Reset
#fuses NOXINST //Disable Extended Instructions
#fuses NOPROTECT //Disable Code Protection
#fuses NOIESO //Disable Two Speed Start-up
#fuses FCMEN //Enable Fail Safe Clock Monitor
#fuses PRIMARY //Set Defaul Clock as PRIMARY
#fuses H4_SW //High Speed Crystal and PLL
#fuses WDT32768 //Set WDT Postscaler to MAX
#fuses CCP2C1 //ECCP2/P2A is Multiplexed with RC1
#use delay( clock = 29,491,200, crystal = 7,372,800 )
#use rs232( stream = UART1, baud = 115,200, UART1, parity = N, BITS = 8 )
const unsigned int8 romString[ 20 ] = { "Hello World" };
unsigned int8 ramString[ 20 ];
void main( void )
{
fprintf( UART1, "\n\r" );
fprintf( UART1, romString );
strcpy( ramString, romString );
fprintf( UART1, "\n\r" );
fprintf( UART1, ramString );
delay_ms( 2000 );
}
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SIM Uart1 output
| Quote: |
Hello World
Hello World
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu May 31, 2007 3:23 pm |
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That method will work in vs. 3.249.
I want to see how vs. 4.039 can create a pointer to a constant string
stored in ROM and pass this pointer as a parameter to a function,
just as if it were a pointer to a RAM array. I'm not directing this
criticism at you. Until I see this in a demo program, I don't think
it's working. I mean full-blown pointers to constant strings, that I
can use just like a RAM pointer. |
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kevcon
Joined: 21 Feb 2007
Posts: 142
Location: Michigan, USA
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| Posted: Thu May 31, 2007 3:54 pm |
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Ok, don't I look stupid with this egg on my face. 
Seems I have been misled again by CCS saying something works when it doesn't.
BTW PCM no offence taken, I will gladly admit when I'm wrong.
Kevin |
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slandry
Joined: 12 Mar 2007
Posts: 7
Location: New Hampshire
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| Posted: Fri Jun 01, 2007 6:23 am |
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In the book "PIC C An introduction to programming the Microchip PIC in CCS C" which I purchased from CCS some time ago, there is an example in section 7.4, "Passing Pointers to Functions". I typed in the example below, the comments are mine:
| Code: |
#include <16C74.h>
void puts(char *p);
void main(void)
{
puts("Microchip is great!"); // This line is where I get the error.
}
void puts(char *p)
{
while(*p)
{
printg("%c", *p);
p++;
}
printf("\n");
}
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This is exactly what I would like to do, but add some code to the "puts()" function above to return an int from a keyboard entry. Like the old "input" statement from basic. One call and you send a prompt and get back the input.
Thanks to everyone that has contributed to this thread. - Steve |
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kevcon
Joined: 21 Feb 2007
Posts: 142
Location: Michigan, USA
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| Posted: Fri Jun 01, 2007 6:56 am |
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You can achieve the result you are looking for in a roundabout way.
| Code: |
#include "18F65J10.h"
#include "string.h"
#case
#fuses DEBUG //Enable Dedicated Debugger Pins
#fuses NOWDT //Disable Watchdog Timer
#fuses STVREN //Enable Stack Over/Underflow Reset
#fuses NOXINST //Disable Extended Instructions
#fuses NOPROTECT //Disable Code Protection
#fuses NOIESO //Disable Two Speed Start-up
#fuses FCMEN //Enable Fail Safe Clock Monitor
#fuses PRIMARY //Set Defaul Clock as PRIMARY
#fuses H4_SW //High Speed Crystal and PLL
#fuses WDT32768 //Set WDT Postscaler to MAX
#fuses CCP2C1 //ECCP2/P2A is Multiplexed with RC1
#use delay( clock = 29,491,200, crystal = 7,372,800 )
//#use delay( clock = 36,684,000, crystal = 9,216,000 )
#use rs232( stream = UART1, baud = 115,200, UART1, parity = N, BITS = 8 )
void display_string(char *ptr);
const unsigned int8 buffer[ 2 ][ * ] = { "Hello World\n", "Goodbye World\n" };
unsigned int8 ramBuffer[ 32 ];
void main( void )
{
strcpy( ramBuffer, buffer[ 0 ] );
display_string( ramBuffer );
strcpy( ramBuffer, buffer[ 1 ] );
display_string( ramBuffer );
while( 1 );
}
void display_string(unsigned int8 *ptr)
{
unsigned int8 c;
while( c = *ptr++ )
putc( c );
}
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Sim window output
| Quote: |
Hello World
Goodbye World
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Ttelmah
Guest
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| Posted: Fri Jun 01, 2007 7:33 am |
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It actually works quite well now!.
'delay_cycles' calls added to allow debugging when I was testing.
| Code: |
//Add processor, clock, and RS232 definitions here
const char cstr[] = "test string";
char space[20];
char *ptr;
void main( void ) {
sprintf(space,"%s",cstr); //can I treat a constant string as normal?.
delay_cycles(1);
ptr=cstr; //works OK.
delay_cycles(1);
sprintf(space,"%s",ptr); //Works OK. Interesting. The pointer
//here, only holds '14', yet the system correctly pulls the data from the
//array in ROM!... space contains 'test string'.
delay_cycles(1);
sprintf(space,"%s","test constant");
//OK 'space', again contains 'test constant'.
puts(cstr);
//Ok outputs 'test string'
delay_cycles(1);
puts(ptr);
//Again works.
delay_cycles(1);
puts("test embedded constant");
//Yahoo. Also works. Outputs 'test embedded constant'.
while (true) ;
}
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Much to my suprise, it does now work. Not as documented (you can't declare the pointer as 'const'), and it handles being aware of the 'type' internally.
Best Wishes |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jun 01, 2007 10:46 am |
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| Quote: | puts(cstr);
//Ok outputs 'test string'
delay_cycles(1);
puts(ptr);
//Again works.
delay_cycles(1);
puts("test embedded constant"); |
The 2nd puts() doesn't give any output when I compile the program
with PCH 4.039 for an 18F452 and run it in MPLAB simulator.
This is the output:
| Quote: |
test string
test embedded constant
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slandry
Joined: 12 Mar 2007
Posts: 7
Location: New Hampshire
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| Posted: Fri Jun 01, 2007 11:44 am |
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I just tested the "puts("test embedded constant");" statement as you did only on real hardware using a PIC18F4680 with v4.039 compiler and the string does go out the serial port. Puts() is a standard built-in function that passes each character in the constant string to the putc() function (also built-in).
I fear there may be some confusion because we have been using "puts()" as the test function name. Below, is more of what I'm trying to compile and maybe someone will see the error of my ways.
| Code: |
void test(char *p)
{
while(*p)
{
printf("%c", *p);
p++;
}
printf("\n");
}
test("Test the function");
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I get the following error message on the last line above that calls the test() function:
: Attempt to create a pointer to a constant
- Steve |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Jun 01, 2007 12:09 pm |
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Email CCS support and ask them what is necessary to make that
routine compile and work, with vs. 4.039. |
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slandry
Joined: 12 Mar 2007
Posts: 7
Location: New Hampshire
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| Posted: Fri Jun 01, 2007 12:33 pm |
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Done, I'll post the reply when I get it.
Thanks
_________________
- Steve |
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