mmatthe@MatteLP:~$ xmodmap -pke | egrep '(Control|Alt)'
keycode  37 = Alt_L NoSymbol Alt_L
keycode  64 = Control_L NoSymbol Control_L
keycode 105 = Control_R NoSymbol Control_R
keycode 204 = NoSymbol Alt_L NoSymbol Alt_L
mmatthe@MatteLP:~$ cat .swapctrlalt
clear control
clear mod1
keycode 64 = Control_L
keycode 37= Alt_L
add control = Control_L
add mod1 = Alt_L
mmatthe@MatteLP:~$ xmodmap .swapctrlalt
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  15
  Current serial number in output stream:  15
mmatthe@MatteLP:~$