Use a variable in a macro before declaration in C-CodePudding

I found the code below in a project and I'm having problems understanding why GCC doesn't complaint since dlc is called before it has been defined.

#define CAN_SET_DLC(dlc) (CANCDMOB |= (dlc))


typedef struct
{
    uint8_t mob_n;
    uint8_t handle;
    long id;
    long id_msk;
    uint8_t dlc;
    uint8_t data_pt[8];
    uint8_t status;
} can_msg;

¿The variable type of dlc should also be defined?:

define CAN_SET_DLC(uint8_t dlc) (CANCDMOB |= (dlc))

CodePudding user response：

The preprocessor has no knowledge of variables or any language construct. It is just a token processor.

In #define a(b) ( b 1 ) this means that anytime the preprocessor encounters a(foo) in the source text, it will replace it with ( foo 1 ). Then the compiler will check whether it is correct C.

CodePudding user response：

This is the normal way to define macro with arguments: https://gcc.gnu.org/onlinedocs/cpp/Macro-Arguments.html#Macro-Arguments

Macros just replace text basically so you have to take care about variable type.

CodePudding user response：

No, macros only substitute text (more precisely tokens) and it is done before actual C code compilation. The preprocessor does not know anything about C language.

The preprocessed source code then is compiled by the C compiler.

#define CAN_SET_DLC(dlc) ((DLC) |= CANCDMOB)

#define CANCMOB 0x34

int foo(uint8_t z)
{
    CAN_SET_DLC(z);
    return z;
}

will result in

int foo(uint8_t z)
{
    ((z) |= 0x34);
    return z;
}