7.3.13 Default Bit-field int Type

The type of a bit-field specified as a plain int is identical to that of one defined using unsigned int. This is quite different from other objects where the types int, signed and signed int are synonymous. It is recommended that the signedness of the bit-field be explicitly stated in all bit-field definitions.

Example

The following shows an example of a structure tag containing bit-fields that are unsigned integers and with the size specified.

struct OUTPUTS {
	int direction :1;
	int parity    :3;
	int value     :4;
};

Differences

When targeting PIC devices, the MPLAB XC8 compiler has issued a warning if an int type was specified for bit-fields, and it has instead implemented the bit-field with an unsigned int type. For other devices, this compiler has implemented bit-fields defined using int as having a signed int type, unless the option -funsigned-bitfields was specified. Use of a signed int type is also true for the MPLAB XC16, XC-DSC, and XC32 compilers.

Migration to the CCI

Any code that defines a bit-field with the plain int type should be reviewed. If the intention was for these to be signed quantities, then the type of these should be changed to signed int. In the following example:

struct WAYPT {
	int log          :3;
	int direction    :4;
};

the bit-field type should be changed to signed int, as in:

struct WAYPT {
	signed int log       :3;
	signed int direction :4;
};