#define inside a struct?

[Daniel Rudy]

What is the purpose of having a #define inside a struct?

typedef struct {
uByte bLength;
uByte bDescriptorType;
uWord wTotalLength;
uByte bNumInterface;
uByte bConfigurationValue;
uByte iConfiguration;
uByte bmAttributes;
#define UC_BUS_POWERED 0x80
#define UC_SELF_POWERED 0x40
#define UC_REMOTE_WAKEUP 0x20
uByte bMaxPower; /* max current in 2 mA units */
#define UC_POWER_FACTOR 2
} UPACKED usb_config_descriptor_t;

The example is from the usb.h header file on FreeBSD. The compiler is gcc.

 

[Robert Harris]

What is the purpose of having a #define inside a struct?

typedef struct {
uByte bLength;
uByte bDescriptorType;
uWord wTotalLength;
uByte bNumInterface;
uByte bConfigurationValue;
uByte iConfiguration;
uByte bmAttributes;
#define UC_BUS_POWERED 0x80
#define UC_SELF_POWERED 0x40
#define UC_REMOTE_WAKEUP 0x20
uByte bMaxPower; /* max current in 2 mA units */
#define UC_POWER_FACTOR 2
} UPACKED usb_config_descriptor_t;

The example is from the usb.h header file on FreeBSD. The compiler is gcc.

The #define's aren't syntactically part of the struct. They are
preprocessed out at a stage before the compiler parses the struct
definitions.

Putting them where they are conveys (to a human) that possible values
for the structure members bmAttributes and bMaxPower are those #define'd
below them.

Robert

 

[Mike Wahler]

What is the purpose of having a #define inside a struct?

Same purpose as having them anywhere else.
I'll guess that in your example they were put inside
the struct definition because those are probably
values to be used for assigning to one or more
members of that struct (iow, put the information
near where it will be used).

-Mike

 

[Daniel Rudy]

At about the time of 10/16/2005 3:33 AM, Robert Harris stated the following:

The #define's aren't syntactically part of the struct. They are
preprocessed out at a stage before the compiler parses the struct
definitions.

Putting them where they are conveys (to a human) that possible values
for the structure members bmAttributes and bMaxPower are those #define'd
below them.

Robert

I knew that there were pre-processed out, but I didn't understand why
they would be placed in the middle of a struct.

Now I know.

Thanks.

 

[Alan Balmer]

What is the purpose of having a #define inside a struct?

typedef struct {
uByte bLength;
uByte bDescriptorType;
uWord wTotalLength;
uByte bNumInterface;
uByte bConfigurationValue;
uByte iConfiguration;
uByte bmAttributes;
#define UC_BUS_POWERED 0x80
#define UC_SELF_POWERED 0x40
#define UC_REMOTE_WAKEUP 0x20
uByte bMaxPower; /* max current in 2 mA units */
#define UC_POWER_FACTOR 2
} UPACKED usb_config_descriptor_t;

The example is from the usb.h header file on FreeBSD. The compiler is gcc.

Presumably the purpose is to aid the reader by having the definitions
near the point of use. For what it's worth, I wouldn't do it that way,
it interferes with the layout of the struct, imo. I would collect the
defines elsewhere, and document the permissible values as an end of
line comment, e.g.

uByte bmAttributes; /* UC_BUS_POWERED, UC_SELF_POWERED,
or UC_REMOTE_WAKEUP */

 

[Daniel Rudy]

At about the time of 10/17/2005 9:16 AM, Alan Balmer stated the following:

Presumably the purpose is to aid the reader by having the definitions
near the point of use. For what it's worth, I wouldn't do it that way,
it interferes with the layout of the struct, imo. I would collect the
defines elsewhere, and document the permissible values as an end of
line comment, e.g.

uByte bmAttributes; /* UC_BUS_POWERED, UC_SELF_POWERED,
or UC_REMOTE_WAKEUP */

I like your idea better myself. This was found in an offical operating
system header file for application use. This was something that I have
never seen before which lead to my confusion.