Support opening files in exclusive mode

Add an option that atomically tests for the existence of a file and
creates it if the file does not exist, by passing the O_EXCL flag
to open() on Unix and CREATE_NEW flag on Windows. Support for O_EXCL
varies across platforms and filesystems.

{ok, Fd} = file:open("/tmp/foo", [write,exclusive]),
{error, eexist} = file:open("/tmp/foo", [write,exclusive]).

1 files changed, 4 insertions, 1 deletions

diff --git a/erts/preloaded/src/prim_file.erl b/erts/preloaded/src/prim_file.erl
index 2d177bf80e..7f24889bb2 100644
--- a/erts/preloaded/src/prim_file.erl
+++ b/erts/preloaded/src/prim_file.erl
@@ -116,9 +116,10 @@
 -define(EFILE_MODE_READ_WRITE, 3).  
 -define(EFILE_MODE_APPEND,     4).
 -define(EFILE_COMPRESSED,      8).
+-define(EFILE_MODE_EXCL,       16).
 
 %% Use this mask to get just the mode bits to be passed to the driver.
--define(EFILE_MODE_MASK, 15).
+-define(EFILE_MODE_MASK, 31).
 
 %% Seek modes for the driver's seek function.
 -define(EFILE_SEEK_SET, 0).
@@ -958,6 +959,8 @@ open_mode([compressed|Rest], Mode, Portopts, Setopts) ->
 open_mode([append|Rest], Mode, Portopts, Setopts) ->
     open_mode(Rest, Mode bor ?EFILE_MODE_APPEND bor ?EFILE_MODE_WRITE, 
 	      Portopts, Setopts);
+open_mode([exclusive|Rest], Mode, Portopts, Setopts) ->
+    open_mode(Rest, Mode bor ?EFILE_MODE_EXCL, Portopts, Setopts);
 open_mode([delayed_write|Rest], Mode, Portopts, Setopts) ->
     open_mode([{delayed_write, 64*1024, 2000}|Rest], Mode,
 	      Portopts, Setopts);