I'm trying to implement my own system call on Linux kernel 2.6.37.3 using this guide. http://enzam.wordpress.com/2011/03/2...nel-ubuntu-os/ Here's the code that I have for kernel/mysystemcalls.c It just takes an int for an argument and then spits out the PIDs for the process that is running on that CPU. Code: #include<linux/linkage.h> #include<linux/cpumask.h> asmlinkage long sys_current_pid(int i) { struct rq *rq; int num_cpu; num_cpu = num_online_cpus(); if(i <= 0 || i > num_cpu) return -1; rq = cpu_rq(i); if(rq->curr != NULL) return rq->curr->pid; else return -1; } But on kernel compilation I get this. Code: kernel/mysystemcalls.c: In function 'sys_current_pid': kernel/mysystemcalls.c:12: error: implicit declaration of function 'cpu_rq' kernel/mysystemcalls.c:12: warning: assignment makes pointer from integer without a cast kernel/mysystemcalls.c:13: error: dereferencing pointer to incomplete type kernel/mysystemcalls.c:14: error: dereferencing pointer to incomplete type make[1]: *** [kernel/mysystemcalls.o] Error 1 make[1]: *** Waiting for unfinished jobs.... The definition for cpu_rq() is in kernel/sched.c. Here's the pertinent piece of Makefile in kernel/Makefile Code: # # Makefile for the linux kernel. # obj-y = sched.o fork.o exec_domain.o panic.o printk.o \ cpu.o exit.o itimer.o time.o softirq.o resource.o \ sysctl.o sysctl_binary.o capability.o ptrace.o timer.o user.o \ signal.o sys.o kmod.o workqueue.o pid.o \ rcupdate.o extable.o params.o posix-timers.o \ kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \ hrtimer.o rwsem.o nsproxy.o srcu.o semaphore.o \ notifier.o ksysfs.o pm_qos_params.o sched_clock.o cred.o \ async.o range.o jump_label.o obj-y += mysystemcalls.o I would think that since sched.o(the first c file that gets compiled) has the definition for the cpu_rq() then mysystemcalls.c should be able to see that function. What am I missing? -Tristan