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tclPreserve.c

/*
 * tclPreserve.c --
 *
 *    This file contains a collection of functions that are used to make
 *    sure that widget records and other data structures aren't reallocated
 *    when there are nested functions that depend on their existence.
 *
 * Copyright (c) 1991-1994 The Regents of the University of California.
 * Copyright (c) 1994-1998 Sun Microsystems, Inc.
 *
 * See the file "license.terms" for information on usage and redistribution of
 * this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * RCS: @(#) $Id: tclPreserve.c,v 1.10 2007/03/21 18:02:51 dgp Exp $
 */

#include "tclInt.h"

/*
 * The following data structure is used to keep track of all the Tcl_Preserve
 * calls that are still in effect. It grows as needed to accommodate any
 * number of calls in effect.
 */

typedef struct {
    ClientData clientData;    /* Address of preserved block. */
    int refCount;       /* Number of Tcl_Preserve calls in effect for
                         * block. */
    int mustFree;       /* Non-zero means Tcl_EventuallyFree was
                         * called while a Tcl_Preserve call was in
                         * effect, so the structure must be freed when
                         * refCount becomes zero. */
    Tcl_FreeProc *freeProc;   /* Function to call to free. */
} Reference;

/*
 * Global data structures used to hold the list of preserved data references.
 * These variables are protected by "preserveMutex".
 */

static Reference *refArray = NULL;  /* First in array of references. */
static int spaceAvl = 0;      /* Total number of structures available at
                         * *firstRefPtr. */
static int inUse = 0;         /* Count of structures currently in use in
                         * refArray. */
TCL_DECLARE_MUTEX(preserveMutex)/* To protect the above statics */

#define INITIAL_SIZE    2     /* Initial number of reference slots to make */

/*
 * The following data structure is used to keep track of whether an arbitrary
 * block of memory has been deleted. This is used by the TclHandle code to
 * avoid the more time-expensive algorithm of Tcl_Preserve(). This mechanism
 * is mainly used when we have lots of references to a few big, expensive
 * objects that we don't want to live any longer than necessary.
 */

typedef struct HandleStruct {
    void *ptr;                /* Pointer to the memory block being tracked.
                         * This field will become NULL when the memory
                         * block is deleted. This field must be the
                         * first in the structure. */
#ifdef TCL_MEM_DEBUG
    void *ptr2;               /* Backup copy of the above pointer used to
                         * ensure that the contents of the handle are
                         * not changed by anyone else. */
#endif
    int refCount;       /* Number of TclHandlePreserve() calls in
                         * effect on this handle. */
} HandleStruct;

/*
 *----------------------------------------------------------------------
 *
 * TclFinalizePreserve --
 *
 *    Called during exit processing to clean up the reference array.
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    Frees the storage of the reference array.
 *
 *----------------------------------------------------------------------
 */

      /* ARGSUSED */
void
TclFinalizePreserve(void)
{
    Tcl_MutexLock(&preserveMutex);
    if (spaceAvl != 0) {
        ckfree((char *) refArray);
        refArray = NULL;
        inUse = 0;
        spaceAvl = 0;
    }
    Tcl_MutexUnlock(&preserveMutex);
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_Preserve --
 *
 *    This function is used by a function to declare its interest in a
 *    particular block of memory, so that the block will not be reallocated
 *    until a matching call to Tcl_Release has been made.
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    Information is retained so that the block of memory will not be freed
 *    until at least the matching call to Tcl_Release.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_Preserve(
    ClientData clientData)    /* Pointer to malloc'ed block of memory. */
{
    Reference *refPtr;
    int i;

    /*
     * See if there is already a reference for this pointer. If so, just
     * increment its reference count.
     */

    Tcl_MutexLock(&preserveMutex);
    for (i=0, refPtr=refArray ; i<inUse ; i++, refPtr++) {
      if (refPtr->clientData == clientData) {
          refPtr->refCount++;
          Tcl_MutexUnlock(&preserveMutex);
          return;
      }
    }

    /*
     * Make a reference array if it doesn't already exist, or make it bigger
     * if it is full.
     */

    if (inUse == spaceAvl) {
      spaceAvl = spaceAvl ? 2*spaceAvl : INITIAL_SIZE;
      refArray = (Reference *) ckrealloc((char *) refArray,
            spaceAvl * sizeof(Reference));
    }

