Вы можете проверить / proc // stat файловой системы, в соответствии с http://www.kernel.org/doc/Documentation/filesystems/proc.txt, вам нужно просто проверить флаг task_cpu.
Как пример без правильных типов и проверки ошибок:
struct pstat
{
int pid; //process id
char tcomm[256];//filename of the executable
char state[2]; //state (R is running, S is sleeping, D is sleeping in an
//uninterruptible wait, Z is zombie, T is traced or stopped)
int ppid;// process id of the parent process
int pgrp;// pgrp of the process
int sid;// session id
int tty_nr;// tty the process uses
int tty_pgrp;// pgrp of the tty
int flags;// task flags
int min_flt;// number of minor faults
int cmin_flt;// number of minor faults with child's
int maj_flt;// number of major faults
int cmaj_flt;// number of major faults with child's
int utime;// user mode jiffies
int stime;// kernel mode jiffies
int cutime;// user mode jiffies with child's
int cstime;// kernel mode jiffies with child's
int priority;// priority level
int nice;// nice level
int num_threads;// number of threads
int it_real_value;// (obsolete, always 0)
int start_time;// time the process started after system boot
int vsize;// virtual memory size
int rss;// resident set memory size
int rsslim;// current limit in bytes on the rss
int start_code;// address above which program text can run
int end_code;// address below which program text can run
int start_stack;// address of the start of the stack
int esp;// current value of ESP
int eip;// current value of EIP
int pending;// bitmap of pending signals
int blocked;// bitmap of blocked signals
int sigign;// bitmap of ignored signals
int sigcatch;// bitmap of catched signals
int wchan;// address where process went to sleep
int i0;// (place holder)
int i1;// (place holder)
int exit_signal;// signal to send to parent thread on exit
int task_cpu;// which CPU the task is scheduled on
int rt_priority;// realtime priority
int policy;// scheduling policy (man sched_setscheduler)
int blkio_ticks;// time spent waiting for block IO
int gtime;// guest time of the task in jiffies
int cgtime;// guest time of the task children in jiffies
} p ;
int main()
{
char name[256];
char state[8];
FILE* f = fopen("/proc/self/stat", "r");
fscanf(f, "%d%s%s%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d",
&p.pid, &p.tcomm, &p.state, &p.ppid, &p.pgrp, &p.sid, &p.tty_nr, &p.tty_pgrp, &p.flags,
&p.min_flt, &p.cmin_flt, &p.maj_flt, &p.cmaj_flt, &p.utime, &p.stime, &p.cutime, &p.cstime,
&p.priority, &p.nice, &p.num_threads, &p.it_real_value, &p.start_time, &p.vsize, &p.rss,
&p.rsslim, &p.start_code, &p.end_code, &p.start_stack, &p.esp, &p.eip, &p.pending, &p.blocked,
&p.sigign, &p.sigcatch, &p.wchan, &p.i0, &p.i1, &p.exit_signal, &p.task_cpu, &p.rt_priority, &p.policy,
&p.blkio_ticks, &p.gtime, &p.cgtime);
printf("CPU %d\n", p.task_cpu);
return 0;
}