In C, can't change a struct attribute except using #define val

Question

In Minix 3.1.2a I've a struct "struct proc" where the PCB of any process stored, but I've a problem when adding new attribute "p_currenthash" in the code below to this struct. I can't change its value except using a constant defined by #define directive; otherwise, the system stops responding. For clarity here is the structure:

struct proc {
  struct stackframe_s p_reg;    /* process' registers saved in stack frame */

#if (CHIP == INTEL)
  reg_t p_ldt_sel;      /* selector in gdt with ldt base and limit */
  struct segdesc_s p_ldt[2+NR_REMOTE_SEGS]; /* CS, DS and remote segments */
#endif 

#if (CHIP == M68000)
/* M68000 specific registers and FPU details go here. */
#endif 

  proc_nr_t p_nr;       /* number of this process (for fast access) */
  struct priv *p_priv;      /* system privileges structure */
  short p_rts_flags;        /* process is runnable only if zero */
  short p_misc_flags;       /* flags that do suspend the process */

  char p_priority;      /* current scheduling priority */
  char p_max_priority;      /* maximum scheduling priority */
  char p_ticks_left;        /* number of scheduling ticks left */
  char p_quantum_size;      /* quantum size in ticks */

  struct mem_map p_memmap[NR_LOCAL_SEGS];   /* memory map (T, D, S) */

  clock_t p_user_time;      /* user time in ticks */
  clock_t p_sys_time;       /* sys time in ticks */

  struct proc *p_nextready; /* pointer to next ready process */
  struct proc *p_caller_q;  /* head of list of procs wishing to send */
  struct proc *p_q_link;    /* link to next proc wishing to send */
  message *p_messbuf;       /* pointer to passed message buffer */
  int p_getfrom_e;      /* from whom does process want to receive? */
  int p_sendto_e;       /* to whom does process want to send? */

  sigset_t p_pending;       /* bit map for pending kernel signals */



  char p_name[P_NAME_LEN];  /* name of the process, including \0 */

  int p_endpoint;       /* endpoint number, generation-aware */

#if DEBUG_SCHED_CHECK
  int p_ready, p_found;
#endif

  char p_currenthash; /* hash */

};

Now, suppose I want to set its value. Initially I use the constant defined below.

#define NONE -1

register struct proc *rp;

rp->p_currenthash=NONE;

That works fine, but this: rp->p_currenthash=0 ; will cause the program to stop responding.

Any suggestions will be appreciated

Here's the main initialization in main():

/* Start the ball rolling. */
  struct boot_image *ip;    /* boot image pointer */
  register struct proc *rp; /* process pointer */
  register struct priv *sp; /* privilege structure pointer */
  register int i, s;
  int hdrindex;         /* index to array of a.out headers */
  phys_clicks text_base;
  vir_clicks text_clicks, data_clicks;
  reg_t ktsb;           /* kernel task stack base */
  struct exec e_hdr;        /* for a copy of an a.out header */

  /* Initialize the interrupt controller. */
  intr_init(1);

  /* Clear the process table. Anounce each slot as empty and set up mappings 
   * for proc_addr() and proc_nr() macros. Do the same for the table with 
   * privilege structures for the system processes. 
   */
  for (rp = BEG_PROC_ADDR, i = -NR_TASKS; rp < END_PROC_ADDR; ++rp, ++i) {
    rp->p_rts_flags = SLOT_FREE;        /* initialize free slot */
    rp->p_nr = i;               /* proc number from ptr */

    rp->p_currenthash=NONE;

    rp->p_endpoint = _ENDPOINT(0, rp->p_nr); /* generation no. 0 */

        (pproc_addr + NR_TASKS)[i] = rp;        /* proc ptr from number */
  }
  for (sp = BEG_PRIV_ADDR, i = 0; sp < END_PRIV_ADDR; ++sp, ++i) {
    sp->s_proc_nr = NONE;           /* initialize as free */
    sp->s_id = i;               /* priv structure index */
    ppriv_addr[i] = sp;         /* priv ptr from number */
  }

  /* Set up proc table entries for processes in boot image.  The stacks of the
   * kernel tasks are initialized to an array in data space.  The stacks
   * of the servers have been added to the data segment by the monitor, so
   * the stack pointer is set to the end of the data segment.  All the
   * processes are in low memory on the 8086.  On the 386 only the kernel
   * is in low memory, the rest is loaded in extended memory.
   */

