Function that returns a string from an array of pointers

Question

I need to declare a function that returns a string from an array of pointers .. I am programming a PIC and I am using as an interface an LCD .. With this code I have what I want:

// CONFIGURATION BITS
#pragma config FOSC = HS   // Oscillator Selection bits (XT oscillator)
#pragma config WDTE = OFF  // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOREN = OFF // Brown-out Reset Enable bit (BOR disabled)
#pragma config LVP = OFF   // Low-Voltage In-Circuit Serial Programming Enable bit
#pragma config CPD = OFF   // Data EEPROM Memory Code Protection bit 
#pragma config WRT = OFF   // Flash Program Memory Write Enable bits 
#pragma config CP = OFF    // Flash Program Memory Code Protection bit

#define _XTAL_FREQ 8000000

// DEFINING LCD PINS
#define RS RD2
#define EN RD3
#define D4 RD4
#define D5 RD5
#define D6 RD6
#define D7 RD7

#include <xc.h>

#include "lcd.h"

#include <stdio.h>
#include <stdlib.h>

void main()
{

    TRISD = 0x00;           //PORTD as output for LCD
    Lcd_Init();
    Lcd_Clear();

    while(1)
    {

        Lcd_Set_Cursor(1,1);

        char *keypress;
        int key = 2;
        char *key2[] = {"3", "4", "5"}; 
        keypress = key2[key];

        Lcd_Write_String(keypress);

        __delay_ms(50);

    }
}

I obtain "5" .. That is what I want .. But when I try to make a function for that..

// CONFIGURATION BITS
#pragma config FOSC = HS   // Oscillator Selection bits (XT oscillator)
#pragma config WDTE = OFF  // Watchdog Timer Enable bit (WDT disabled)
#pragma config PWRTE = OFF // Power-up Timer Enable bit (PWRT disabled)
#pragma config BOREN = OFF // Brown-out Reset Enable bit (BOR disabled)
#pragma config LVP = OFF   // Low-Voltage In-Circuit Serial Programming Enable bit
#pragma config CPD = OFF   // Data EEPROM Memory Code Protection bit 
#pragma config WRT = OFF   // Flash Program Memory Write Enable bits 
#pragma config CP = OFF    // Flash Program Memory Code Protection bit

#define _XTAL_FREQ 8000000

// DEFINING LCD PINS
#define RS RD2
#define EN RD3
#define D4 RD4
#define D5 RD5
#define D6 RD6
#define D7 RD7

#include <xc.h>

#include "lcd.h"

#include <stdio.h>
#include <stdlib.h>

char kbd_getc();

char kbd_getc(){

    int key = 2;
    char *key2[] = {"3", "4", "5"}; 
    return key2[key];

}

void main()
{

    TRISD = 0x00;           //PORTD as output for LCD
    Lcd_Init();
    Lcd_Clear();

    while(1)
    {

        Lcd_Set_Cursor(1,1);

        char *keypress;

        keypress = kbd_getc();

        Lcd_Write_String(keypress);

        __delay_ms(50);

    }
}

I don't understand why do I get mm .. I am not sure if I am declaring the function incorrectly or I am doing something wrong with the pointers, thanks

The root of the problem here is that the infamous PIC compiler "MPLAB", or whatever that crap is called nowadays, is bad. A working C compiler would have pointed out the language violation at return key2[key];, which comes from accidentally using char kbd_getc() rather than as intended: char* kbd_getc(). — Lundin
– Lundin, Commented Jul 5, 2018 at 6:30
As a side note, pointers to string literals should be declared as const char*. They will be stored in flash. — Lundin
– Lundin, Commented Jul 5, 2018 at 6:32
As another side note, void main() is not standard-compliant. main() should return an int. — schaiba
– schaiba, Commented Jul 5, 2018 at 7:43

LoztInSpace · Accepted Answer · 2018-07-05 05:45:07Z

2

Your function returns char but you return a char *.

Change the return type to char *

And up your compiler warning to catch those things.

edited Jul 5, 2018 at 5:45

answered Jul 5, 2018 at 5:30

LoztInSpace

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3 Comments

LoztInSpace Over a year ago

I don't know enough about the specifics of your application, I just told you what is wrong with your C. You (I expect) need to change the return to char *. Which did you do?

Nau Over a year ago

I change: char kbd_getc(){ to: char * kbd_getc(){ Now it works, thanks

Lundin Over a year ago

There is no need to "up" compiler warnings, the code contains a constraint violation and a conforming compiler must give a diagnostic message. So the solution is rather to change to a standard C compiler.

H.S. · Accepted Answer · 2018-07-05 05:53:37Z

    char *key2[] = {"3", "4", "5"}; 
    return key2[key];

Here, key2 is an array of char pointers and key2[key] is pointer at index key of array key2 which is pointing to first character of a string.
Now, look at the return type of function kbd_getc():

char kbd_getc(){
^^^^

Its a char type. That means, you are returning a string from function which is suppose to return a character.
If I write a simple program to check your kbd_getc(), I am getting following warning message on statement return key2[key]; -

warning: incompatible pointer to integer conversion returning 'char *' from a function with result type 'char';

If I change the return type to char *

char * kbd_getc(){

the warning message disappear and I get the desired output:

$ ./a.out 
5

Sivabalakrishnan · Accepted Answer · 2018-07-05 05:37:40Z

-2

Just try this,

char kbd_getc(){

int key = 2;
char *key2[] = {"3", "4", "5"}; 
char a = key2[key];
return a ;

}

I think That u r returning array.

You need to return a char.

Thank you

answered Jul 5, 2018 at 5:37

Sivabalakrishnan

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2 Comments

LoztInSpace Over a year ago

Same problem, same bug. Your code returns a char * from a char function. I think the function should return char *

Sivabalakrishnan Over a year ago

Ya , i think that key pressed is char.. plz change that to char* , if then change the method name to char *

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