-The FFI library allows calling external C functions and the use -of C data structures from pure Lua code. + +The FFI library allows calling external C functions and +using C data structures from pure Lua code. +

+ The FFI library largely obviates the need to write tedious manual -Lua/C bindings in C. It doesn't require learning a separate binding -language — it parses plain C declarations, which can be +Lua/C bindings in C. No need to learn a separate binding language +— it parses plain C declarations! These can be cut-n-pasted from C header files or reference manuals. It's up to the task of binding large libraries without the need for dealing with fragile binding generators. +

The FFI library is tightly integrated into LuaJIT (it's not available @@ -83,26 +93,30 @@ Please use the FFI sub-topics in the navigation bar to learn more.

It's really easy to call an external C library function:

-local ffi = require("ffi") --①
-ffi.cdef[[ //②
++①
+②
+
+
+③local ffi = require("ffi")
+ffi.cdef[[
 int printf(const char *fmt, ...);
 ]]
-ffi.C.printf("Hello %s!", "world") --③
+ffi.C.printf("Hello %s!", "world")
 
 
 So, let's pick that apart:
 
 
-① Load the FFI library.
+① Load the FFI library.
 
 
-② Add a C declaration
+② Add a C declaration
 for the function. The part inside the double-brackets (in green) is
 just standard C syntax.
 
 
-③ Call the named
+③ Call the named
 C function — Yes, it's that simple!
 
 
@@ -198,25 +212,42 @@ need of a simple example ...
 And here's the FFI version. The modified parts have been marked in
 bold:
 
--local ffi = require("ffi") --①
-ffi.cdef[[
++①
+
+
+
+
+
+②
+
+③
+④
+
+
+
+
+
+
+③
+⑤local ffi = require("ffi")
+ffi.cdef[[
 typedef struct { uint8_t red, green, blue, alpha; } rgba_pixel;
 ]]
 
 local function image_ramp_green(n)
-  local img = ffi.new("rgba_pixel[?]", n) --②
+  local img = ffi.new("rgba_pixel[?]", n)
   local f = 255/(n-1)
-  for i=0,n-1 do --③
-    img[i].green = i*f --④
+  for i=0,n-1 do
+    img[i].green = i*f
     img[i].alpha = 255
   end
   return img
 end
 
 local function image_to_grey(img, n)
-  for i=0,n-1 do --③
-    local y = 0.3*img[i].red + 0.59*img[i].green + 0.11*img[i].blue --⑤
+  for i=0,n-1 do
+    local y = 0.3*img[i].red + 0.59*img[i].green + 0.11*img[i].blue
     img[i].red = y; img[i].green = y; img[i].blue = y
   end
 end
@@ -231,30 +262,30 @@ end
 Ok, so that wasn't too difficult:
 
 
-① First, load the FFI
+① First, load the FFI
 library and declare the low-level data type. Here we choose a
 struct which holds four byte fields, one for each component
 of a 4x8 bit RGBA pixel.
 
 
-② Creating the data
+② Creating the data
 structure with ffi.new() is straightforward — the
 '?' is a placeholder for the number of elements of a
 variable-length array.
 
 
-③ C arrays are
+③ C arrays are
 zero-based, so the indexes have to run from 0 to
 n-1. One might want to allocate one more element instead to
 simplify converting legacy code.
 
 
-④ Since ffi.new()
+④ Since ffi.new()
 zero-fills the array by default, we only need to set the green and the
 alpha fields.
 
 
-⑤ The calls to
+⑤ The calls to
 math.floor() can be omitted here, because floating-point
 numbers are already truncated towards zero when converting them to an
 integer. This happens implicitly when the number is stored in the
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