diff --git a/nsprpub/pr/include/prio.h b/nsprpub/pr/include/prio.h
index 49e5fdd2ecc..5df417eb263 100644
--- a/nsprpub/pr/include/prio.h
+++ b/nsprpub/pr/include/prio.h
@@ -315,8 +315,8 @@ typedef PRInt32 (PR_CALLBACK *PRWriteFN)(PRFileDesc *fd, const void *buf, PRInt3
 typedef PRInt32 (PR_CALLBACK *PRAvailableFN)(PRFileDesc *fd);
 typedef PRInt64 (PR_CALLBACK *PRAvailable64FN)(PRFileDesc *fd);
 typedef PRStatus (PR_CALLBACK *PRFsyncFN)(PRFileDesc *fd);
-typedef PRInt32 (PR_CALLBACK *PRSeekFN)(PRFileDesc *fd, PRInt32 offset, PRSeekWhence how);
-typedef PRInt64 (PR_CALLBACK *PRSeek64FN)(PRFileDesc *fd, PRInt64 offset, PRSeekWhence how);
+typedef PROffset32 (PR_CALLBACK *PRSeekFN)(PRFileDesc *fd, PROffset32 offset, PRSeekWhence how);
+typedef PROffset64 (PR_CALLBACK *PRSeek64FN)(PRFileDesc *fd, PROffset64 offset, PRSeekWhence how);
 typedef PRStatus (PR_CALLBACK *PRFileInfoFN)(PRFileDesc *fd, PRFileInfo *info);
 typedef PRStatus (PR_CALLBACK *PRFileInfo64FN)(PRFileDesc *fd, PRFileInfo64 *info);
 typedef PRInt32 (PR_CALLBACK *PRWritevFN)(
@@ -746,14 +746,14 @@ typedef enum PRFileType
 
 struct PRFileInfo {
     PRFileType type;        /* Type of file */
-    PRUint32 size;          /* Size, in bytes, of file's contents */
+    PROffset32 size;        /* Size, in bytes, of file's contents */
     PRTime creationTime;    /* Creation time per definition of PRTime */
     PRTime modifyTime;      /* Last modification time per definition of PRTime */
 };
 
 struct PRFileInfo64 {
     PRFileType type;        /* Type of file */
-    PRUint64 size;          /* Size, in bytes, of file's contents */
+    PROffset64 size;        /* Size, in bytes, of file's contents */
     PRTime creationTime;    /* Creation time per definition of PRTime */
     PRTime modifyTime;      /* Last modification time per definition of PRTime */
 };
@@ -857,7 +857,7 @@ PR_EXTERN(PRStatus) PR_Access(const char *name, PRAccessHow how);
  * INPUTS:
  *     PRFileDesc *fd
  *         Pointer to a PRFileDesc object.
- *     PRInt32, PRInt64 offset
+ *     PROffset32, PROffset64 offset
  *         Specifies a value, in bytes, that is used in conjunction
  *         with the 'whence' parameter to set the file pointer.  A
  *         negative value causes seeking in the reverse direction.
@@ -873,7 +873,7 @@ PR_EXTERN(PRStatus) PR_Access(const char *name, PRAccessHow how);
  *                          file plus the value of the offset parameter.
  * OUTPUTS:
  *     None.
- * RETURN: PRInt32, PRInt64
+ * RETURN: PROffset32, PROffset64
  *     Upon successful completion, the resulting pointer location,
  *     measured in bytes from the beginning of the file, is returned.
  *     If the PR_Seek() function fails, the file offset remains
@@ -882,8 +882,8 @@ PR_EXTERN(PRStatus) PR_Access(const char *name, PRAccessHow how);
  *************************************************************************
  */
 
-PR_EXTERN(PRInt32) PR_Seek(PRFileDesc *fd, PRInt32 offset, PRSeekWhence whence);
-PR_EXTERN(PRInt64) PR_Seek64(PRFileDesc *fd, PRInt64 offset, PRSeekWhence whence);
+PR_EXTERN(PROffset32) PR_Seek(PRFileDesc *fd, PROffset32 offset, PRSeekWhence whence);
+PR_EXTERN(PROffset64) PR_Seek64(PRFileDesc *fd, PROffset64 offset, PRSeekWhence whence);
 
