diff --git a/src/core/counter.lua b/src/core/counter.lua
index 64d5b75b3d..fe3c368f78 100644
--- a/src/core/counter.lua
+++ b/src/core/counter.lua
@@ -46,13 +46,14 @@ local numbers = {} -- name -> number
 function open (name, readonly)
    if numbers[name] then error("counter already opened: " .. name) end
    local n = #public+1
-   numbers[name] = n
-   public[n] = shm.map(name, counter_t, readonly)
    if readonly then
+      public[n] = shm.open(name, counter_t, readonly)
       private[n] = public[#public] -- use counter directly
    else
+      public[n] = shm.create(name, counter_t)
       private[n] = ffi.new(counter_t)
    end
+   numbers[name] = n
    return private[n]
 end
 
@@ -86,7 +87,7 @@ function selftest ()
    print("selftest: core.counter")
    local a  = open("core.counter/counter/a")
    local b  = open("core.counter/counter/b")
-   local a2 = shm.map("core.counter/counter/a", counter_t, true)
+   local a2 = shm.create("core.counter/counter/a", counter_t, true)
    set(a, 42)
    set(b, 43)
    assert(read(a) == 42)
diff --git a/src/core/link.lua b/src/core/link.lua
index 8337626a9e..ed825e2606 100644
--- a/src/core/link.lua
+++ b/src/core/link.lua
@@ -24,7 +24,7 @@ max        = C.LINK_MAX_PACKETS
 local counternames = {"rxpackets", "txpackets", "rxbytes", "txbytes", "txdrop"}
 
 function new (name)
-   local r = shm.map("links/"..name, "struct link")
+   local r = shm.create("links/"..name, "struct link")
    for _, c in ipairs(counternames) do
       r.stats[c] = counter.open("counters/"..name.."/"..c)
    end
diff --git a/src/core/shm.lua b/src/core/shm.lua
index 631cbd96f7..05a2bf53a3 100644
--- a/src/core/shm.lua
+++ b/src/core/shm.lua
@@ -3,8 +3,12 @@
 -- shm.lua -- shared memory alternative to ffi.new()
 
 -- API:
---   shm.map(name, type[, readonly]) => ptr
---     Map a shared object into memory via a heirarchical name.
+--   shm.create(name, type) => ptr
+--     Map a shared object into memory via a hierarchical name, creating it
+--     if needed.
+--   shm.open(name, type[, readonly]) => ptr
+--     Map a shared object into memory via a hierarchical name.  Fail if
+--     the shared object does not already exist.
 --   shm.unmap(ptr)
 --     Delete a memory mapping.
 --   shm.unlink(path)
@@ -49,11 +53,11 @@
 -- of the 'path' variable. Here are examples of names and how they are
 -- resolved:
 --   Fully qualified:
---     //snabb/1234/foo/bar => /var/run/snabb/1234/foo/bar
+--     //1234/foo/bar => /var/run/snabb/1234/foo/bar
 --   Path qualified:
---     /foo/bar             => /var/run/snabb/$pid/foo/bar
+--     /foo/bar       => /var/run/snabb/$pid/foo/bar
 --   Local:
---     bar                  => /var/run/snabb/$pid/$path/bar
+--     bar            => /var/run/snabb/$pid/$path/bar
 -- .. where $pid is the PID of this process and $path is the current
 -- value of the 'path' variable in this module.
 
@@ -72,7 +76,7 @@ path = ""
 mappings = {}
 
 -- Map an object into memory.
-function map (name, type,  readonly)
+local function map (name, type, readonly, create)
    local path = resolve(name)
    local mapmode = readonly and 'read' or 'read, write'
    local ctype = ffi.typeof(type)
@@ -82,11 +86,20 @@ function map (name, type,  readonly)
       print(("shm warning: resizing %s from %d to %d bytes")
             :format(path, stat.size, size))
    end
-   -- Create the parent directories. If this fails then so will the open().
-   mkdir(path)
-   local fd, err = S.open(root..'/'..path, "creat, rdwr", "rwxu")
+   local fd, err
+   if create then
+      -- Create the parent directories. If this fails then so will the open().
+      mkdir(path)
+      fd, err = S.open(root..'/'..path, "creat, rdwr", "rwxu")
+   else
+      fd, err = S.open(root..'/'..path, readonly and "rdonly" or "rdwr")
+   end
    if not fd then error("shm open error ("..path.."):"..tostring(err)) end
-   assert(fd:ftruncate(size), "shm: ftruncate failed")
+   if create then
+      assert(fd:ftruncate(size), "shm: ftruncate failed")
+   else
+      assert(fd:fstat().size == size, "shm: unexpected size")
+   end
    local mem, err = S.mmap(nil, size, mapmode, "shared", fd, 0)
    fd:close()
    if mem == nil then error("mmap failed: " .. tostring(err)) end
@@ -94,6 +107,14 @@ function map (name, type,  readonly)
    return ffi.cast(ffi.typeof("$&", ctype), mem)
 end
 
+function create (name, type)
+   return map(name, type, false, true)
+end
+
+function open (name, type, readonly)
+   return map(name, type, readonly, false)
+end
+
 function resolve (name)
    local q, p = name:match("^(/*)(.*)") -- split qualifier (/ or //)
    local result = p
@@ -169,10 +190,10 @@ function selftest ()
    path = 'shm/selftest'
    local name = "obj"
    print("create "..name)
-   local p1 = map(name, "struct { int x, y, z; }")
-   local p2 = map(name, "struct { int x, y, z; }")
+   local p1 = create(name, "struct { int x, y, z; }")
+   local p2 = create(name, "struct { int x, y, z; }")
    alias(name, name..".alias")
-   local p3 = map(name..".alias", "struct { int x, y, z; }")
+   local p3 = create(name..".alias", "struct { int x, y, z; }")
    assert(p1 ~= p2)
    assert(p1.x == p2.x)
    p1.x = 42
@@ -188,7 +209,7 @@ function selftest ()
    local n = 10000
    local objs = {}
    for i = 1, n do
-      table.insert(objs, map("obj/"..i, "uint64_t[1]"))
+      table.insert(objs, create("obj/"..i, "uint64_t[1]"))
    end
    print(n.." objects created")
    for i = 1, n do unmap(objs[i]) end