Hi,
I seem not to be able to properly instruct gcc for producing struct
bytes aligned the way I expect.
I am using gcc 4.7.0, from the mingw32-64 distribution under Windows
7. My machine is a little-endian x86_64.
First let me state my expectations.
I have a memory region with these physical contents:
450002cf9fe5000040115a9fc0a8fe...
And I have a pointer to the struct defined below initialized to the
beginning of that region (the byte with contents "45" above).
typedef struct ip4 {
unsigned int ihl :4;
unsigned int version :4;
uint8_t tos;
uint16_t tot_len;
uint16_t id;
uint16_t frag_off; // flags=3 bits, offset=13 bits
uint8_t ttl;
uint8_t protocol;
uint16_t check;
uint32_t saddr;
uint32_t daddr;
/*The options start here. */
} ip4_t;
Now, gdb shows the following situation in runtime:
ip4: address=0x8da36ce
ip4->ihl: address=0x8da36ce, value=0x5
ip4->version: address=0x8da36ce, value=0x4
ip4->tos: address=0x8da36d2, value=0x9f
ip4->tot_len: address=0x8da36d4, value=0x0
But my expectations (basing on how the 32-bit-compiled version of this
code behaves) are:
ip4->tos: address=0x8da36cf, value=0x0
ip4->tot_len: address=0x8da36d0, value=0x02cf.
It seems gcc aligns the ihl and version bitfields to a 4-byte
boundary. That causes subsequent fields to be wrongly mapped. My goal
is to configure the padding/aligment so that the physical size of the
structure will be in agreement with the logical size.
I have tried the following options:
* -fpack-struct
* The aligned attribute
* The packed attribute
* The pragma pack (1)
* Combinations of the above
With no visible effects so far.
I will appreciate any hints on this issue.
Best regards,
D.
