This is a follow up on a private mail exchange about C++ and C user space
programs seeing diferent things in the same mbuff shared area.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARRRRGGGGHHHHH !!!!
I was telling about it in Vienna. You have used high optimization -O6 and gcc
and g++ have aligned your structures differently. Thus the difference in
sizeof and layout of your structures. No wonder each program sees the same
raw data, but understands it differently.
typedef struct {
bool inuse; /* only used inside shm */
/* g++ - no space, gcc - bool is 4 bytes! */B
bool valid;
/* here g++ 2 bytes padding, gcc - bool is 4 bytes! */
/* the LUT itself with Distance [um] */
double um[NI_PCI_MIO_XE10_RES+1];
} shm_slut_t;
reading directly slut from shm (each 4096 Bytes)
Debug: Reading from Shared Memory Region <SensorLUT>
0 0.0000 off:4
4095 4095.0000 off:32764
inuse: 0 valid:1
crds:/home/motyl/mbuff/petzold/mbuff_test3# ./test_shm -r
Shared Memory for Sensor LUT at 0x400f7000
reading directly slut from shm (each 4096 Bytes)
0 0.0000 off:8
4095 0.0000 off:32768
sizeof(bool) == 4
inuse: 0 valid:4
Even using
typedef struct {
..........
} __attribute__((packed)) shm_slut_t;
does not save the problem, because sizeof(bool)==4 in gcc and 1 in g++.
I would like to use this opportunity to stress once again the importance to
use the same compiler flags, volatile, and __attribute__((packed)) to get
right any shared structures.
This can be used to check the position of the fields, I will add it to the
next version of mbuff.
#define FIELD_OFFSET(strptr,field) ((char*)&((strptr)->field)-(char*)strptr)
Such problems are very difficult to find, and I have seen some bugs of type
"works for me but not for you" traced to different layout of kernel's and
module's structures.
I have no idea what has caused your problems with memcpy. Please mail me
again if this problem was caused by something else.
Small remark: you are probably doing too much copying. Shared memory is
designed to avoid it.
Best regards,
--
Tomasz Motylewski
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