    /*
     * Make a new entry for the new reference.
     */

    refPtr = &refArray[inUse];
    refPtr->clientData = clientData;
    refPtr->refCount = 1;
    refPtr->mustFree = 0;
    refPtr->freeProc = TCL_STATIC;
    inUse += 1;
    Tcl_MutexUnlock(&preserveMutex);
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_Release --
 *
 *    This function is called to cancel a previous call to Tcl_Preserve,
 *    thereby allowing a block of memory to be freed (if no one else cares
 *    about it).
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    If Tcl_EventuallyFree has been called for clientData, and if no other
 *    call to Tcl_Preserve is still in effect, the block of memory is freed.
 *
 *----------------------------------------------------------------------
 */

void
Tcl_Release(
    ClientData clientData)    /* Pointer to malloc'ed block of memory. */
{
    Reference *refPtr;
    int i;

    Tcl_MutexLock(&preserveMutex);
    for (i=0, refPtr=refArray ; i<inUse ; i++, refPtr++) {
      int mustFree;
      Tcl_FreeProc *freeProc;

      if (refPtr->clientData != clientData) {
          continue;
      }

      if (--refPtr->refCount != 0) {
          Tcl_MutexUnlock(&preserveMutex);
          return;
      }

      /*
       * Must remove information from the slot before calling freeProc to
       * avoid reentrancy problems if the freeProc calls Tcl_Preserve on the
       * same clientData. Copy down the last reference in the array to
       * overwrite the current slot.
       */

      freeProc = refPtr->freeProc;
      mustFree = refPtr->mustFree;
      inUse--;
      if (i < inUse) {
          refArray[i] = refArray[inUse];
      }

      /*
       * Now committed to disposing the data. But first, we've patched up
       * all the global data structures so we should release the mutex now.
       * Only then should we dabble around with potentially-slow memory
       * managers...
       */

      Tcl_MutexUnlock(&preserveMutex);
      if (mustFree) {
          if (freeProc == TCL_DYNAMIC) {
            ckfree((char *) clientData);
          } else {
            (*freeProc)((char *) clientData);
          }
      }
      return;
    }
    Tcl_MutexUnlock(&preserveMutex);

    /*
     * Reference not found. This is a bug in the caller.
     */

    Tcl_Panic("Tcl_Release couldn't find reference for 0x%x", clientData);
}

/*
 *----------------------------------------------------------------------
 *
 * Tcl_EventuallyFree --
 *
 *    Free up a block of memory, unless a call to Tcl_Preserve is in effect
 *    for that block. In this case, defer the free until all calls to
 *    Tcl_Preserve have been undone by matching calls to Tcl_Release.
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    Ptr may be released by calling free().
 *
 *----------------------------------------------------------------------
 */

void
Tcl_EventuallyFree(
    ClientData clientData,    /* Pointer to malloc'ed block of memory. */
    Tcl_FreeProc *freeProc)   /* Function to actually do free. */
{
    Reference *refPtr;
    int i;

    /*
     * See if there is a reference for this pointer. If so, set its "mustFree"
     * flag (the flag had better not be set already!).
     */

    Tcl_MutexLock(&preserveMutex);
    for (i = 0, refPtr = refArray; i < inUse; i++, refPtr++) {
      if (refPtr->clientData != clientData) {
          continue;
      }
      if (refPtr->mustFree) {
          Tcl_Panic("Tcl_EventuallyFree called twice for 0x%x",
                clientData);
        }
        refPtr->mustFree = 1;
      refPtr->freeProc = freeProc;
      Tcl_MutexUnlock(&preserveMutex);
        return;
    }
    Tcl_MutexUnlock(&preserveMutex);

    /*
     * No reference for this block.  Free it now.
     */

    if (freeProc == TCL_DYNAMIC) {
      ckfree((char *) clientData);
    } else {
      (*freeProc)((char *)clientData);
    }
}

/*
 *---------------------------------------------------------------------------
 *
 * TclHandleCreate --
 *
 *    Allocate a handle that contains enough information to determine if an
 *    arbitrary malloc'd block has been deleted. This is used to avoid the
 *    more time-expensive algorithm of Tcl_Preserve().
 *
 * Results:
 *    The return value is a TclHandle that refers to the given malloc'd
 *    block. Doubly dereferencing the returned handle will give back the
 *    pointer to the block, or will give NULL if the block has been deleted.
 *
 * Side effects:
 *    The caller must keep track of this handle (generally by storing it in
 *    a field in the malloc'd block) and call TclHandleFree() on this handle
 *    when the block is deleted. Everything else that wishes to keep track
 *    of whether the malloc'd block has been deleted should use calls to
 *    TclHandlePreserve() and TclHandleRelease() on the associated handle.
 *
 *---------------------------------------------------------------------------
 */