 /*
  ************************************************************************
@@ -1698,7 +1698,7 @@ PR_EXTERN(PRFileMap *) PR_CreateFileMap(
 
 PR_EXTERN(void *) PR_MemMap(
     PRFileMap *fmap,
-    PRInt64 offset,  /* must be aligned and sized to whole pages */
+    PROffset64 offset,  /* must be aligned and sized to whole pages */
     PRUint32 len);
 
 PR_EXTERN(PRStatus) PR_MemUnmap(void *addr, PRUint32 len);
diff --git a/nsprpub/pr/include/private/primpl.h b/nsprpub/pr/include/private/primpl.h
index 200773cd900..88d26566e68 100644
--- a/nsprpub/pr/include/private/primpl.h
+++ b/nsprpub/pr/include/private/primpl.h
@@ -1196,10 +1196,10 @@ extern void *_PR_MD_GET_SP(PRThread *thread);
 *************************************************************************/
 /************************************************************************/
 
-extern PRInt32 _PR_MD_LSEEK(PRFileDesc *fd, PRInt32 offset, PRSeekWhence whence);
+extern PROffset32 _PR_MD_LSEEK(PRFileDesc *fd, PROffset32 offset, PRSeekWhence whence);
 #define    _PR_MD_LSEEK _MD_LSEEK
 
-extern PRInt64 _PR_MD_LSEEK64(PRFileDesc *fd, PRInt64 offset, PRSeekWhence whence);
+extern PROffset64 _PR_MD_LSEEK64(PRFileDesc *fd, PROffset64 offset, PRSeekWhence whence);
 #define    _PR_MD_LSEEK64 _MD_LSEEK64
 
 extern PRInt32 _PR_MD_GETFILEINFO(const char *fn, PRFileInfo *info);
@@ -1799,7 +1799,7 @@ extern PRStatus _PR_MD_CREATE_FILE_MAP(PRFileMap *fmap, PRInt64 size);
 
 extern void * _PR_MD_MEM_MAP(
     PRFileMap *fmap,
-    PRInt64 offset,
+    PROffset64 offset,
     PRUint32 len);
 #define _PR_MD_MEM_MAP _MD_MEM_MAP
 
diff --git a/nsprpub/pr/include/prtypes.h b/nsprpub/pr/include/prtypes.h
index fd3d084274f..8aaf594ea75 100644
--- a/nsprpub/pr/include/prtypes.h
+++ b/nsprpub/pr/include/prtypes.h
@@ -316,6 +316,15 @@ typedef double          PRFloat64;
 ************************************************************************/
 typedef size_t PRSize;
 
+
+/************************************************************************
+** TYPES:       PROffset32, PROffset64
+** DESCRIPTION:
+**  A type for representing byte offsets from some location. 
+************************************************************************/
+typedef PRInt32 PROffset32;
+typedef PRInt64 PROffset64;
+
 /************************************************************************
 ** TYPES:       PRPtrDiff
 ** DESCRIPTION:
diff --git a/nsprpub/pr/src/io/prfile.c b/nsprpub/pr/src/io/prfile.c
index fa05e0bdd99..ab16e27a8af 100644
--- a/nsprpub/pr/src/io/prfile.c
+++ b/nsprpub/pr/src/io/prfile.c
@@ -95,20 +95,20 @@ static PRInt32 PR_CALLBACK FileWrite(PRFileDesc *fd, const void *buf, PRInt32 am
     return count;
 }
 
-static PRInt32 PR_CALLBACK FileSeek(PRFileDesc *fd, PRInt32 offset, PRSeekWhence whence)
+static PROffset32 PR_CALLBACK FileSeek(PRFileDesc *fd, PROffset32 offset, PRSeekWhence whence)
 {
-    PRInt32 result;
+    PROffset32 result;
 
     result = _PR_MD_LSEEK(fd, offset, whence);
     return result;
 }
 
-static PRInt64 PR_CALLBACK FileSeek64(PRFileDesc *fd, PRInt64 offset, PRSeekWhence whence)
+static PROffset64 PR_CALLBACK FileSeek64(PRFileDesc *fd, PROffset64 offset, PRSeekWhence whence)
 {
 #ifdef XP_MAC
 #pragma unused( fd, offset, whence )
 #endif
-    PRInt64 result;
+    PROffset64 result;
 
     result = _PR_MD_LSEEK64(fd, offset, whence);
     return result;
diff --git a/nsprpub/pr/src/io/prmmap.c b/nsprpub/pr/src/io/prmmap.c
index 9383c151987..351adb1aac5 100644
--- a/nsprpub/pr/src/io/prmmap.c
+++ b/nsprpub/pr/src/io/prmmap.c
@@ -52,7 +52,7 @@ PR_IMPLEMENT(PRFileMap *) PR_CreateFileMap(
 
