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Asked: 2026-06-09T09:16:10+00:00

I am using this function to copy some structures to the kernel. But, the

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I am using this function to copy some structures to the kernel.
But, the problem is that I have to copy three data structures which are part of a bigger data structure. NOTE: the 3 data structures are contiguous in the bigger data structure.

SO, In my copy user function I pass the pointer of the 1st data structure and give the length of all the 3 data structure. But, when I go to user-space and print the 1st element of the 2nd data structure it gives some other value.

SO, what am I doing wrong.
As, a solution I made 3 copt_to_user calls and to my surprise it works fine. Its the problem when I make a single copy_to_user call.

Please, let me know what could be the reason.

Hey guys thanks for the answer it was a alignment issue , but, going further, if I want to pad an internal structure how do I do it..?
Example-

structure d{

struct b;

struct c; //I want to make this structure a padded one, how to go about it?

struct d;

}

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1 Answer

  1. Editorial Team

    As mentioned in the comments, this really seems to be an alignment problem. Gcc will probably add some padding between the structures a, b and c in struct d. Depending on how you instantiated the one in userland, it could be a problem. You can force gcc to not generate padding, using __atribute__ ((packed)) on your structure, but unless this structure maps to hardware registers, it’s usually a bad idea as it will lead to worse performance when accessing fields of that structure.

    Another possible problem would be if your kernel is 64 bits and your userland program is 32 bits, in this case you need to use fixed size types to be sure to have the same layout.

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