# Passing Julia Callback Functions to C

One of the great strengths of Julia is that it is so easy to call C
code
natively, with no special “glue” routines or overhead to marshal
arguments and convert return values. For example, if you want to call
GNU GSL to compute a special function
like a Debye integral, it is as easy as:

debye_1(x) = ccall((:gsl_sf_debye_1,:libgsl), Cdouble, (Cdouble,), x)


at which point you can compute debye_1(2), debye_1(3.7), and so
on. (Even easier would be to use Jiahao Chen’s
GSL package for Julia, which has
already created such wrappers for you.) This makes a vast array of
existing C libraries accessible to you in Julia (along with Fortran
libraries and other languages with C-accessible calling conventions).

In fact, you can even go the other way around, passing Julia routines
to C, so that C code is calling Julia code in the form of callback
functions. For example, a C library for numerical integration might
expect you to pass the integrand as a function argument, which the
library will then call to evaluate the integrand as many times as
needed to estimate the integral. Callback functions are also natural
for optimization, root-finding, and many other numerical tasks, as well
as in many non-numerical problems. The purpose of this blog post is to
illustrate the techniques for passing Julia functions as callbacks to
C routines, which is straightforward and efficient but requires some
lower-level understanding of how functions and other values are passed as
arguments.

The code in this post requires Julia 0.2 (or a recent git facsimile
thereof); the key features needed for callback functions (especially
unsafe_pointer_to_objref) are not available in Julia 0.1.

## Sorting with qsort

Perhaps the most well-known example of a callback parameter is
provided by the
qsort
function, part of the ANSI C standard library and declared in C as:

void qsort(void *base, size_t nmemb, size_t size,
int(*compare)(const void *a, const void *b));


The base argument is a pointer to an array of length nmemb, with
elements of size bytes each. compare is a callback function which
takes pointers to two elements a and b and returns an integer
less/greater than zero if a should appear before/after b (or zero
if any order is permitted). Now, suppose that we have a 1d array A
of values in Julia that we want to sort using the qsort function
(rather than Julia’s built-in sort function). Before we worry about
calling qsort and passing arguments, we need to write a comparison
function that works for some arbitrary type T, e.g.

function mycompare{T}(a_::Ptr{T}, b_::Ptr{T})
return a < b ? cint(-1) : a > b ? cint(+1) : cint(0)
end
cint(n) = convert(Cint, n)


Notice that we use the built-in function unsafe_load to fetch the
values pointed to by the arguments a_ and b_ (which is “unsafe”
because it will crash if these are not valid pointers, but qsort
will always pass valid pointers). Also, we have to be a little
careful about return values: qsort expects a function returning a C
int, so we must be sure to return Cint (the corresponding type in
Julia) via a call to convert.

Now, how do we pass this to C? A function pointer in C is essentially
just a pointer to the memory location of the machine code implementing
that function, whereas a function value mycompare (of type
Function) in Julia is quite different. Thanks to Julia’s JIT
compilation

approach,a Julia function may not even be compiled until the first
time it is called, and in general the same Julia function may be
compiled into multiple machine-code instantiations, which are
specialized for arguments of different types (e.g. different T in
this case). So, you can imagine that mycompare must internally
point to a rather complicated data structure (a jl_function_t in
julia.h, if you are interested), which holds information about the
argument types, the compiled versions (if any), and so on. In
general, it must store a
closure
with information about the environment in which the function was
very different object than a simple pointer to machine code for one
set of argument types. Fortunately, we can get the latter simply by
calling a built-in Julia function called cfunction:

const mycompare_c = cfunction(mycompare, Cint, (Ptr{Cdouble}, Ptr{Cdouble}))


Here, we pass cfunction three arguments: the function mycompare,
the return type Cint, and a tuple of the argument types, in this case to
sort an array of Cdouble (Float64) elements. Julia compiles a version of
mycompare specialized for these argument types (if it has not done
so already), and returns a Ptr{Void} holding the address of the
machine code, exactly what we need to pass to qsort. We are
now ready to call qsort on some sample data:

A = [1.3, -2.7, 4.4, 3.1]
ccall(:qsort, Void, (Ptr{Cdouble}, Csize_t, Csize_t, Ptr{Void}),
A, length(A), sizeof(eltype(A)), mycompare_c)


After this executes, A is changed to the sorted array [ -2.7, 1.3, 3.1, 4.4]. Note that Julia knows how to convert an array
A::Vector{Cdouble} into a Ptr{Cdouble}, how to compute the sizeof
a type in bytes (identical to C’s sizeof operator), and so on. For fun,
try inserting a println("mycompare($a,$b)") line into mycompare, which
will allow you to see the comparisons that qsort is performing (and to verify
that it is really calling the Julia function that you passed to it).

