An application: foreign function bindings C int puts(const char - - PowerPoint PPT Presentation

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An application: foreign function bindings C int puts(const char - - PowerPoint PPT Presentation

Sep 02, 2022 190 likes •459 views

An application: foreign function bindings C int puts(const char *s); 1/ 19 C in two minutes object types function types numeric types built from object types int , char , float , . . . n arguments, one return type pointers int(const char

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SLIDE 1

An application: foreign function bindings

C

int puts(const char *s);

1/ 19

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SLIDE 2

C in two minutes

  • bject types

numeric types

int, char, float, . . .

pointers

int *, char *, int **, . . .

structures and unions

struct t { int x, char y };

arrays

int x[3] = { 1, 2, 3};

Operations address, sizeof, read, write, . . . function types built from object types n arguments, one return type

int(const char *); char *(char *, char *);

Operations call

2/ 19

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SLIDE 3

Talking to C: two challenges

Conversions between value representations

Long_val, Val_long, caml_copy_double, . . .

Interactions with the garbage collector Protect locals & parameters against disappearance & destruction

value puts_stub(value s) { CAMLparam1(s); const char *p = String_val(s); int n = puts(p); CAMLreturn(Val_int(n)); }

3/ 19

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SLIDE 4

Representing types

4/ 19

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SLIDE 5

Representing object types

C object types

type ::= int char type * . . .

Representing C object types

type _ typ = Int : int typ | Char : char typ | Ptr : ’a typ → ’a ptr typ | . . . | View : (’a → ’b) * (’b → ’a) * ’a typ → ’b typ | . . . let string : string typ = View (ptr_of_string , string_of_ptr , Ptr Char)

5/ 19

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SLIDE 6

Operations on object types

Low-level operations

val read : ’a typ → address → ’a val write : ’a typ → ’a → address → unit val sizeof : ’a typ → int let read : type a. a typ → address → a = fun typ addr → match typ with | Int → read_int address | Char → read_char address | . . .

Higher-level operations

type ’a ptr (* = ’a typ * address *) val (!@) : ’a ptr → ’a val (@+) : ’a ptr → int → ’a ptr

6/ 19

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SLIDE 7

Representing function types

C function types

ftype ::= type(type , type , . . ., type)

Representing C function types

type _ fn = Returns : ’a typ → ’a fn | Function: ’a typ * ’b fn → (’a → ’b) fn let (@→) a b = Function (a,b) and returning v = Returns v

Example

Ptr Char @→ Int @→ returning Int

represents

int(char *, int)

7/ 19

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SLIDE 8

Operations on function types

val foreign : string → (’a → ’b) fn → (’a → ’b)

Example

let puts = foreign "puts" (string @→ returning int)

produces

val puts : string → int

8/ 19

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SLIDE 9

Anatomy of a binding

let puts = foreign "puts" (str ing @→ returning int) puts "Hello , world"

  • 1. resolve the name
  • 2. create a buffer with enough space
  • 3. convert and write arguments
  • 4. apply function
  • 5. read results

9/ 19

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SLIDE 10

Anatomy of a binding

let puts = foreign "puts" (str ing @→ returning int) puts "Hello , world"

  • 1. resolve the name

"puts" ⇝ 0x7f0d1eebcf60

  • 2. create a buffer with enough space
  • 3. convert and write arguments
  • 4. apply function
  • 5. read results

9/ 19

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SLIDE 11

Anatomy of a binding

let puts = foreign "puts" (str ing @→ returning int) puts "Hello , world"

  • 1. resolve the name

"puts" ⇝ 0x7f0d1eebcf60

  • 2. create a buffer with enough space
  • 3. convert and write arguments
  • 4. apply function
  • 5. read results

9/ 19

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SLIDE 12

Anatomy of a binding

let puts = foreign "puts" (str ing @→ returning int) puts "Hello , world"

  • 1. resolve the name

"puts" ⇝ 0x7f0d1eebcf60

  • 2. create a buffer with enough space
  • 3. convert and write arguments

ff ea 23 22

  • 4. apply function
  • 5. read results

9/ 19

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SLIDE 13

Anatomy of a binding

let puts = foreign "puts" (str ing @→ returning int) puts "Hello , world"

