Added SharedMemory::MapFrom.

base/shared_memory_posix.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/shared_memory.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/mman.h>
 #include <sys/stat.h>
@@ skipping 201 matching lines @@
 
   read_only_ = read_only;
 
   const char *mode = read_only ? "r" : "r+";
   FILE *fp = file_util::OpenFile(path, mode);
   return PrepareMapFile(fp);
 }
 
 #endif  // !defined(OS_ANDROID)
 
-bool SharedMemory::Map(size_t bytes) {
+bool SharedMemory::MapAt(off_t offset, size_t bytes) {
   if (mapped_file_ == -1)
     return false;
 
   if (bytes > static_cast<size_t>(std::numeric_limits<int>::max()))
     return false;
 
 #if defined(OS_ANDROID)
   if (bytes == 0) {
     int ashmem_bytes = ashmem_get_size_region(mapped_file_);
     if (ashmem_bytes < 0)
       return false;
 
     DCHECK_GE(static_cast<uint32>(ashmem_bytes), bytes);
     // The caller wants to determine the map region size from ashmem.
-    bytes = ashmem_bytes;
+    bytes = ashmem_bytes - offset;
     // TODO(port): we set the created size here so that it is available in
     // transport_dib_android.cc. We should use ashmem_get_size_region()
     // in transport_dib_android.cc.
-    created_size_ = bytes;
+    created_size_ = ashmem_bytes;
   }
 #endif
 
   memory_ = mmap(NULL, bytes, PROT_READ | (read_only_ ? 0 : PROT_WRITE),
-                 MAP_SHARED, mapped_file_, 0);
+                 MAP_SHARED, mapped_file_, offset);
 
   bool mmap_succeeded = memory_ != (void*)-1 && memory_ != NULL;
   if (mmap_succeeded) {
     mapped_size_ = bytes;
     DCHECK_EQ(0U, reinterpret_cast<uintptr_t>(memory_) &
         (SharedMemory::MAP_MINIMUM_ALIGNMENT - 1));
   } else {
     memory_ = NULL;
   }
 
@@ skipping 123 matching lines @@
   new_handle->fd = new_fd;
   new_handle->auto_close = true;
 
   if (close_self)
     Close();
 
   return true;
 }
 
 }  // namespace base