## The problem with closures

We aren’t done yet, however. If you start passing callback functions to
C routines, it won’t be long before you discover that cfunction doesn’t
always work. For example, suppose we tried to declare our comparison
function inline, via:

mycomp = cfunction((a_,b_) -> unsafe_load(a_) < unsafe_load(b_) ?
cint(-1) : cint(+1),
Cint, (Ptr{Cdouble}, Ptr{Cdouble}))


Julia barfs on this, printing ERROR: function is not yet c-callable. In
general, cfunction only works for “top-level” functions: named
functions defined in the top-level (global or module) scope, but not
anonymous (args -> value) functions and not functions defined within
other functions (“nested” functions). The reason for this stems from
one important concept in computer science: a
closure.

To understand the need for closures, and the difficulty they pose for
callback functions, suppose that we wanted to provide a nicer interface
for qsort, one which permitted the user to simply pass a lessthan function
returning true or false while hiding all of the low-level business with
pointers, Cint, and so on. We might like to do something of the form:

function qsort!{T}(A::Vector{T}, lessthan::Function)
function mycompare(a_::Ptr{T}, b_::Ptr{T})
return lessthan(a, b) ? cint(-1) : cint(+1)
end
mycompare_c = cfunction(mycompare, Cint, (Ptr{T}, Ptr{T}))
ccall(:qsort, Void, (Ptr{T}, Csize_t, Csize_t, Ptr{Void}),
A, length(A), sizeof(T), mycompare_c)
A
end


Then we could simply call qsort!([1.3, -2.7, 4.4, 3.1], <) to sort
in ascending order using the built-in < comparison, or any other
comparison function we wanted. Unfortunately cfunction will again
barf when you try to call qsort!, and it is no longer so difficult
to understand why. Notice that the nested mycompare function is no
longer self-contained: it uses the variable lessthan from the
surrounding scope. This is a common pattern for nested functions and
anonymous functions: often, they are parameterized by local variables
in the environment where the function is defined. Technically, the
ability to have this kind of dependency is provided by lexical
scoping
in
a programming language like Julia, and is typical of any language in
which functions are
first-class
objects. In order to support lexical scoping, a Julia Function
object needs to internally carry around a pointer to the variables in
the enclosing environment, and this encapsulation is called a
closure.

In contrast, a C function pointer is not a closure. It doesn’t
enclose a pointer to the environment in which the function was
defined, or anything else for that matter; it is just the address of a
stream of instructions. This makes it hard, in C, to write functions
to transform other functions (higher-order
functions
) or to
parameterize functions by local variables. This apparently leaves us
with two options, neither of which is especially attractive:

• We could store lessthan in a global variable, and reference that
from a top-level mycompare function. (This is the traditional solution
for C programmers calling qsort with parameterized comparison functions.)
The problem with this strategy is that it is not re-entrant: it prevents us from calling qsort!
recursively (e.g. if the comparison function itself needs to do a sort, for
some complicated datastructure), or from calling qsort! from multiple
threads (when a future Julia version supports shared-memory parallelism).
Still, this is better than nothing.

• Every time qsort! is called, Julia could JIT-compile a new version
of mycompare, which hard-codes the reference to the lessthan
argument passed on that call. This is technically possible and has
been implemented in some languages (e.g. reportedly GNU
Guile

and Lua
do something like this). However, this strategy
comes at a price: it requires that callbacks be recompiled every time
a parameter in them changes, which is not true of the global-variable
strategy. Anyway, it is not implemented yet in Julia.

Fortunately, there is often a third option, because C programmers
long ago recognized these limitations of function pointers, and
devised a workaround: most modern C callback interfaces allow
arbitrary data to be passed through to the callback via a
“pass-through” (or “thunk”) pointer parameter. As explained in the
next section, we can exploit this technique in Julia to pass a “true”
closure as a callback.

