source: branches/minix3-book/servers/pm/break.c@ 26

/* The MINIX model of memory allocation reserves a fixed amount of memory for
 * the combined text, data, and stack segments.  The amount used for a child
 * process created by FORK is the same as the parent had.  If the child does
 * an EXEC later, the new size is taken from the header of the file EXEC'ed.
 *
 * The layout in memory consists of the text segment, followed by the data
 * segment, followed by a gap (unused memory), followed by the stack segment.
 * The data segment grows upward and the stack grows downward, so each can
 * take memory from the gap.  If they meet, the process must be killed.  The
 * procedures in this file deal with the growth of the data and stack segments.
 *
 * The entry points into this file are:
 *   do_brk:      BRK/SBRK system calls to grow or shrink the data segment
 *   adjust:      see if a proposed segment adjustment is allowed
 *   size_ok:     see if the segment sizes are feasible
 */

#include "pm.h"
#include <signal.h>
#include "mproc.h"
#include "param.h"

#define DATA_CHANGED       1    /* flag value when data segment size changed */
#define STACK_CHANGED      2    /* flag value when stack size changed */

/*===========================================================================*
 *                              do_brk                                       *
 *===========================================================================*/
PUBLIC int do_brk()
{
/* Perform the brk(addr) system call.
 *
 * The call is complicated by the fact that on some machines (e.g., 8088),
 * the stack pointer can grow beyond the base of the stack segment without
 * anybody noticing it.
 * The parameter, 'addr' is the new virtual address in D space.
 */

  register struct mproc *rmp;
  int r;
  vir_bytes v, new_sp;
  vir_clicks new_clicks;

  rmp = mp;
  v = (vir_bytes) m_in.addr;
  new_clicks = (vir_clicks) ( ((long) v + CLICK_SIZE - 1) >> CLICK_SHIFT);
  if (new_clicks < rmp->mp_seg[D].mem_vir) {
        rmp->mp_reply.reply_ptr = (char *) -1;
        return(ENOMEM);
  }
  new_clicks -= rmp->mp_seg[D].mem_vir;
  if ((r=get_stack_ptr(who, &new_sp)) != OK)    /* ask kernel for sp value */
        panic(__FILE__,"couldn't get stack pointer", r);
  r = adjust(rmp, new_clicks, new_sp);
  rmp->mp_reply.reply_ptr = (r == OK ? m_in.addr : (char *) -1);
  return(r);                    /* return new address or -1 */
}

/*===========================================================================*
 *                              adjust                                       *
 *===========================================================================*/
PUBLIC int adjust(rmp, data_clicks, sp)
register struct mproc *rmp;     /* whose memory is being adjusted? */
vir_clicks data_clicks;         /* how big is data segment to become? */
vir_bytes sp;                   /* new value of sp */
{
/* See if data and stack segments can coexist, adjusting them if need be.
 * Memory is never allocated or freed.  Instead it is added or removed from the
 * gap between data segment and stack segment.  If the gap size becomes
 * negative, the adjustment of data or stack fails and ENOMEM is returned.
 */

  register struct mem_map *mem_sp, *mem_dp;
  vir_clicks sp_click, gap_base, lower, old_clicks;
  int changed, r, ft;
  long base_of_stack, delta;    /* longs avoid certain problems */

  mem_dp = &rmp->mp_seg[D];     /* pointer to data segment map */
  mem_sp = &rmp->mp_seg[S];     /* pointer to stack segment map */
  changed = 0;                  /* set when either segment changed */

  if (mem_sp->mem_len == 0) return(OK); /* don't bother init */

  /* See if stack size has gone negative (i.e., sp too close to 0xFFFF...) */
  base_of_stack = (long) mem_sp->mem_vir + (long) mem_sp->mem_len;
  sp_click = sp >> CLICK_SHIFT; /* click containing sp */
  if (sp_click >= base_of_stack) return(ENOMEM);        /* sp too high */

  /* Compute size of gap between stack and data segments. */
  delta = (long) mem_sp->mem_vir - (long) sp_click;
  lower = (delta > 0 ? sp_click : mem_sp->mem_vir);

  /* Add a safety margin for future stack growth. Impossible to do right. */
#define SAFETY_BYTES  (384 * sizeof(char *))
#define SAFETY_CLICKS ((SAFETY_BYTES + CLICK_SIZE - 1) / CLICK_SIZE)
  gap_base = mem_dp->mem_vir + data_clicks + SAFETY_CLICKS;
  if (lower < gap_base) return(ENOMEM); /* data and stack collided */

  /* Update data length (but not data orgin) on behalf of brk() system call. */
  old_clicks = mem_dp->mem_len;
  if (data_clicks != mem_dp->mem_len) {
        mem_dp->mem_len = data_clicks;
        changed |= DATA_CHANGED;
  }

  /* Update stack length and origin due to change in stack pointer. */
  if (delta > 0) {
        mem_sp->mem_vir -= delta;
        mem_sp->mem_phys -= delta;
        mem_sp->mem_len += delta;
        changed |= STACK_CHANGED;
  }

  /* Do the new data and stack segment sizes fit in the address space? */
  ft = (rmp->mp_flags & SEPARATE);
  r = (rmp->mp_seg[D].mem_vir + rmp->mp_seg[D].mem_len > 
          rmp->mp_seg[S].mem_vir) ? ENOMEM : OK;
  if (r == OK) {
        if (changed) sys_newmap((int)(rmp - mproc), rmp->mp_seg);
        return(OK);
  }

  /* New sizes don't fit or require too many page/segment registers. Restore.*/
  if (changed & DATA_CHANGED) mem_dp->mem_len = old_clicks;
  if (changed & STACK_CHANGED) {
        mem_sp->mem_vir += delta;
        mem_sp->mem_phys += delta;
        mem_sp->mem_len -= delta;
  }
  return(ENOMEM);
}