4.4.11 Packing Objects

The __pack specifier can be used to indicate that structures should not use memory gaps to align structure members, or that individual structure members should not be aligned.

Use the native keywords discussed in the Differences section to look up information on the semantics of this specifier.

Some compilers cannot pad structures with alignment gaps for some devices, and use of this specifier for such devices is ignored.

Example

The following shows a structure qualified using __pack, as well as a structure where one member has been explicitly packed.

__pack struct DATAPOINT {
  unsigned char type;
  int value;
} x-point;
struct LINETYPE {
  unsigned char type;
  __pack int start;
  long total;
} line;

Differences

The __pack specifier is a new CCI specifier that is available with MPLAB XC8. This specifier has no apparent effect since the device memory is byte addressable for all data objects.

The 16- and 32-bit compilers have used the packed attribute to indicate that a structure member was not aligned with a memory gap.

Migration to the CCI

No migration is required for MPLAB XC8.

For 16- and 32-bit compilers, change any occurrence of the packed attribute, for example, from:

struct DOT
{
 char a;
 int x[2] __attribute__ ((packed));
};

to:

struct DOT
{
 char a;
 __pack int x[2];
};

Alternatively, you can pack the entire structure, if required.

Caveats

None.