## Passing closures via pass-through pointers

The qsort interface is nowadays considered rather antiquated. Years
ago, it was supplemented on BSD-Unix systems, and eventually in GNU
libc, by a function called qsort_r that solves the problem of passing
parameters to the callback in a re-entrant way. This is how the BSD (e.g. MacOS)
qsort_r function is defined:

void qsort_r(void *base, size_t nmemb, size_t size, void *thunk,
int (*compare)(void *thunk, const void *a, const void *b));


Compared to qsort, there is an extra thunk parameter, and this is
passed through to the compare function as its first argument. In this
way, you can pass a pointer to arbitrary data through to your callback,
and we can exploit this to pass a closure through for an arbitrary Julia
callback.

All we need is a way to convert a Julia Function into an opaque
Ptr{Void} so that we can pass it through to our callback, and then a
way to convert the opaque pointer back into a Function. The former
is automatic if we simply declare the ccall argument as type Any
(which passes the argument as an opaque Julia object pointer), and the
latter is accomplished by the built-in function
unsafe_pointer_to_objref. (Technically, we could use type
Function or an explicit call to pointer_from_objref instead of
Any.) Using these, we can now define a working high-level qsort!
function that takes an arbitrary lessthan comparison-function
argument:

function qsort!_compare{T}(lessthan_::Ptr{Void}, a_::Ptr{T}, b_::Ptr{T})
lessthan = unsafe_pointer_to_objref(lessthan_)::Function
return lessthan(a, b) ? cint(-1) : cint(+1)
end

function qsort!{T}(A::Vector{T}, lessthan::Function=<)
compare_c = cfunction(qsort!_compare, Cint, (Ptr{Void}, Ptr{T}, Ptr{T}))
ccall(:qsort_r, Void, (Ptr{T}, Csize_t, Csize_t, Any, Ptr{Void}),
A, length(A), sizeof(T), lessthan, compare_c)
return A
end


qsort!_compare is a top-level function, so cfunction has no
problem with it, and it will only be compiled once per type T to be
sorted (rather than once per call to qsort! or per lessthan
function). We use the explicit ::Function assertion to tell
the compiler that we will only pass Function pointers in
lessthan_. Note that we gave the lessthan argument a default value
of < (default arguments being a recent
feature
Julia).

We can now do qsort!([1.3, -2.7, 4.4, 3.1]) and it will
return the array sorted in ascending order, or qsort!([1.3, -2.7, 4.4, 3.1], >) to sort in descending order.

#### Warning: qsort_r is not portable

The example above has one major problem that has nothing to do with
Julia: the qsort_r function is not portable. The above example
won’t work on Windows, since the Windows C library doesn’t define
qsort_r (instead, it has a function called
qsort_s,
which of course uses an argument order incompatible with both the
BSD and GNU qsort_r functions). Worse, it will crash on GNU/Linux
systems, which do provide qsort_r but with an
incompatible
calling
convention
. And
as a result it is difficult to use qsort_r in a way that does not
crash either on GNU/Linux or BSD (e.g. MacOS) systems. This is how
glibc’s qsort_r is defined:

void qsort_r(void *base, size_t nmemb, size_t size,
int (*compare)(const void *a, const void *b, void *thunk),
void *thunk);


Note that the position of the thunk argument is moved, both in qsort_r itself and
in the comparison function. So, the corresponding qsort! Julia code on
GNU/Linux systems should be:

function qsort!_compare{T}(a_::Ptr{T}, b_::Ptr{T}, lessthan_::Ptr{Void})
lessthan = unsafe_pointer_to_objref(lessthan_)::Function
return lessthan(a, b) ? cint(-1) : cint(+1)
end

function qsort!{T}(A::Vector{T}, lessthan::Function=<)
compare_c = cfunction(qsort!_compare, Cint, (Ptr{T}, Ptr{T}, Ptr{Void}))
ccall(:qsort_r, Void, (Ptr{T}, Csize_t, Csize_t, Ptr{Void}, Any),
A, length(A), sizeof(T), compare_c, lessthan)
return A
end


If you really needed to call qsort_r from Julia, you could use the
above definitions if OS_NAME == :Linux and the BSD definitions
otherwise, with a third version using qsort_s on Windows, but
fortunately there is not much need as Julia comes with its own
perfectly adequate sort and sort! routines.

## Passing closures in data structures

As another example that is oriented more towards numerical
computations, we’ll examine how we might call the numerical integration
routines in the GNU Scientific Library
(GSL)
. There is already a GSL
package
that handles the wrapper
work below for you, but it is instructive to look at how this is
implemented because GSL simulates closures in a slightly different
way, with data structures.

