topic Discrepancy in values when struct is accessed via a pointer. in i.MX RT Crossover MCUs

Discrepancy in values when struct is accessed via a pointer.

dmckeever — Wed, 03 Apr 2024 16:11:36 GMT

Hello

I am using the iMXRT1064 processor on a custom board with an eMMC card fitted

MCUXpresso is the development environment.

I am using the file fsl_mmc.c from the SDK for library functions to access the eMMC card.

The eMMC card descriptor is a C struct allocated at a certain memory address.

mmc_card_t g_mmc;

The descriptor is initialised by calling the library function, MMC_Init(mmc_card_t *card) passing a pointer to the struct.

After the return from MMC_Init, the fields of the struct have "bad" values, eg blockSize is 17017344 instead of 512.

Inside MMC_Init, when the struct is accessed directly (as a test), most of the fields show different values than when it is accessed via the pointer; so that the initialisation done via the pointer is placing bad values into the actual struct in memory.

The attached file g_mmc.docx shows two screenshots taken just before the return from MMC_Init. This shows the "good" values via the pointer, but "bad" values in the actual memory locations.
( In successive executions, the "bad" values are always the same.)

NOTE. For those familiar with mmc_card_t, I experimented by moving one field to the end of the struct.

I have tried various alignments of g_mmc but without success.

Can anyone suggest how accessing the same struct directly and via a pointer can yield different values?

Thank you in anticipation.

Damien

Re: Discrepancy in values when struct is accessed via a pointer.

dmckeever — Thu, 04 Apr 2024 10:55:26 GMT

The behavior is even more bizarre.

Example.

mmc_card_t *card = (mmc_card_t*)&g_mmc;

...

Execute the following statement, with a breakpoint on the second line:

MMCdeviceSectors = g_mmc.userPartitionBlocks;

Success = true;

Expression windows in the debugger show that the value of g_mmc.userPartitionBlocks shown in the struct (ie 7733248) is not being assigned to the variable MMCdeviceSectors ( ie. 131072)

The value assigned (131072) is the "bad" value shown when the g_mmc struct is accessed via the pointer card .

The attached file Expression Discrepancy.docx shows this with screenshots.

Help please!

Damien

Re: Discrepancy in values when struct is accessed via a pointer.

Kan_Li — Thu, 11 Apr 2024 02:46:32 GMT

Hi @dmckeever ,

May I have the IDE tool version? From here it seems ok. Please kindly refer to the following for details.

If your IDE version is lower than above, maybe you have to update it.

Hope that helps,

Have a great day,
Kan

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