limit the syscalls to track on examples/systrack.lua

* see #96

examples/systrack.lua

@@ -21,18 +21,19 @@
 -- SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 --
 
-local linux   = require("linux")
-local probe   = require("probe")
-local device  = require("device")
-local syscall = require("syscall.table")
+local linux  = require("linux")
+local probe  = require("probe")
+local device = require("device")
+local systab = require("syscall.table")
 
-local track = {}
+local syscalls = {"openat", "read", "write", "readv", "writev", "close"}
 
 local function nop() end -- do nothing
 
 local s = linux.stat
 local driver = {name = "systrack", open = nop, release = nop, mode = s.IRUGO}
 
+local track = {}
 local toggle = true
 function driver:read()
 	local log = ""
@@ -45,9 +46,12 @@ function driver:read()
 	return log
 end
 
-for symbol, address in pairs(syscall) do
+for _, symbol in ipairs(syscalls) do
+	local address = systab[symbol]
+	track[symbol] = 0
+
 	local function handler()
-		track[symbol] = (track[symbol] or 0) + 1
+		track[symbol] = track[symbol] + 1
 	end
 
 	probe.new(address, {pre = handler, post = nop})