text: #include "types.h"
  #include "x86.h"
  #include "defs.h"
  #include "param.h"
  #include "memlayout.h"
  #include "mmu.h"
  #include "proc.h"
  
  int
  sys_fork(void)
  {
    return fork();
  }
  
  int
  sys_exit(void)
  {
    exit();
    return 0;  // not reached
  }
  
  int
  sys_wait(void)
  {
    return wait();
  }
  
  int
  sys_kill(void)
  {
    int pid;
  
    if(argint(0, &pid) < 0)
      return -1;
    return kill(pid);
  }
  
  int
  sys_getpid(void)
  {
    return proc->pid;
  }
  
  int
  sys_sbrk(void)
  {
    int addr;
    int n;
  
    if(argint(0, &n) < 0)
      return -1;
    addr = proc->sz;
    if(growproc(n) < 0)
      return -1;
    return addr;
  }
  
  int
  sys_sleep(void)
  {
    int n;
    uint ticks0;
  
    if(argint(0, &n) < 0)
      return -1;
    acquire(&tickslock);
    ticks0 = ticks;
    while(ticks - ticks0 < n){
      if(proc->killed){
        release(&tickslock);
        return -1;
      }
      sleep(&ticks, &tickslock);
    }
    release(&tickslock);
    return 0;
  }
  
  // return how many clock tick interrupts have occurred
  // since start.
  int
  sys_uptime(void)
  {
    uint xticks;
  
    acquire(&tickslock);
    xticks = ticks;
    release(&tickslock);
    return xticks;
  }
  
  
  // set custom signal handler
  // returns the previous signal handler
  sighandler_t
  sys_signal(void)
  {
    int signum;
    sighandler_t handler;
  
    if (argint(0, &signum) < 0)
      return (sighandler_t)-1;
    if(argfunc(1, &handler))
      return (sighandler_t)-1;
  
    return signal(signum, handler);
  }
  