  • 1. resolve the name

"puts" ⇝ 0x7f0d1eebcf60

  • 2. create a buffer with enough space
  • 3. convert and write arguments

ff ea 23 22

  • 4. apply function

ff ea 23 22 00 1b

  • 5. read results

9/ 19

slide-14
SLIDE 14

Anatomy of a binding

let puts = foreign "puts" (str ing @→ returning int) puts "Hello , world"

  • 1. resolve the name

"puts" ⇝ 0x7f0d1eebcf60

  • 2. create a buffer with enough space
  • 3. convert and write arguments

ff ea 23 22

  • 4. apply function

ff ea 23 22 00 1b

  • 5. read results

puts "Hello, world" ⇝ 13

9/ 19

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SLIDE 15

More type interpretations

10/ 19

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SLIDE 16

Drawbacks of dynamism

No type safety Name lookup may fail dynamically Interpretive overhead Can’t use standard tools (nm, objdump, ldd, . . . )

11/ 19

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SLIDE 17

Staged binding

module Bindings(F: FOREIGN) = struct

  • pen F

let puts = foreign "puts" (string @→ returning int) end value puts_stub(value s) { char *p = Ptr_val(s); int n = puts(p); return Val_int(n); } external puts_stub : address → int = "puts_stub" let foreign nm fn = match nm , fn with | "puts", Function (View . . . Bindings( Generated_ML ) 12/ 19

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SLIDE 18

Staged binding

module Bindings(F: FOREIGN) = struct

  • pen F

let puts = foreign "puts" (string @→ returning int) end value puts_stub(value s) { char *p = Ptr_val(s); int n = puts(p); return Val_int(n); } external puts_stub : address → int = "puts_stub" let foreign nm fn = match nm , fn with | "puts", Function (View . . . Bindings( Generated_ML ) 12/ 19

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SLIDE 19

Staged binding: abstracting the interpretation

module type FOREIGN = sig type _ result val foreign: string → (’a→’b) → (’a→’b) result end

Example

module Bindings(F: FOREIGN) = struct

  • pen F

let puts = foreign "puts" (string @→ returning int) end

13/ 19

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SLIDE 20

Staged binding: recovering the dynamic interpretation

module Foreign_dynamic = struct type ’a result = ’a let foreign = foreign (* i.e. implementation above *) end

Example

Bindings( Foreign_dynamic )

produces

sig val puts : string → int end

14/ 19

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SLIDE 21

Staged binding: generating C

val generate_C : string → ’a fn → unit module Foreign_generate_C = struct type ’a result = unit let foreign = generate_C end

Example

Bindings( Foreign_generate_C )

  • utputs

value puts_stub(value s) { char *p = Ptr_val(s); int n = puts(p); return Val_int(n); }

15/ 19

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SLIDE 22

Staged binding: generating ML

val generate_ML : string → ’a fn → unit module Foreign_generate_ML = struct type ’a result = unit let foreign = generate_ML end

Example

Bindings( Foreign_generate_ML )

  • utputs

external puts_stub : address → int = "puts_stub" let foreign nm fn = match nm , fn with | "puts", Function (View (_, froms , Ptr Char)) → fun s → puts_stub (froms s) | "puts", fn → fail "type mismatch" | name , _ → fail "unexpected name"

16/ 19

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SLIDE 23

Staged binding: linking

module Bindings(F: FOREIGN) = struct

  • pen F

let puts = foreign "puts" (string @→ returning int) end module Generated_ML : FOREIGN with type ’a result = ’a = (* code generated on previous slide *) Bindings(Generated_ML )

Type safe linking via type refinement!

17/ 19

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SLIDE 24

(Some details omitted)

concurrency

C

remote calls

C

function pointers

void (*)(int , float);

more object types

struct s x[3];

determining object layout

struct t { int x, y; };

0: 1: 2: 3: 4: 5: 6: 7: 8:

x y

inverted bindings C

18/ 19

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SLIDE 25

Next time: overloading

val (=) : {E:EQ} → E.t → E.t → bool

19/ 19