TclHandle
TclHandleCreate(
    void *ptr)                /* Pointer to an arbitrary block of memory to
                         * be tracked for deletion. Must not be
                         * NULL. */
{
    HandleStruct *handlePtr;

    handlePtr = (HandleStruct *) ckalloc(sizeof(HandleStruct));
    handlePtr->ptr = ptr;
#ifdef TCL_MEM_DEBUG
    handlePtr->ptr2 = ptr;
#endif
    handlePtr->refCount = 0;
    return (TclHandle) handlePtr;
}

/*
 *---------------------------------------------------------------------------
 *
 * TclHandleFree --
 *
 *    Called when the arbitrary malloc'd block associated with the handle is
 *    being deleted. Modifies the handle so that doubly dereferencing it
 *    will give NULL. This informs any user of the handle that the block of
 *    memory formerly referenced by the handle has been freed.
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    If nothing is referring to the handle, the handle will be reclaimed.
 *
 *---------------------------------------------------------------------------
 */

void
TclHandleFree(
    TclHandle handle)         /* Previously created handle associated with a
                         * malloc'd block that is being deleted. The
                         * handle is modified so that doubly
                         * dereferencing it will give NULL. */
{
    HandleStruct *handlePtr;

    handlePtr = (HandleStruct *) handle;
#ifdef TCL_MEM_DEBUG
    if (handlePtr->refCount == 0x61616161) {
      Tcl_Panic("using previously disposed TclHandle %x", handlePtr);
    }
    if (handlePtr->ptr2 != handlePtr->ptr) {
      Tcl_Panic("someone has changed the block referenced by the handle %x\nfrom %x to %x",
            handlePtr, handlePtr->ptr2, handlePtr->ptr);
    }
#endif
    handlePtr->ptr = NULL;
    if (handlePtr->refCount == 0) {
      ckfree((char *) handlePtr);
    }
}

/*
 *---------------------------------------------------------------------------
 *
 * TclHandlePreserve --
 *
 *    Declare an interest in the arbitrary malloc'd block associated with
 *    the handle.
 *
 * Results:
 *    The return value is the handle argument, with its ref count
 *    incremented.
 *
 * Side effects:
 *    For each call to TclHandlePreserve(), there should be a matching call
 *    to TclHandleRelease() when the caller is no longer interested in the
 *    malloc'd block associated with the handle.
 *
 *---------------------------------------------------------------------------
 */

TclHandle
TclHandlePreserve(
    TclHandle handle)         /* Declare an interest in the block of memory
                         * referenced by this handle. */
{
    HandleStruct *handlePtr;

    handlePtr = (HandleStruct *) handle;
#ifdef TCL_MEM_DEBUG
    if (handlePtr->refCount == 0x61616161) {
      Tcl_Panic("using previously disposed TclHandle %x", handlePtr);
    }
    if ((handlePtr->ptr != NULL) && (handlePtr->ptr != handlePtr->ptr2)) {
      Tcl_Panic("someone has changed the block referenced by the handle %x\nfrom %x to %x",
            handlePtr, handlePtr->ptr2, handlePtr->ptr);
    }
#endif
    handlePtr->refCount++;

    return handle;
}

/*
 *---------------------------------------------------------------------------
 *
 * TclHandleRelease --
 *
 *    This function is called to release an interest in the malloc'd block
 *    associated with the handle.
 *
 * Results:
 *    None.
 *
 * Side effects:
 *    The ref count of the handle is decremented. If the malloc'd block has
 *    been freed and if no one is using the handle any more, the handle will
 *    be reclaimed.
 *
 *---------------------------------------------------------------------------
 */

void
TclHandleRelease(
    TclHandle handle)         /* Unregister interest in the block of memory
                         * referenced by this handle. */
{
    HandleStruct *handlePtr;

    handlePtr = (HandleStruct *) handle;
#ifdef TCL_MEM_DEBUG
    if (handlePtr->refCount == 0x61616161) {
      Tcl_Panic("using previously disposed TclHandle %x", handlePtr);
    }
    if ((handlePtr->ptr != NULL) && (handlePtr->ptr != handlePtr->ptr2)) {
      Tcl_Panic("someone has changed the block referenced by the handle %x\nfrom %x to %x",
            handlePtr, handlePtr->ptr2, handlePtr->ptr);
    }
#endif
    handlePtr->refCount--;
    if ((handlePtr->refCount == 0) && (handlePtr->ptr == NULL)) {
      ckfree((char *) handlePtr);
    }
}

/*
 * Local Variables:
 * mode: c
 * c-basic-offset: 4
 * fill-column: 78
 * End:
 */

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