[postgis-devel] Memory context question

Stephen Woodbridge woodbri at swoodbridge.com
Tue Apr 23 07:32:58 PDT 2013

Hi Guys,

I have created the function below. At the end of the function I add a 
new standardizer to my cache. As part of doing this I need to copy the 
table names and attach them to the cache entry and do this with pstrdup().

Because pstrdup() does a palloc(), I think this needs to be done in the 
memory context of the cache entry. Am I correct in assuming that I 
should do a MemoryContextSwitchTo(STDMemoryContext) before the pstrdup() 
and then switch back to the old_context afterwards?

static void
AddToStdPortalCache(StdPortalCache *STDCache, char *lextab, char 
*gaztab, char *rultab)
     MemoryContext STDMemoryContext;

     std = CreateStd(lextab, gaztab, rultab);
     if (!std)
             "AddToStdPortalCache: could not create address standardizer 
for '%s', '%s', '%s'", lextab, gaztab, rultab);

     /* if the NextSlot in the cache is used, then delete it */
     if (STDCache->StdCache[STDCache->NextSlot].std != NULL) {
         StdCacheItem *ci = &STDCache->StdCache[STDCache->NextSlot];
         DBG("Removing item from STD cache ('%s', '%s', '%s') index %d", 
ci->lextab, ci->gaztab, ci->rultab, STDCache->NextSlot);

     DBG("Adding item to STD cache ('%s', '%s', '%s') index %d", lextab, 
gaztab, rultab, STDCache->NextSlot);

     STDMemoryContext = MemoryContextCreate(T_AllocSetContext, 8192,
                                            "PAGC STD Memory Context");

     /* Create the backend hash if it doesn't already exist */
     if (!StdHash)
         StdHash = CreateStdHash();

      * Add the MemoryContext to the backend hash so we can
      * clean up upon portal shutdown
     DBG("Adding standardizer obj (%p) to hash table with MemoryContext 
key (%p)", std, STDMemoryContext);

     AddStdHashEntry(STDMemoryContext, std);

     // TODO: do we need to change memory context before the pstrdup ????

     STDCache->StdCache[STDCache->NextSlot].lextab = pstrdup(lextab);
     STDCache->StdCache[STDCache->NextSlot].gaztab = pstrdup(gaztab);
     STDCache->StdCache[STDCache->NextSlot].rultab = pstrdup(rultab);
     STDCache->StdCache[STDCache->NextSlot].std = std;
     STDCache->StdCache[STDCache->NextSlot].std_mcxt = STDMemoryContext;
     STDCache->NextSlot = (STDCache->NextSlot + 1) % STD_CACHE_ITEMS;

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