 PR_IMPLEMENT(void *) PR_MemMap(
     PRFileMap *fmap,
-    PRInt64 offset,
+    PROffset64 offset,
     PRUint32 len)
 {
     return _PR_MD_MEM_MAP(fmap, offset, len);
diff --git a/nsprpub/pr/src/md/mac/macio.c b/nsprpub/pr/src/md/mac/macio.c
index df9e905fc2a..c12f39b27b9 100644
--- a/nsprpub/pr/src/md/mac/macio.c
+++ b/nsprpub/pr/src/md/mac/macio.c
@@ -425,7 +425,7 @@ ErrorExit:
 
 /* _MD_CLOSE_FILE, _MD_READ, _MD_WRITE, _MD_GET_FILE_ERROR are defined in _macos.h */
 
-PRInt32 _MD_LSeek(PRFileDesc *fd, PRInt32 offset, PRSeekWhence how)
+PROffset32 _MD_LSeek(PRFileDesc *fd, PROffset32 offset, PRSeekWhence how)
 {
 	PRInt32 refNum = fd->secret->md.osfd;
 	OSErr 	err = noErr;
@@ -1880,7 +1880,7 @@ PRStatus _MD_CreateFileMap(PRFileMap *fmap, PRInt64 size)
 
 void * _MD_MemMap(
     PRFileMap *fmap,
-    PRInt64 offset,
+    PROffset64 offset,
     PRUint32 len)
 {
 #pragma unused (fmap, offset, len)
diff --git a/nsprpub/pr/src/md/os2/os2misc.c b/nsprpub/pr/src/md/os2/os2misc.c
index a74eca86eb6..de675e43167 100644
--- a/nsprpub/pr/src/md/os2/os2misc.c
+++ b/nsprpub/pr/src/md/os2/os2misc.c
@@ -487,7 +487,7 @@ PRStatus _MD_CreateFileMap(PRFileMap *fmap, PRInt64 size)
 
 void * _MD_MemMap(
     PRFileMap *fmap,
-    PRInt64 offset,
+    PROffset64 offset,
     PRUint32 len)
 {
     PR_ASSERT(!"Not implemented");
diff --git a/nsprpub/pr/src/md/unix/unix.c b/nsprpub/pr/src/md/unix/unix.c
index aa3cf45aa4b..8c3dca00a4b 100644
--- a/nsprpub/pr/src/md/unix/unix.c
+++ b/nsprpub/pr/src/md/unix/unix.c
@@ -2256,9 +2256,9 @@ static void sigbushandler() {
 
 #endif  /* !defined(_PR_PTHREADS) */
 
-PRInt32 _MD_lseek(PRFileDesc *fd, PRInt32 offset, PRSeekWhence whence)
+PROffset32 _MD_lseek(PRFileDesc *fd, PROffset32 offset, PRSeekWhence whence)
 {
-    PRInt32 rv, where;
+    PROffset32 rv, where;
 
     switch (whence) {
         case PR_SEEK_SET:
@@ -2285,10 +2285,10 @@ done:
     return(rv);
 }
 
-PRInt64 _MD_lseek64(PRFileDesc *fd, PRInt64 offset, PRSeekWhence whence)
+PROffset64 _MD_lseek64(PRFileDesc *fd, PROffset64 offset, PRSeekWhence whence)
 {
     PRInt32 where;
-    PRInt64 rv;
+    PROffset64 rv;
 
     switch (whence)
     {
@@ -2661,10 +2661,10 @@ static PRIntn _MD_solaris25_stat64(const char *fn, _MDStat64 *buf)
 
 #if defined(_PR_NO_LARGE_FILES) || defined(SOLARIS2_5)
 
