typedef enum explanation in c

Question

I'm looking at a header file for an ADC on a micro controller and the following code is in it.

/**
 * ADC channels type.
*/

typedef enum {
    ADC_CH_0,
    ADC_CH_1,
    ADC_CH_2,
    ADC_CH_3,
    ADC_CH_4,
    ADC_CH_5,
    ADC_CH_6, 
} adc_channel_t;

And in the main.c for the ADC there is the following line of code

adc_channel_t channels[] = {ADC_CH_4, ADC_CH_5};

I was wondering why would you need declare new data types for the ADC? and what does typedef enum mean?

Thanks

Answer 1

As a complement to the answer by artm the typedef is added in front of the enum, to ease the use of the enum. Had the declaration looked like this instead:

enum adc_channel_t {
    ADC_CH_0,
    ADC_CH_1,
    ADC_CH_2,
    ADC_CH_3,
    ADC_CH_4,
    ADC_CH_5,
    ADC_CH_6, 
};

Then the line adc_channel_t channels[] = {ADC_CH_4, ADC_CH_5}; would have to be written as:

enum adc_channel_t channels[] = {ADC_CH_4, ADC_CH_5};

The typedef allows us to ignore the enum at every use of the type.

Using useful constants is often preferred over "magic numbers", though it might seem a bit strange in this case the constants give little extra information. It can however be useful since the enumerator serves as extra description. For instance your IDE which will expect a value of type adc_channel_t will be able to suggest the channels: ADC_CH_0 through ADC_CH_6 that might be the valid range of values, instead of simply telling you to use a number.