How to check for a single field being set in a C bitfield -

i have following style of union - defined in interface not easy change.

i want check if foo field being set. , don't want itemizing other fields.

so immediate thoughts construct mask, bitfield doing it's best hide details position of named field.

i couldn't think of better creating variable 1 field set , inverting raw field. there neater solution?

typedef union    struct {         unsigned char user:1;         unsigned char zero:1;         unsigned char foo:1;         unsigned char bar:1;         unsigned char blah:1;         unsigned char unused:3;     };     unsigned char raw; } flags_t; 

bitwise xor negation of want:

  11011111 ^ 00100000 = 11111111 

then check value == 255. can make clean using own struct build negation setting bar->foo = 0 , else 1.

edit: little elaboration because feel bad not being pretty when that's you're asking for:

struct {     unsigned char bad:1;     unsigned char bad:1;      unsigned char foo;      unsigned char other:1;     unsigned char bad:1;     unsigned char things:3; } state_checker;  int some_checking_function(flags_t possible_foo) {     result = possible_foo ^ state_checker;     result = !(result - 255u);     return result; } 

ideally make constant uses same struct value you're checking uses, make sure nothing wonky happens @ compile time, basic idea.