Calling C and Fortran Code

Though most code can be written in Julia, there are many high-quality, mature libraries for numerical computing already written in C and Fortran. To allow easy use of this existing code, Julia makes it simple and efficient to call C and Fortran functions. Julia has a “no boilerplate” philosophy: functions can be called directly from Julia without any “glue” code, code generation, or compilation — even from the interactive prompt. This is accomplished just by making an appropriate call with ccall syntax, which looks like an ordinary function call.

The code to be called must be available as a shared library. Most C and Fortran libraries ship compiled as shared libraries already, but if you are compiling the code yourself using GCC (or Clang), you will need to use the -shared and -fPIC options. The machine instructions generated by Julia’s JIT are the same as a native C call would be, so the resulting overhead is the same as calling a library function from C code. (Non-library function calls in both C and Julia can be inlined and thus may have even less overhead than calls to shared library functions. When both libraries and executables are generated by LLVM, it is possible to perform whole-program optimizations that can even optimize across this boundary, but Julia does not yet support that. In the future, however, it may do so, yielding even greater performance gains.)

Shared libraries and functions are referenced by a tuple of the form (:function, "library") or ("function", "library") where function is the C-exported function name. library refers to the shared library name: shared libraries available in the (platform-specific) load path will be resolved by name, and if necessary a direct path may be specified.

A function name may be used alone in place of the tuple (just :function or "function"). In this case the name is resolved within the current process. This form can be used to call C library functions, functions in the Julia runtime, or functions in an application linked to Julia.

Finally, you can use ccall to actually generate a call to the library function. Arguments to ccall are as follows:

  1. (:function, “library”) pair (must be a constant, but see below).
  2. Return type, which may be any bits type, including Int32, Int64, Float64, or Ptr{T} for any type parameter T, indicating a pointer to values of type T, or Ptr{Void} for void* “untyped pointer” values.
  3. A tuple of input types, like those allowed for the return type. The input types must be written as a literal tuple, not a tuple-valued variable or expression.
  4. The following arguments, if any, are the actual argument values passed to the function.

As a complete but simple example, the following calls the clock function from the standard C library:

julia> t = ccall( (:clock, "libc"), Int32, ())

julia> t

julia> typeof(ans)

clock takes no arguments and returns an Int32. One common gotcha is that a 1-tuple must be written with a trailing comma. For example, to call the getenv function to get a pointer to the value of an environment variable, one makes a call like this:

julia> path = ccall( (:getenv, "libc"), Ptr{Uint8}, (Ptr{Uint8},), "SHELL")
Ptr{Uint8} @0x00007fff5fbffc45

julia> bytestring(path)

Note that the argument type tuple must be written as (Ptr{Uint8},), rather than (Ptr{Uint8}). This is because (Ptr{Uint8}) is just Ptr{Uint8}, rather than a 1-tuple containing Ptr{Uint8}:

julia> (Ptr{Uint8})

julia> (Ptr{Uint8},)

In practice, especially when providing reusable functionality, one generally wraps ccall uses in Julia functions that set up arguments and then check for errors in whatever manner the C or Fortran function indicates them, propagating to the Julia caller as exceptions. This is especially important since C and Fortran APIs are notoriously inconsistent about how they indicate error conditions. For example, the getenv C library function is wrapped in the following Julia function in env.jl:

function getenv(var::String)
  val = ccall( (:getenv, "libc"),
              Ptr{Uint8}, (Ptr{Uint8},), var)
  if val == C_NULL
    error("getenv: undefined variable: ", var)

The C getenv function indicates an error by returning NULL, but other standard C functions indicate errors in various different ways, including by returning -1, 0, 1 and other special values. This wrapper throws an exception clearly indicating the problem if the caller tries to get a non-existent environment variable:

julia> getenv("SHELL")

julia> getenv("FOOBAR")
getenv: undefined variable: FOOBAR

Here is a slightly more complex example that discovers the local machine’s hostname:

function gethostname()
  hostname = Array(Uint8, 128)
  ccall( (:gethostname, "libc"), Int32,
        (Ptr{Uint8}, Uint),
        hostname, length(hostname))
  return bytestring(convert(Ptr{Uint8}, hostname))

This example first allocates an array of bytes, then calls the C library function gethostname to fill the array in with the hostname, takes a pointer to the hostname buffer, and converts the pointer to a Julia string, assuming that it is a NUL-terminated C string. It is common for C libraries to use this pattern of requiring the caller to allocate memory to be passed to the callee and filled in. Allocation of memory from Julia like this is generally accomplished by creating an uninitialized array and passing a pointer to its data to the C function.

When calling a Fortran function, all inputs must be passed by reference.

A prefix & is used to indicate that a pointer to a scalar argument should be passed instead of the scalar value itself. The following example computes a dot product using a BLAS function.

function compute_dot(DX::Vector{Float64}, DY::Vector{Float64})
  assert(length(DX) == length(DY))
  n = length(DX)
  incx = incy = 1
  product = ccall( (:ddot_, "libLAPACK"),
                  (Ptr{Int32}, Ptr{Float64}, Ptr{Int32}, Ptr{Float64}, Ptr{Int32}),
                  &n, DX, &incx, DY, &incy)
  return product

The meaning of prefix & is not quite the same as in C. In particular, any changes to the referenced variables will not be visible in Julia. However, it will never cause any harm for called functions to attempt such modifications (that is, writing through the passed pointers). Since this & is not a real address operator, it may be used with any syntax, such as &0 or &f(x).