Like most modern C libraries accepting callbacks, GSL uses a void* pass-through
parameter to allow arbitrary data to be passed through to the callback routine,
and we can use that to support arbitrary closures in Julia. Unlike qsort_r,
however, GSL wraps both the C function pointer and the pass-through pointer in
a data structure called gsl_function:

struct {
double (*function)(double x, void *params);
void *params;
} gsl_function;


Using the techniques above, we can easily declare a GSL_Function type in Julia
that mirrors this C type, and with a constructor GSL_Function(f::Function) that
creates a wrapper around an arbitrary Julia function f:

function gsl_function_wrap(x::Cdouble, params::Ptr{Void})
f = unsafe_pointer_to_objref(params)::Function
convert(Cdouble, f(x))::Cdouble
end
const gsl_function_wrap_c = cfunction(gsl_function_wrap,
Cdouble, (Cdouble, Ptr{Void}))

type GSL_Function
func::Ptr{Void}
params::Any
GSL_Function(f::Function) = new(gsl_function_wrap_c, f)
end


One subtlety with the above code is that we need to explicitly
convert the return value of f to a Cdouble (in case the caller’s
code returns some other numeric type for some x, such as an Int).
Moreover, we need to explicitly assert (::Cdouble) that the result
of the convert was a Cdouble. As with the qsort example, this
is because cfunction only works if Julia can guarantee that
gsl_function_wrap returns the specified Cdouble type, and
Julia cannot infer the return type of convert since it does not
know the return type of f(x).

Given the above definitions, it is a simple matter to pass this to the
GSL

routines in a wrapper function gsl_integration_qag:

function gsl_integration_qag(f::Function, a::Real, b::Real, epsrel::Real=1e-12,
maxintervals::Integer=10^7)
s = ccall((:gsl_integration_workspace_alloc,:libgsl), Ptr{Void}, (Csize_t,),
maxintervals)
result = Array(Cdouble,1)
abserr = Array(Cdouble,1)
ccall((:gsl_integration_qag,:libgsl), Cint,
(Ptr{GSL_Function}, Cdouble,Cdouble, Cdouble, Csize_t, Cint, Ptr{Void},
Ptr{Cdouble}, Ptr{Cdouble}),
&GSL_Function(f), a, b, epsrel, maxintervals, 1, s, result, abserr)
ccall((:gsl_integration_workspace_free,:libgsl), Void, (Ptr{Void},), s)
return (result[1], abserr[1])
end


Note that &GSL_Function(f) passes a pointer to a GSL_Function
“struct” containing a pointer to gsl_function_wrap_c and f, corresponding
to the gsl_function* argument in C. The return value is a tuple of the estimated
integral and an estimated error.

For example, gsl_integration_qag(cos, 0, 1) returns
(0.8414709848078965,9.34220461887732e-15), which computes the
correct integral sin(1) to machine precision.

## Taking out the trash (or not)

In the above examples, we pass an opaque pointer (object reference) to a
Julia Function into C. Whenever one passes pointers to Julia data into C
code, one has to ensure that the Julia data is not garbage-collected until
the C code is done with it, and functions are no exception to this rule.
An anonymous function that is no longer referred to by any Julia variable
may be garbage collected, at which point any C pointers to it become invalid.

This sounds scary, but in practice you don’t need to worry about it very often,
because Julia guarantees that ccall arguments won’t be garbage-collected until
the ccall exits. So, in all of the above examples, we are safe: the Function
only needs to live as long as the ccall.

The only danger arises when you pass a function pointer to C and the C code
saves the pointer in some data structure which it will use in a later ccall.
In that case, you are responsible for ensuring that the Function variable lives
(is referred to by some Julia variable) as long as the C code might need it.

