from my previous experience i understood the following:
// if i have structure in big-endian system, look like this:
typedef struct
{
unsigned long
a: t1,
b: t2,
c: t3,
d: t4,
//...
z: tn;
} TType;
// i can adapt this for little-endian so:
typedef struct
{
unsigned long
z:tn,
//...
d: t4,
c: t3,
b: t2,
a: t1;
} TType;
// and i get identical mapping to memory
or following:
// if i have next structure:
typedef struct
{
unsigned long
a : 2,
b : 5,
c : 6,
d : 3;
} TType2;
// ...
TType2 test;
test.a = 0x2;
test.b = 0x0E;
test.c = 0x3A;
test.d = 0x6;
printf("*(unsigned short *)&test = 0x%04X\n", *(unsigned short *)&test);
// in little-endian system i get: 0xDD3A , or mapping to memory :
// c'-|-----b-----|-a-| |--d--|------c----
// 0 0 1 1 _ 1 0 1 0 _|_ 1 1 0 1 _ 1 1 0 1 _
// in big-endian system i get: 0xD69D , or mapping to memory :
// -----c-----|--d--| |-a-|------b----|-c'
// 1 1 0 1 _ 0 1 1 0 _|_ 1 0 0 1 _ 1 1 0 1 _
If i am not right - please correct me.
My embedded-system is 32-bit little-endian. This device have big-endian hardware, connected via SPI. Program, that i must adapt for my system, later work with this hardware in 32-bit big-endian system via parallel bus.
I begin to adapt the library, which intends for building and analyzing ethernet framers and something else.
I met the following code (which crash my mind):
#pragma pack(1)
//...
typedef unsigned short word;
//...
#ifdef _MOTOROLA_CPU
typedef struct
{
word ip_ver : 4;
word ihl : 4;
word ip_tos : 8;
word tot_len;
word identification;
word flags : 3;
word fragment_ofs: 13;
word time_to_live: 8;
word protocol : 8;
word check_sum;
IP_ADDRESS_T src;
IP_ADDRESS_T dest;
} IP_MSG_HEADER_T, *IP_MSG_HEADER_P;
#else // Intel CPU.
typedef struct
{
word ip_tos : 8;
word ihl : 4;
word ip_ver : 4;
word tot_len;
word identification;
word fragment_ofs: 13;
word flags : 3;
word protocol : 8;
word time_to_live: 8;
word check_sum;
IP_ADDRESS_T src;
IP_ADDRESS_T dest;
} IP_MSG_HEADER_T, *IP_MSG_HEADER_P;
#endif
But i met and the following:
typedef struct
{
word formid : 5;
word padding_formid : 3;
word TS_in_bundle : 5;
word padding_ts : 3;
word cell_per_frame : 8;
word padding : 8;
} SERVICE_SPEC_OLD_FIELD_T, *SERVICE_SPEC_OLD_FIELD_P
#else // Intel CPU.
typedef struct
{
word padding : 8;
word cell_per_frame : 8;
word padding_ts : 3;
word TS_in_bundle : 5;
word padding_formid : 3;
word formid : 5;
} SERVICE_SPEC_OLD_FIELD_T, *SERVICE_SPEC_OLD_FIELD_P;
#endif /*_MOTOROLA_CPU*/
Similar vagueness everywhere in this library. I don't see here logic. Is me really stupid, or this code - nonsense?
Also: Whether I am right in the following:
// Two structures
typedef struct
{
unsigned long
a : 1,
b : 3,
c : 12;
unsigned short word;
} Type1;
// and
typedef struct
{
unsigned long
a : 1,
b : 3,
c : 12,
word : 16;
} Type2;
// will be identical in memory for little-endian and different for big-endian?
Thanks in advance!
