sizeof *data, when data isn't initalized, is that right?

I came across this code:

        struct mcp23s08_driver_data     *data;
	...
	data = kzalloc(sizeof *data + chips * sizeof(struct mcp23s08),
                        GFP_KERNEL);

Since data wasn't initialized when `sizeof *data' is called, wasn't this a non-initialized pointer dereference?

Cheers

Hi,

As I understand it, sizeof(*data) is the same as sizeof(struct mcp23s08_driver_data). There should be no pointer dereference here, so I would say it isn't an issue. That notation is quite confusing though. Note that data is surrounded by parenthesis, at least in Linux 3.18, in gpio-mcp23s08.c
Regards,

Hubert

Le 22/01/2015 19:19, Daniel Hilst Selli a écrit :

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Hi,

As I understand it, sizeof(*data) is the same as sizeof(struct mcp23s08_driver_data). There should be no pointer dereference here, so I would say it isn't an issue. That notation is quite confusing though. Note that data is surrounded by parenthesis, at least in Linux 3.18, in gpio-mcp23s08.c
Regards,

Hubert

Le 22/01/2015 19:19, Daniel Hilst Selli a écrit :

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In most cases (always?) sizeof() is a compile-time operator; it is up to the compiler to perform the calculation of an object's size and simply place that constant into the code. This is why the target of a sizeof() operation needs to be fully defined at compile time.

Therefore, since this calculation takes place at compile time, there is no run-time pointer dereference taking place.

This idiom is quite common, so it would be good to become familiar with it. Without using this idiom one would have to write:

sizeof (struct mpc23s08_driver_data)

which is fine... until someone else comes along and changes "data"'s type (defined way up at the start of the function) to, say, mpc23s08_driver_data_2. Then someone would need to go through all the code and find all instances of

sizeof (struct mpc23s08_driver_data)

and change all of them to

    sizeof (struct mpc23s08_driver_data_2)

As you might guess, this is prone to error. However, if the original developer used

sizeof (*data)

throughout their code, no such subsequent changes would be needed. The compiler is smart enough to figure out that *data is an object of struct mpc23s08_driver_data and use that in its sizeof calculation. If/when "data"'s definition gets changed, the compiler is then able to use mpc23s08_driver_data_2 in its calculation, without any necessary subsequent changes to the code.