c preprocessor commands & return types(?)

kpachopoulos · 11-09-2005, 11:29 AM

Hi,
I don't think exactly, that C preprocessor commands-defines return something, but see that:

Code:

447 #define IW_EVENT_CAPA_INDEX(cmd)        (IW_EVENT_CAPA_BASE(cmd) >> 5)
448 #define IW_EVENT_CAPA_MASK(cmd)         (1 << (IW_EVENT_CAPA_BASE(cmd) & 0x1F))

Here is the IW_EVENT_CAPA_BASE(cmd) definition:

Code:

441 /* Event capability macros - in (struct iw_range *)->event_capa
442  * Because we have more than 32 possible events, we use an array of
443  * 32 bit bitmasks. Note : 32 bits = 0x20 = 2^5. */
444 #define IW_EVENT_CAPA_BASE(cmd)         ((cmd >= SIOCIWFIRSTPRIV) ? \
445                                          (cmd - SIOCIWFIRSTPRIV + 0x60) : \
446                                          (cmd - SIOCSIWCOMMIT))

when a preprocessor "define" uses bitwise operators on another preprocessor "define", surely the latter must return a value. How do i understand this value? In this occassion, it is of course an arithmetic one, but what kind (int, long, etc)?

naf · 11-09-2005, 12:08 PM

These are just macros. There is no 'type' associated with the argument. It performs text substitution and the code is compiled. The resulting code will dictate type. This is why some macros are not safe when types are important.

For a simple answer, the 'type' is the same type as 'cmd'. If you pass a 64-bit integer, then the result will be based on that (by promoting all other arguments.) However, in this case, it seems that the operation is expecting 32-bit integers (platform-specific)! Using 64-bit will work for the arguments, because these two macros is just telling what 32-bit word (macro IW_EVENT_CAPA_INDEX) and what bit within this 32 bit word as a mask ( IW_EVENT_CAPA_MASK ). Just make sure that the variable accepting the MASK result is at least 32 bits.

phanirajiv · 12-03-2005, 11:49 AM

i jus have a question in mind whether char can be used as a return type in C

vivekr · 12-04-2005, 02:15 AM

Why not?
And u should have started a separate thread if u wanted to get the response from all

naf · 12-06-2005, 09:18 AM

Any defined type can be used as a return type.
This includes all basic types:
char
unsigned char
long
unsigned long
short
unsigned short
int
unsigned int
float
double
void * (any kind of pointer works too)

Complex types including:
enumerations:

Code:

typedef enum { VAL_1, VAL_2, VAL_3 } MyEnum;

MyEmum AFunction( ... );

structures:

Code:

typedef struct my_struct { long a; char b; double c; } MyStruct;

MyStruct BFunction( ... );  /* <-- Using the typedef  */
struct my_struct BFunction( ... );  /* <-- Using the fully qualified name. */

unions (see struct above with union instead of struct)

All these return types can also apply the const qualifier. Pointers can have a pointer to what is pointed to and the pointer itself.

C++ offers three more possibilities: two types (bool and class) and references.