Bit-fields can be used to allow flags or other integer values with small ranges to be packed together to save storage space.
It is implementation - defined whether the specifier int designates the same type as signed int or the same type as unsigned int for bit-fields. According to C99 [ISO/IEC 9899:1999], C integer promotions also require that "If if an int can represent all values of the original type, the value is converted to an int; otherwise, it is converted to an unsigned int."
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Bit-field types other than _Bool, int, signed int, and unsigned int are implementation - defined. They still obey the integer promotions quoted above when the specified width is at least as narrow as CHAR_BIT*sizeof(int), but wider bit-fields are not portable.
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