Added SharedMemory::MapFrom.

base/shared_memory_posix.cc

Index: base/shared_memory_posix.cc
diff --git a/base/shared_memory_posix.cc b/base/shared_memory_posix.cc
index 7862f42c194fe2701293673efb6417bf2d7140a6..dcd55756f98257d57cdee2f135097d20bbfcefef 100644
--- a/base/shared_memory_posix.cc
+++ b/base/shared_memory_posix.cc
@@ -16,6 +16,7 @@
 #include "base/threading/platform_thread.h"
 #include "base/safe_strerror_posix.h"
 #include "base/synchronization/lock.h"
+#include "base/sys_info.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/utf_string_conversions.h"
 
@@ -219,7 +220,7 @@ bool SharedMemory::Open(const std::string& name, bool read_only) {
 
 #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;
 
@@ -242,8 +243,10 @@ bool SharedMemory::Map(size_t bytes) {
   }
 #endif
 
+  DCHECK_EQ(static_cast<size_t>(0),

[jar (doing other things), 2013/01/15 22:20:29]

I assume this call is not made that often... so how about doing a real check,
rather then a DCHECK (or at least returning false when this is violated)?

[Vitaly Buka (NO REVIEWS), 2013/01/15 22:58:54]

it will return false because memory_ will be invalid.

+            offset & (SysInfo::VMAllocationGranularity() - 1));

[Mark Mentovai, 2013/01/15 22:26:02]

My point was that this is sort of an implementation detail. There are a few
different reasons that memory_ might wind up NULL. Why are you singling this one
out and DCHECKing for it? mmap parameter validity checks are mmap’s
responsibility. If we want to crash when mmap returns NULL, we should
CHECK(memory_). Otherwise, why are you picking on one parameter and writing your
own debug-only check for it?

[Vitaly Buka (NO REVIEWS), 2013/01/15 22:58:54]

I don't believe we want crash on mmap returns null, it may happen on reasonable
runtime conditions.

So then I just remove dcheck.

   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) {