The following type attributes are supported:
The packed attribute is supported for struct and union types if the --relaxed_ansi option is used.
Members of a packed structure are stored as closely to each other as possible, omitting additional bytes of padding usually added to preserve word-alignment. For example, assuming a word-size of 4 bytes ordinarily has 3 bytes of padding between members c1 and i, and another 3 bytes of trailing padding after member c2, leading to a total size of 12 bytes:
struct unpacked_struct { char c1; int i; char c2;};
However, the members of a packed struct are byte-aligned. Thus the following does not have any bytes of padding between or after members and totals 6 bytes:
struct __attribute__((__packed__)) packed_struct { char c1; int i; char c2; };
Subsequently, packed structures in an array are packed together without trailing padding between array elements.
Bit fields of a packed structure are bit-aligned. The byte alignment of adjacent struct members that are not bit fields does not change. However, there are no bits of padding between adjacent bit fields.
The packed attribute can only be applied to the original definition of a structure or union type. It cannot be applied with a typedef to a non-packed structure that has already been defined, nor can it be applied to the declaration of a struct or union object. Therefore, any given structure or union type can only be packed or non-packed, and all objects of that type will inherit its packed or non-packed attribute.
The packed attribute is not applied recursively to structure types that are contained within a packed structure. Thus, in the following example the member s retains the same internal layout as in the first example above. There is no padding between c and s, so s falls on an unaligned boundary:
struct __attribute__((__packed__)) outer_packed_struct { char c; struct unpacked_struct s; };
It is illegal to implicitly or explicitly cast the address of a packed struct member as a pointer to any non-packed type except an unsigned char. In the following example, p1, p2, and the call to foo are all illegal.
void foo(int *param);
struct packed_struct ps;
int *p1 = &ps.i;
int *p2 = (int *)&ps.i;
foo(&ps.i);
However, it is legal to explicitly cast the address of a packed struct member as a pointer to an unsigned char:
unsigned char *pc = (unsigned char *)&ps.i;
The packed attribute can also be applied to enumerated types. On an enum, packed indicates that the smallest integral type should be used.
The TI compiler also supports an unpacked attribute for an enumeration type to allow you to indicate that the representation is to be an integer type that is no smaller than int; in other words, it is not packed.