Assigning value in C macro-CodePudding

Is it ok to assign a value in a C macro (i.e. have an lvalue in a macro)? For example, I would like to set a bit of a number to 1 based on the specified bit position. Is it ok to do this:

#define SET_BIT(data, pos)  ((data) |= (1U << (pos)))

I've tested this and it works, but I feel like I'm walking on thin ice here and if I get used to macros like this, I would eventually run into problems.

CodePudding user response：

Macros won't exist once the preprocessor has finished expanding them in the code. If you have a statement like

SET_BIT(some_data, some_pos);

then it will be replaced by

((some_data) |= (1U << (some_pos)));

in the code that is parsed and compiled.

In short, doing assignments in macros is perfectly fine.

The problem with the first SET_BIT_TO_VAL macro:

#define SET_BIT_TO_VAL(data, pos, val)  (CLEAR_BIT(data, pos); (data) |= (1U << (pos)))

is that you put statements inside parentheses, like they were expressions. So that will not build.

The common way to group statements in a macro is to wrap them in a do { ... } while(0) loop:

#define SET_BIT_TO_VAL(data, pos, val) \
    do {                               \
        CLEAR_BIT(data, pos);          \
        SET_BIT(data, pos);            \
    } while (0)