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In structs, each field is associated with an offset, which is relative to the beginning of the struct. This is the offset used when reading and writing the field from/to the IO space when mapping.
The offset is reset to zero bits at the beginning of the struct type, and it is increased by the size of the fields:
struct { /* Offset: 0#b */ uint<1> f1; /* Offset: 1#b */ sint<7> f2; /* Offset: 1#B */ int f3; /* Offset: 5#B */ Bar f4; /* Offset: 23#B */ }
It is possible to specify an alternative offset for a field using a field label.
Consider for example an entry in an ELF symbol table. Each entry has
a st_info
field which is 64-bits long, that in turn can be
interpreted as two fields st_bind
and st_type
.
The obvious solution is to encode st_info
as a sub-struct that
is integral, like this:
struct { elf32_word st_name; struct uint<64> { uint<60> st_bind; uint<4> st_type; } st_info; }
This makes the value of st_info
easily accessible as an
integral value. But we may prefer to use labels instead:
struct { elf32_word st_name; elf64_word st_info; uint<60> st_bind @ 4#B; uint<4> st_type @ 4#B + 60#b; }
The resulting struct has fields st_info
, st_bind
and
st_type
, with the last two overlapping the first.
Field labels are also useful in the rare cases where a variable part of a struct depends on some data that comes after it. This happens for example in the COFF “line number” structures, which are defined like this in C:
struct external_lineno { union { char l_symndx[4]; /* function name symbol index, iff l_lnno == 0*/ char l_paddr[4]; /* (physical) address of line number */ } l_addr; char l_lnno[L_LNNO_SIZE]; /* line number */ };
i.e. the l_addr
field is to be interpreted as an index in a
table or as a physical address depending on the value of
l_lnno
.
This is one way to denote the above data structure using Poke struct labels:
type COFF_LINENO_32 = struct { uint<32> l_lnno 4#B; /* Line number. */ union { uint<32> l_symndx : l_lnno == 0; offset<uint<32>,B> l_paddr; } l_addr 0#B; };
Note how the l_lnno
field is stored after l_addr
even if
they appear in reverse order in the struct definition. This allows us
to refer to l_nno
in the constraints for the union fields.
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