I encountered a statement like "char var : 3". What does this C statement do?

Question

While going through some C code, I encountered statements like

char var1 : num1, char var2: num2;

From the context, it seems like the number i.e. num1 is the byte size. I am unable to find any explanation.

Answer 1

This could be part of what is called a bit-field in the C programming language. Bit-fields can only be declared inside a struct, e.g.

struct {
   unsigned int flag  : 1;    /* A one bit flag */
   unsigned int value : 5;    /* A 5 bit value */
} option;

if (option.flag == 1)
    option.value = 7;

About everything on bit-fields is implementation-defined. The intention is to have bit-fields arranged as compact as possible by the compiler. E.g. the above could well fit in one byte.