CODEWITHSUNDEEP
c
P0W
P0W

Reputation: 47824

Subscripting functions in C

I saw this code (pulled from device driver source, changed variable names, added comments)

What does the subscript [] do inside the driver_funcs static array of pointer to void ?

More importantly what's purpose of DECLFUNC here macro ? 

#define DECLFUNC(x)  [DRIVER_##x - DRIVER_IOCTL_MIN]  f_##x
enum 
{
    DRIVER_IOCTL_MIN = 300,
    DRIVER_GET_PARTITION_STATUS = DRIVER_IOCTL_MIN,
    DRIVER_SET_PARTITION_MODE,
    DRIVER_GET_PROCESS_ID,
    //...
};


static int f_GET_PARTITION_STATUS( int a, int b, int c )
{
    //...
    return 1;
}

static int f_SET_PARTITION_MODE( int a, int b, int c )
{
    //...
    return 2;
}

static int f_GET_PROCESS_ID( int a, int b, int c )
{
    //...
    return 3;
}

static void *driver_funcs[] = 
{
    DECLFUNC(GET_PARTITION_STATUS),  
     //=> [0] f_GET_PARTITION_STATUS,  => *(f_GET_PARTITION_STATUS+0)   ???
    DECLFUNC(SET_PARTITION_MODE),    
     //=> [1] f_SET_PARTITION_MODE,   =>  *(f_SET_PARTITION_MODE+1)     ???
    DECLFUNC(GET_PROCESS_ID),        
    //=> [2] f_GET_PROCESS_ID,       =>  *(f_GET_PROCESS_ID+2)          ???
    //....
};

driver_funcs is used in normal way

func = driver_funcs[cmd - DRIVER_IOCTL_MIN];
func(1,2,3);

This code is compiled using gcc, if that matters (gcc extension ?). Also if its duplicate, please let me know I'll delete this post.

Upvotes: 2

Views: 145

Answers (1)

Some programmer dude
Some programmer dude

Reputation: 409374

It's a feature in the C99 standard, to be able to place items at arbitrary positions an in array initializer. See e.g. this GCC manual page.

What it does is to place a function pointer for e.g. GET_PARTITION_STATUS at index zero in the array.

The C99 standard mandates that there should be a = between the index and the value, GCC as an extension of the language allows you to omit it.

Upvotes: 4

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