For example, in the GSL one-dimensional minimization
interface
,
you don’t simply pass your objective function to a minimization
routine and wait until it is minimized. Instead, you call a GSL
routine to create a “minimizer object”, store your function pointer in this
object, call routines to iterate the minimization, and then deallocate the
minimizer when you are done. The Julia function must not be garbage-collected
until this process is complete. The easiest way to ensure this is to create
a Julia wrapper type around the minimizer object that stores an explicit
reference to the Julia function, like this:

type GSL_Minimizer
m::Ptr{Void} # the gsl_min_fminimizer pointer
f::Any  # explicit reference to objective, to prevent garbage-collection
function GSL_Minimizer(t)
m = ccall((:gsl_min_fminimizer_alloc,:libgsl), Ptr{Void}, (Ptr{Void},), t)
p = new(m, nothing)
finalizer(p, p -> ccall((:gsl_min_fminimizer_free,:libgsl),
Void, (Ptr{Void},), p.m))
p
end
end


This wraps around a gsl_min_fminimizer object of type t, with a
placeholder f to store a reference to the objective function (once
it is set below), including a finalizer to deallocate the GSL object
when the GSL_Minimizer is garbage-collected. The parameter t is
used to specify the minimization algorithm, which could default to
Brent’s algorithm via:

const gsl_brent = unsafe_load(cglobal((:gsl_min_fminimizer_brent,:libgsl), Ptr{Void}))
GSL_Minimizer() = GSL_Minimizer(gsl_brent)


(The call to cglobal yields a pointer to the
gsl_min_fminimizer_brent global variable in GSL, which we then
dereference to get the actual pointer via unsafe_load.)

Then, when we set the function to minimize (the “objective”), we store
an extra reference to it in the GSL_Minimizer to prevent
garbage-collection for the lifetime of the GSL_Minimizer, again
using the GSL_Function type defined above to wrap the callback:

function gsl_minimizer_set!(m::GSL_Minimizer, f, x0, xmin, xmax)
ccall((:gsl_min_fminimizer_set,:libgsl), Cint,
(Ptr{Void}, Ptr{GSL_Function}, Cdouble, Cdouble, Cdouble),
m.m, &GSL_Function(f), x0, xmin, xmax)
m.f = f
m
end


There are then various GSL routines to iterate the minimizer and to check the
current x, objective value, or bounds on the minimum, which are convenient to wrap:

gsl_minimizer_iterate!(m::GSL_Minimizer) =
ccall((:gsl_min_fminimizer_iterate,:libgsl), Cint, (Ptr{Void},), m.m)

gsl_minimizer_x(m::GSL_Minimizer) =
ccall((:gsl_min_fminimizer_x_minimum,:libgsl), Cdouble, (Ptr{Void},), m.m)

gsl_minimizer_f(m::GSL_Minimizer) =
ccall((:gsl_min_fminimizer_f_minimum,:libgsl), Cdouble, (Ptr{Void},), m.m)

gsl_minimizer_xmin(m::GSL_Minimizer) =
ccall((:gsl_min_fminimizer_x_lower,:libgsl), Cdouble, (Ptr{Void},), m.m)
gsl_minimizer_xmax(m::GSL_Minimizer) =
ccall((:gsl_min_fminimizer_x_upper,:libgsl), Cdouble, (Ptr{Void},), m.m)


Putting all of these together, we can minimize a simple function sin(x) in
the interval [-3,1], with a starting guess -1, via:

m = GSL_Minimizer()
gsl_minimizer_set!(m, sin, -1, -3, 1)
while gsl_minimizer_xmax(m) - gsl_minimizer_xmin(m) > 1e-6
println("iterating at x = $(gsl_minimizer_x(m))") gsl_minimizer_iterate!(m) end println("found minimum$(gsl_minimizer_f(m)) at x = \$(gsl_minimizer_x(m))")


After a few iterations, it prints found minimum -1.0 at x = -1.5707963269964016, which is the correct minimum (−π/2) to

At this point, I will shamelessly plug my own NLopt
package
for Julia, which wraps
around my free/open-source NLopt library
to provide many more optimization algorithms than GSL, with perhaps a nicer
interface. However, the techniques used to pass callback functions to
NLopt are actually quite similar to those used for GSL.

An even more complicated version of these techniques can be found in
the PyCall package to call
Python from Julia. In order to pass a Julia function to Python, we
again use cfunction on a wrapper function that handles the type
conversions and so on, and pass the actual Julia closure through via a
pass-through pointer. But in that case, the pass-through pointer
consists of a Python object that has been created with a new type that
allows it to wrap a Julia object, and garbage-collection is deferred
by storing the Julia object in a global dictionary of saved objects
(removing it via the Python destructor of the new type). That is all
somewhat tricky stuff and beyond the scope of this blog post; I only
mention it to illustrate the fact that it is possible to implement
quite complex inter-language calling behaviors purely in Julia by
building on the above techniques.