Question

I m working with Rad Hat 8.0, trying to make changes to the kernel, and I m at the compilation stage. I have a header in include/linux where I define wrapper functions, and they use errno. I included errno.h using #include <errno.h>. When I try to compile, it tells me "errno.h no such file or directory". When I try #include <linux/errno.h> it finds it but complains that I did not declare errno the variable before use. I looked at errno.h and it really doesn t have it declared, which is confusing because I was under the impression that this is where it is defined.

Am I looking in the wrong places? How do I make use of errno?

Answer 1

For kernel code, #include <linux/errno.h>.

The extern declaration for errno is in #include <linux/unistd.h>.

Answer 2

In accordance to this, errno variable is set automaticly when you return a negative value.

Example:

asmlinkage long sys_foo() {
    ...
    if(error)
        return -(errorcode);
    ...
}

In this case the returning result in userspace will be -1 and errno variable will have value of errorcode.

Answer 3

errno is defined as an extern in

/usr/src/lib/linux/errno.c

you have to compile this file with yours

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