  /* Task stacks. */
  ktsb = (reg_t) t_stack;

  for (i=0; i < NR_BOOT_PROCS; ++i) {
    ip = &image[i];             /* process' attributes */
    rp = proc_addr(ip->proc_nr);        /* get process pointer */
    ip->endpoint = rp->p_endpoint;      /* ipc endpoint */
    rp->p_max_priority = ip->priority;  /* max scheduling priority */
    rp->p_priority = ip->priority;      /* current priority */
    rp->p_quantum_size = ip->quantum;   /* quantum size in ticks */
    rp->p_ticks_left = ip->quantum;     /* current credit */
    strncpy(rp->p_name, ip->proc_name, P_NAME_LEN); /* set process name */
    (void) get_priv(rp, (ip->flags & SYS_PROC));    /* assign structure */
    priv(rp)->s_flags = ip->flags;          /* process flags */
    priv(rp)->s_trap_mask = ip->trap_mask;      /* allowed traps */
    priv(rp)->s_call_mask = ip->call_mask;      /* kernel call mask */
    priv(rp)->s_ipc_to.chunk[0] = ip->ipc_to;   /* restrict targets */
    if (iskerneln(proc_nr(rp))) {       /* part of the kernel? */ 
        if (ip->stksize > 0) {      /* HARDWARE stack size is 0 */
            rp->p_priv->s_stack_guard = (reg_t *) ktsb;
            *rp->p_priv->s_stack_guard = STACK_GUARD;
        }
        ktsb += ip->stksize;    /* point to high end of stack */
        rp->p_reg.sp = ktsb;    /* this task's initial stack ptr */
        text_base = kinfo.code_base >> CLICK_SHIFT;
                    /* processes that are in the kernel */
        hdrindex = 0;       /* all use the first a.out header */
    } else {
        hdrindex = 1 + i-NR_TASKS;  /* servers, drivers, INIT */
    }

    /* The bootstrap loader created an array of the a.out headers at
     * absolute address 'aout'. Get one element to e_hdr.
     */
    phys_copy(aout + hdrindex * A_MINHDR, vir2phys(&e_hdr),
                        (phys_bytes) A_MINHDR);
    /* Convert addresses to clicks and build process memory map */
    text_base = e_hdr.a_syms >> CLICK_SHIFT;
    text_clicks = (e_hdr.a_text + CLICK_SIZE-1) >> CLICK_SHIFT;
    if (!(e_hdr.a_flags & A_SEP)) text_clicks = 0;     /* common I&D */
    data_clicks = (e_hdr.a_total + CLICK_SIZE-1) >> CLICK_SHIFT;
    rp->p_memmap[T].mem_phys = text_base;
    rp->p_memmap[T].mem_len  = text_clicks;
    rp->p_memmap[D].mem_phys = text_base + text_clicks;
    rp->p_memmap[D].mem_len  = data_clicks;
    rp->p_memmap[S].mem_phys = text_base + text_clicks + data_clicks;
    rp->p_memmap[S].mem_vir  = data_clicks; /* empty - stack is in data */

    /* Set initial register values.  The processor status word for tasks 
     * is different from that of other processes because tasks can
     * access I/O; this is not allowed to less-privileged processes 
     */
    rp->p_reg.pc = (reg_t) ip->initial_pc;
    rp->p_reg.psw = (iskernelp(rp)) ? INIT_TASK_PSW : INIT_PSW;

    /* Initialize the server stack pointer. Take it down one word
     * to give crtso.s something to use as "argc".
     */
    if (isusern(proc_nr(rp))) {     /* user-space process? */ 
        rp->p_reg.sp = (rp->p_memmap[S].mem_vir +
                rp->p_memmap[S].mem_len) << CLICK_SHIFT;
        rp->p_reg.sp -= sizeof(reg_t);
    }

    /* Set ready. The HARDWARE task is never ready. */
    if (rp->p_nr != HARDWARE) {
        rp->p_rts_flags = 0;        /* runnable if no flags */
        lock_enqueue(rp);       /* add to scheduling queues */
    } else {
        rp->p_rts_flags = NO_MAP;   /* prevent from running */
    }

    /* Code and data segments must be allocated in protected mode. */
    alloc_segments(rp);
  }

Answer 1

register struct proc *rp;

rp->p_currenthash=NONE;

rp is an uninitialized pointer; it isn't pointing to a valid struct proc object, and so dereferencing it leads to undefined behavior. The fact that this didn't crash when assigning -1 was pure luck. (bad luck, because it misled you to believe you were doing something meaningful)

Answer 2

acutually the problem not solved ,first the "p_currenthash" initialized in the main as shown above ,later in a function called pic_proc ,i've this code :

register struct proc **xpp;         /* iterate over queue */
  register struct proc *rp;         /* process to run */
  int q;
    for (q=0; q < NR_SCHED_QUEUES; q++) 
      {
       if ( (rp = rdy_head[q]) != NIL_PROC)
        {
         for (xpp = &rdy_head[q]; *xpp != NIL_PROC; xpp = &(*xpp)->p_nextready) 
             {
                 if ((*xpp)->p_currenthash==NONE) 
                     {
                         pick_val++;    

                     }
             }
         }
    }

that code works fine ,no inside the if if i changed the attribute "p_currenthash" the problem ocuured as bellow:

 register struct proc **xpp;            /* iterate over queue */
  register struct proc *rp;         /* process to run */
  int q;
    for (q=0; q < NR_SCHED_QUEUES; q++) 
      {
       if ( (rp = rdy_head[q]) != NIL_PROC)
        {
         for (xpp = &rdy_head[q]; *xpp != NIL_PROC; xpp = &(*xpp)->p_nextready) 
             {
                 if ((*xpp)->p_currenthash==NONE) 
                     {
                         pick_val++;    
                        (*xpp)->p_currenthash=1;
                     }
             }
         }
    }

i really can't realize the problem.