value assignment to a structure member inside the structure

[nisarga]

Is it possible to assign a value to structure member inside the structure.like. ....

struct control{
 
char tbi:2 =0;
char res:1 =0;
};

[donbock]

Not like that. The struct command defines a new type. Putting the assignments in the struct command makes it look like you want these to be the initial values of those fields for every variable of this structure type. That sounds like a C++ constructor; but C doesn't have feature like that.

[stdq]

I just wanted to point that, as far as I know, you can't use the ':' character to name a variable.

[Banfa]

@stdq - you'd be wrong because it is not naming a field it is declaring a bit field. Now bit fields are a bit platform dependant and really just syntactic sugar for using bit masks but they can be useful (particularly mapping data in microprocessor peripheral registers).

Effectively the code in the OP declares a field of 2 bits and a field of 1 bit, which will be followed by padding bits to bring the total size up to the number of bits in a char.

See http://en.wikipedia.org/wiki/Bit_field

[donbock]

In the OP code, the only initial values we see happen to be zero. If there are indeed no nonzero initial values then this effect can be achieved by declaring the structure variables with static duration. Variables with static duration are implicitly initialized to all zero before the first instruction in your program executes.

Static storage duration is any variable defined with the static keyword or any global variable (explicit or implicit extern keyword).

[donbock]

@Banfa - Standard C requires bit fields be of type unsigned int, signed int, int, or _Bool (C99 only). Notice that whether an int bit field is signed or unsigned is implementation-dependent -- quite different from an int variable, which is always signed.

I'm told that allowing other integer types (including char) for bit fields is a nonstandard extension found in some compilers. Needless to say, nonstandard extensions should be avoided.

Blast from the past: pre-Standard K&R C only supported bit fields of type unsigned.

[Banfa]

@donbock you are of course correct on all counts and I probably should have pointed that out myself (I did think about it while I was typing if that counts for anything :D).

It is a relatively common non-standard extension so I assumed that is what was being used in this case (because actually that is the only thing that makes sense given the code).