Why C function free() blows up on MVS, but works fine on NT and UNIX?

Here is the function that I'm working on:

long svgParse (STYH styh, char *fileName, char **pOutBuf, long imID,
    long *sizeX, long *sizeY, SS_NLSCharTab *charTab,
    ImageDescPtr imagePtr)
{
  R1H           r1h;
  t_xmlhctx     *xmlhctx = NULL;
  long          rc;
  t_xmlf        *xmlapi;
  void          *xmlh;
  t_ntsnlep     ntsnlep;
  void          *xmltree;
  t_ntsnPub     *root;
  SvgContext    svgCtx;
  int isTransparent;
  unsigned char transpIndex;
  RgbColorType *colorMap;
  unsigned char **screenBuffer;
  int screenWidth;
  int screenHeight;
 
  svgCtx.styh = styh;
  initSvgContext(&svgCtx);
 
  r1h = mbxR(R1H_MBX, MBX_AGN);
  r1h->r1lodctx->r1lodli( r1h->r1lodctx, "XMLH    ",
    (Pfun *)&xmlhctx, &rc );
  if (*xmlhctx == NULL) {
    printf("**** r1lodli() failed\n");
    goto badRet;
  }
 
  xmlapi = (t_xmlf *) fmalloc(sizeof(t_xmlf));
  if (xmlapi == NULL)
    goto badRet;
 
  ntsnLoadLib(r1h, &ntsnlep, &rc);  /* init NTMAS */
  if (rc != 0) {
    printf("**** ntsnLoadLib() failed\n");
    goto badRet;
  }
 
  (*xmlhctx)(xmlapi);
  xmlh = xmlapi->xmlini(r1h, sysctx()->ioh, &ntsnlep, NULL, NULL);
  if (xmlh == NULL) {
    printf("**** xmlini() failed\n");
    goto badRet;
  }
 
  /* xmltree = xmlapi->xmlparse(xmlh, IOH_XML, XML_SOURCE_BUFFER, xmlInput); */
  xmltree = xmlapi->xmlparse(xmlh, IOH_SVG, XML_SOURCE_FILE,
    fileName, 8);
  if (xmltree == NULL) {
    printf("**** xmlparse() failed\n");
    xmlapi->xmldel(xmlh);
    goto badRet;
  }
 
  xmlapi->xmlprtree(xmlh, xmltree, "The XML Tree");
 
  root = xmlapi->GetRealRoot(xmlh, xmltree);
 
  svgWalk(&svgCtx, xmlh, xmlapi, root, 0);
 
  xmlapi->xmlfrtree(xmlh, xmltree);
  xmlapi->xmldel(xmlh);
 
  *sizeX = svgCtx.svgTagAttrib.width;
  *sizeY = svgCtx.svgTagAttrib.height;
 
  imagePtr->xPixels = svgCtx.svgTagAttrib.xPixels;
  imagePtr->yPixels = svgCtx.svgTagAttrib.yPixels;
 
  isTransparent = FALSE;
  transpIndex = 0;
  colorMap = NULL;
  screenBuffer = &(svgCtx.outBuf.buffer);
  screenWidth = *sizeX;
  screenHeight = *sizeY;
 
  dumpImage(styh, SVG_FORMAT, screenBuffer, screenWidth, screenHeight,
      pOutBuf, imID, isTransparent, transpIndex, charTab, colorMap);
 
  rc = 0;
  goto done;
 
badRet:
  rc = 1;
 
done:
  ffree(svgCtx.outBuf.buffer);  
  ffree(xmlapi);  /* Program crashes at this line on MVS */
  return rc;
}

********************************************************
The program crashes on MVS when trying to do a free() at the end of
svgParse() as follows:
ffree(xmlapi);   /* Program crashes at this line on MVS */

The function ffree() is defined as follows:
#define  ffree(p) do{ if((p)!=NULL) {free(p);(p)=NULL;} } while(0)

The program works fine on NT workstation 4.0.  Since this usually indicates some kind of memory problem, such as overrunning an allocated area, I tested it on UNIX using Purify program, but Purify program didn't report any problems.

Could you please let me know why it crashes on MVS?

Thank you for your help!

Starkman