error: implicit declaration of function 'rdtscl' [-Werror=implicit-function-declaration] (but no error when running on older kernel version)

Question

I'm new to all of this and am trying to compile a program on a virtual machine with kernel version 3.10.0-957.el7.x86_64. But I get this error:

/home/../../../isr_demux.c: In function 'demux0_isr':
/home/../../../ isr_demux.c: 782:3: error: implicit declaration of function 'rdtscl' [-Werror=implicit-function-declaration]`

I pulled up isr_demux.c and saw that the snippet of code it said was causing the error is this:

The third to last line is line 782. What's interesting is that this same file compiles fine on a virtual machine with an older kernel version 3.10.0-327.el7.x86_64.

I searched online a lot but am truly stuck. I saw another post and checked my header files, and included linux/proc_fs.h along with many other ones. Someone said I could be missing a function prototype or definition, but adding a prototype did not fix this, and defining the function led to a redefinition error.

Any ideas about whether I'm still missing a header file somewhere? Or if the problem is with the newer kernel itself and I might need to find some sort of patch? Thanks!

Answer 1

The website elixir.bootlin.com is pretty useful for these things. The following link is a search across the entire kernel sources tree for version 3.10.108 for the symbol rdtscl.

https://elixir.bootlin.com/linux/v3.10.108/ident/rdtscl

rdtscl is only defined as a macro, so looks like the macro is not defined where you are getting your error about implicit declaration of rdtscl.

The two places where rdtscl is defined as a macro are: https://elixir.bootlin.com/linux/v3.10.108/source/arch/x86/include/asm/msr.h#L182 https://elixir.bootlin.com/linux/v3.10.108/source/arch/x86/include/asm/paravirt.h#L182

So the fix is to include the correct headers files if you are building for x86 (the headers defining those macros are in arch/x86), or replace the call to rdtscl with something else.