To give you an example on the different alignment strategy of (for example)
PL/1 and C:
let's assume a structure
typedef struct
{
short a;
int b;
short c;
int d;
}
example;
in PL/1;
DCL 1 EXAMPLE,
3 A BIN FIXED (15),
3 B BIN FIXED (31),
3 C BIN FIXED (15),
3 D BIN FIXED (31);
These two definitions will not match, because:
in C there are two padding bytes after a and after c; the whole structure
starts at an address multiple of 4, because there are ints inside the
structure.
In PL/1, B has to be on a 4 byte boundary. To save padding bytes, the
beginning
of the structure will be in the middle of a full word, that is: on an
offset that has
remainder 2 when divided by 4. So the offsets of the structure
components are
different; when the structures are overlaid, the fields don't match.
To get around this, we put dummy double precisions fields (with the
highest possible
alignment needs) in the front of every such structure. Then the offsets
and paddings
of the remaining fields will be in sync.
(BTW: if there are multiple instances of the PL/1 structure, for example
in an array,
the PL/1 technique does not save anything, because the padding fields
now appear
at the end of the structure ... before the next element begins).
Kind regards
Bernd
Am 23.02.2015 um 08:37 schrieb Bernd Oppolzer:
Hello Ze'ev,
unless the structure is defined with the nonstandard extension Packed
(or _Packed, I don't recall it exactly), all ints or longs will be
aligned on
a 4 byte boundary, that is, in a sequence of int, short, int, short,
you will
get two padding bytes after every short field (which is in fact 2
bytes long).
Same goes for PL/1 with a sequence of BIN FIXED(31) and BIN FIXED(15)
fields. The rules for alignment with PL/1 are more complicated than
with C.
Structures in C always start with the highest needed alignment
required inside
the structure, while in PL/1 there is some work done to start the
structure at
an offset which minimizes the padding bytes in the middle of the
structure
(short story, the long explanation in the PL/1 books needs many pages).
You can synchronize alignment between PL/1 and C, if you put a dummy
double precision field (which has alignment requirement of 8) in front
of the structure.
The the two languages do the same. A similar technique should work for
COBOL, too.
Kind regards
Bernd
Am 23.02.2015 um 03:44 schrieb Ze'ev Atlas:
__off_t is clearly defined earlier as a 32 bit entity but mbstate_t
is defined as short which I assumed is a 16 bit entity. Either short
is NOT 16 bits but 32 bit entity as well, or the C compiler leaves 2
bytes of zeroes in order to keep the correct integral boundary.
However, in REAL LIFE there are 32 bits between rm_so and rm_eo. I
did not bother to investigate too much ...
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