-static PRInt64 _MD_Unix_lseek64(PRIntn osfd, PRInt64 offset, PRIntn whence)
+static PROffset64 _MD_Unix_lseek64(PRIntn osfd, PROffset64 offset, PRIntn whence)
 {
     PRUint64 maxoff;
-    PRInt64 rv = minus_one;
+    PROffset64 rv = minus_one;
     LL_I2L(maxoff, 0x7fffffff);
     if (LL_CMP(offset, <=, maxoff))
     {
diff --git a/nsprpub/pr/src/md/windows/ntio.c b/nsprpub/pr/src/md/windows/ntio.c
index 47c1efebef7..8225da9bd99 100644
--- a/nsprpub/pr/src/md/windows/ntio.c
+++ b/nsprpub/pr/src/md/windows/ntio.c
@@ -2460,10 +2460,10 @@ _PR_MD_SOCKETAVAILABLE(PRFileDesc *fd)
     return result;
 }
 
-PRInt32
-_PR_MD_LSEEK(PRFileDesc *fd, PRInt32 offset, int whence)
+PROffset32
+_PR_MD_LSEEK(PRFileDesc *fd, PROffset32 offset, int whence)
 {
-    PRInt32 rv;
+    PROffset32 rv;
 
     rv = SetFilePointer((HANDLE)fd->secret->md.osfd, offset, NULL, whence);
     /*
@@ -2476,8 +2476,8 @@ _PR_MD_LSEEK(PRFileDesc *fd, PRInt32 offset, int whence)
     return rv;
 }
 
-PRInt64
-_PR_MD_LSEEK64(PRFileDesc *fd, PRInt64 offset, int whence)
+PROffset64
+_PR_MD_LSEEK64(PRFileDesc *fd, PROffset64 offset, int whence)
 {
     PRUint64 result;
     PRUint32 position, uhi;
@@ -2516,7 +2516,7 @@ _PR_MD_LSEEK64(PRFileDesc *fd, PRInt64 offset, int whence)
     PR_ASSERT((PRInt64)result >= 0);
     result += position;
     PR_ASSERT((PRInt64)result >= 0);
-    return (PRInt64)result;
+    return (PROffset64)result;
 }
 
 /*
diff --git a/nsprpub/pr/src/md/windows/ntmisc.c b/nsprpub/pr/src/md/windows/ntmisc.c
index 6ef02f6d8ed..6a60814c241 100644
--- a/nsprpub/pr/src/md/windows/ntmisc.c
+++ b/nsprpub/pr/src/md/windows/ntmisc.c
@@ -669,7 +669,7 @@ PRStatus _MD_CreateFileMap(PRFileMap *fmap, PRInt64 size)
 extern PRLogModuleInfo *_pr_shma_lm;
 void * _MD_MemMap(
     PRFileMap *fmap,
-    PRInt64 offset,
+    PROffset64 offset,
     PRUint32 len)
 {
     DWORD dwHi, dwLo;
diff --git a/nsprpub/pr/src/md/windows/w95io.c b/nsprpub/pr/src/md/windows/w95io.c
index af8c63e3db5..2b21efb983e 100644
--- a/nsprpub/pr/src/md/windows/w95io.c
+++ b/nsprpub/pr/src/md/windows/w95io.c
@@ -225,11 +225,11 @@ _PR_MD_WRITE(PRFileDesc *fd, const void *buf, PRInt32 len)
     return bytes;
 } /* --- end _PR_MD_WRITE() --- */
 
-PRInt32
-_PR_MD_LSEEK(PRFileDesc *fd, PRInt32 offset, PRSeekWhence whence)
+PROffset32
+_PR_MD_LSEEK(PRFileDesc *fd, PROffset32 offset, PRSeekWhence whence)
 {
     DWORD moveMethod;
-    PRInt32 rv;
+    PROffset32 rv;
 
     switch (whence) {
         case PR_SEEK_SET:
@@ -258,8 +258,8 @@ _PR_MD_LSEEK(PRFileDesc *fd, PRInt32 offset, PRSeekWhence whence)
     return rv;
 }
 
-PRInt64
-_PR_MD_LSEEK64(PRFileDesc *fd, PRInt64 offset, PRSeekWhence whence)
+PROffset64
+_PR_MD_LSEEK64(PRFileDesc *fd, PROffset64 offset, PRSeekWhence whence)
 {
     DWORD moveMethod;
     LARGE_INTEGER li;