C中的枚举类型警告

Question

I am writing a code on lpc1788 ARM Cortex M3. I came across a strange warning when I tried to configure the ports as GPIO. Despite of the warning, the code works absolutely fine, but to learn why this warning come, I am putting forward this post here. Following is the code that I have written.

static uint32_t * PIN_GetPointer(uint8_t portnum, uint8_t pinnum)  
{  
    uint32_t *pPIN = NULL;  
    pPIN = (uint32_t *)(LPC_IOCON_BASE + ((portnum * 32 + pinnum)*sizeof(uint32_t)));  
    return pPIN;  
}    

void PINSEL_SetPinMode ( uint8_t portnum, uint8_t pinnum, PinSel_BasicMode modenum)  
{  
    uint32_t *pPIN = NULL;  
    pPIN = PIN_GetPointer(portnum, pinnum);  
    *(uint32_t *)pPIN &= ~(3<<3);    //Clear function bits  
    *(uint32_t *)pPIN |= (uint32_t)(modenum<<3);  
}  

int main(void)  
{  
    PINSEL_SetPinMode(1,15,0);  //this gave a warning: enumerated type mixed with another type  
    PINSEL_SetPinMode(1,18,PINSEL_BASICMODE_NPLU_NPDN);    //this doesnt give any warning  

   /* Following is the enum present in a GPIO related header file, putting it here in comments so that   
       those who are going through this post, can see the enum  

            typedef enum
            {
                PINSEL_BASICMODE_NPLU_NPDN  = 0, // Neither Pull up nor pull down        
                PINSEL_BASICMODE_PULLDOWN,       // Pull-down enabled
                PINSEL_BASICMODE_PULLUP,         // Pull-up enabled (default)         
                PINSEL_BASICMODE_REPEATER        // Repeater mode          
            }PinSel_BasicMode;        
   */

    return 0;  
}     

Answer 1

You are using int type where enum PinSel_BasicMode type is required. While enums and ints are usually interchangeable, they are different types.

Value 0 is not an enum value. PINSEL_BASICMODE_NPLU_NPDN is. It is only 0 through definition.

Should the enum declaration change and PINSEL_BASICMODE_NPLU_NPDN was equal to 1, your code would be invalid.

Answer 2

1. You're passing an int where a enum value is expected. So either, cast it to the correct enum, or better: use the correct enum value directly:

PINSEL_SetPinMode(1, 15, (PinSel_BasicMode)0);
PINSEL_SetPinMode(1, 15, PINSEL_BASICMODE_NPLU_NPDN);

2. You're using the bit shift operator on a enum value. I think you need to cast before and after the bit shift to make the compiler happy:

void PINSEL_SetPinMode ( uint8_t portnum, uint8_t pinnum, PinSel_BasicMode modenum)  
{  
    uint32_t *pPIN = NULL;  
    pPIN = PIN_GetPointer(portnum, pinnum);  
    *pPIN &= ~(3<<3);    //Clear function bits  
    *pPIN |= (uint32_t)((uint32_t)modenum << 3);
}

Before: because you want to shift an integer value instead of the enum value.

After: because the output type of a shift operation is not necessarily the same as the input type.

Answer 3

Enums are defined type and hence here warning is of incomatible type with int, so you can typecast to avoid warning.

But as here 0 is defined for your enum so it doesnot cause your code to give wrong result.

Hope it helps.....