can a linux kernel module call libc functions?

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# 1  
Old 11-12-2011
can a linux kernel module call libc functions?

can a linux kernel module call libc functions, such as printf(), strcpy(), etc...?
# 2  
Old 11-12-2011
No, absolutely not. There are internal, kernel-specific functions for some of those. If you're asking this question, you must absolutely read the O'Reilley published book on writing Linux device drivers.
# 3  
Old 11-14-2011
How can you do printf() when you don't even have terminals or files? The kernel is what makes libc possible.

Last edited by Corona688; 11-14-2011 at 01:39 PM..
# 4  
Old 11-15-2011
I mean in a DIY kernel module that is not significant to the kernel.
How printk print messages when it have no terminals?
Can the kernel module call syscalls?
# 5  
Old 11-15-2011
printk goes to a buffer inside the kernel. Things inside userspace read from it (try dmesg). The log system also reads from it. Nothing in this scheme requires the kernel to have access to system calls or libc.

It's possible to use a hardware serial port(probably not a USB one) in an extremely limited way by setting it as an output for printk messages with options on the kernel commandline. Again, neither libc nor system calls are used.

The kernel really, genuinely doesn't have libc or system calls. The kernel is what makes them possible, if you're not running in user mode you don't have them. We're not joking, kidding, or lying to you.
# 6  
Old 11-16-2011
I know it is the kernel which makes syscalls and libc possible.
But, what makes the kernel can not call syscalls which inside itself?
# 7  
Old 11-16-2011
But, what makes the kernel can not call syscalls which inside itself?
(If you received mail of this post, it was premature. Disregards)

Linux (AFAIK) does not support a 're-entrant' model. Once you're inside the kernel, you can't thread a new process, or go again into the kernel (you're already there). Allowing a module to do a syscall would make things incredibly complicated and would more than often result in a deadlock or worse -- a infinite recursive loop, filling up the stack.

Some UNIX's (Solaris 2.5+) are re-entrant and theoretically could support a kernel module making a syscall, but again, the problems are similar - extreme care has to be taken to ensure that bad things dont happen.

If you want to have a kernel module do some nifty things, the common practice is to have it communicate with a userland process via a pipe or ioctl. That is, make your module act as a dummy device driver which fills the dummy device with data that it wants to send to the userland process. The userland process is then blocking-read on this device and acts appropriately when the data is available.

There is also the issue of calling symantics. On most devices (ie, CPUs), invoking a system call involves setting registers and triggering an interrupt, which the kernel then handles. If your code is running in the Kernel, this interrupt has to be masked to prevent another system call from interrupting the currently executing one. (Unless you're a re-entrant kernel like Solaris; then your code has to be re-entrant too). In other instances, the system call entry point is through a specific CPU instruction -- one which requires a mode bit to indicate the currently executing process is in "userland" mode. Once the control is handed to the kernel, the kernel sets that bit accordingly, so once again, the CPU wont let a kernel thread invoke a kernel thread.

Last edited by otheus; 11-16-2011 at 05:40 AM.. Reason: first response was already answered by corona
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