Microchip Technology XC8 Standard Compiler (Workstation) SW006021-1 SW006021-1 User Manual

 2012 Microchip Technology Inc.

DS52053A-page 23

2.4.13.2

DIFFERENCES

The 8-bit compilers have previously issued a warning if type int was used for bit-fields, 
but would implement the bit-field with an unsigned int type.

The 16- and 32-bit compilers have implemented bit-fields defined using int as having 
a signed int type, unless the option -funsigned-bitfields was specified.

2.4.13.3

MIGRATION TO THE CCI

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

, for example, in:

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;

};

Whether a bit-field can straddle a storage unit boundary is implementation-defined 
behavior in the standard. In the CCI, bit-fields will not straddle a storage unit boundary; 
a new storage unit will be allocated to the structure, and padding bits will fill the gap.

Note that the size of a storage unit differs with each compiler as this is based on the 
size of the base data type (e.g., int) from which the bit-field type is derived. On 8-bit 
compilers this unit is 8-bits in size; for 16-bit compilers, it is 16 bits; and for 32-bit com-
pilers, it is 32 bits in size.

2.4.14.1

EXAMPLE

The following shows a structure containing bit-fields being defined.

struct {

        unsigned first  : 6;

        unsigned second :6;

} order;

Under the CCI and using XC8, the storage allocation unit is byte sized. The bit-field 
second

, will be allocated a new storage unit since there are only 2 bits remaining in 

the first storage unit in which first is allocated. The size of this structure, order, will 
be 2 bytes.

2.4.14.2

DIFFERENCES

This allocation is identical with that used by all previous compilers.

2.4.14.3

MIGRATION TO THE CCI

No action required.

The memory ordering of bit-fields into their storage unit is not specified by the ANSI C 
Standard. In the CCI, the first bit defined will be the least significant bit of the storage 
unit in which it will be allocated.