Note that no C header files are used anywhere in the process. Currently, it is not possible to pass structs and other non-primitive types from Julia to C libraries. However, C functions that generate and use opaque struct types by passing pointers to them can return such values to Julia as Ptr{Void}, which can then be passed to other C functions as Ptr{Void}. Memory allocation and deallocation of such objects must be handled by calls to the appropriate cleanup routines in the libraries being used, just like in any C program.

Mapping C Types to Julia

Julia automatically inserts calls to the convert function to convert each argument to the specified type. For example, the following call:

ccall( (:foo, "libfoo"), Void, (Int32, Float64),
      x, y)

will behave as if the following were written:

ccall( (:foo, "libfoo"), Void, (Int32, Float64),
      convert(Int32, x), convert(Float64, y))

When a scalar value is passed with & as an argument of type Ptr{T}, the value will first be converted to type T.

Array conversions

When an array is passed to C as a Ptr{T} argument, it is never converted: Julia simply checks that the element type of the array matches T, and the address of the first element is passed. This is done in order to avoid copying arrays unnecessarily.

Therefore, if an Array contains data in the wrong format, it will have to be explicitly converted using a call such as int32(a).

To pass an array A as a pointer of a different type without converting the data beforehand (for example, to pass a Float64 array to a function that operates on uninterpreted bytes), you can either declare the argument as Ptr{Void} or you can explicitly call convert(Ptr{T}, pointer(A)).

Type correspondences

On all systems we currently support, basic C/C++ value types may be translated to Julia types as follows. Every C type also has a corresponding Julia type with the same name, prefixed by C. This can help for writing portable code (and remembering that an int in C is not the same as an Int in Julia).


signed char   Int8
unsigned char Cuchar Uint8
short Cshort Int16
unsigned short Cushort Uint16
int Cint Int32
unsigned int Cuint Uint32
long long Clonglong Int64
unsigned long long Culonglong Uint64
float Cfloat Float32
double Cdouble Float64
ptrdiff_t Cptrdiff_t Int
ssize_t Cssize_t Int
size_t Csize_t Uint
complex float Ccomplex_float (future addition)
complex double Ccomplex_double (future addition)
void   Void
void*   Ptr{Void}
char* (or char[], e.g. a string) Ptr{Uint8}
char** (or *char[]) Ptr{Ptr{Uint8}}
struct T* (where T represents an appropriately defined bits type) Ptr{T} (call using &variable_name in the parameter list)
struct T (where T represents an appropriately defined bits type) T (call using &variable_name in the parameter list)
jl_value_t* (any Julia Type) Ptr{Any}

Julia’s Char type is 32 bits, which is not the same as the wide character type (wchar_t or wint_t) on all platforms.

A C function declared to return void will give nothing in Julia.


char Cchar

Int8 (x86, x86_64)

Uint8 (powerpc, arm)

long Clong

Int (UNIX)

Int32 (Windows)

unsigned long Culong

Uint (UNIX)

Uint32 (Windows)

wchar_t Cwchar_t

Int32 (UNIX)

Uint16 (Windows)

For string arguments (char*) the Julia type should be Ptr{Uint8}, not ASCIIString. C functions that take an argument of the type char** can be called by using a Ptr{Ptr{Uint8}} type within Julia. For example, C functions of the form:

int main(int argc, char **argv);

can be called via the following Julia code:

argv = [ "a.out", "arg1", "arg2" ]
ccall(:main, Int32, (Int32, Ptr{Ptr{Uint8}}), length(argv), argv)

For wchar_t* arguments, the Julia type should be Ptr{Wchar_t}, and data can be converted to/from ordinary Julia strings by the wstring(s) function (equivalent to either utf16(s) or utf32(s) depending upon the width of Cwchar_t. Note also that ASCII, UTF-8, UTF-16, and UTF-32 string data in Julia is internally NUL-terminated, so it can be passed to C functions expecting NUL-terminated data without making a copy.

Accessing Data through a Pointer

The following methods are described as “unsafe” because they can cause Julia to terminate abruptly or corrupt arbitrary process memory due to a bad pointer or type declaration.

Given a Ptr{T}, the contents of type T can generally be copied from the referenced memory into a Julia object using unsafe_load(ptr, [index]). The index argument is optional (default is 1), and performs 1-based indexing. This function is intentionally similar to the behavior of getindex() and setindex!() (e.g. [] access syntax).

The return value will be a new object initialized to contain a copy of the contents of the referenced memory. The referenced memory can safely be freed or released.

If T is Any, then the memory is assumed to contain a reference to a Julia object (a jl_value_t*), the result will be a reference to this object, and the object will not be copied. You must be careful in this case to ensure that the object was always visible to the garbage collector (pointers do not count, but the new reference does) to ensure the memory is not prematurely freed. Note that if the object was not originally allocated by Julia, the new object will never be finalized by Julia’s garbage collector. If the Ptr itself is actually a jl_value_t*, it can be converted back to a Julia object reference by unsafe_pointer_to_objref(ptr). (Julia values v can be converted to jl_value_t* pointers, as Ptr{Void}, by calling pointer_from_objref(v).)

The reverse operation (writing data to a Ptr{T}), can be performed using unsafe_store!(ptr, value, [index]). Currently, this is only supported for bitstypes or other pointer-free (isbits) immutable types.

Any operation that throws an error is probably currently unimplemented and should be posted as a bug so that it can be resolved.

If the pointer of interest is a plain-data array (bitstype or immutable), the function pointer_to_array(ptr,dims,[own]) may be more useful. The final parameter should be true if Julia should “take ownership” of the underlying buffer and call free(ptr) when the returned Array object is finalized. If the own parameter is omitted or false, the caller must ensure the buffer remains in existence until all access is complete.

Arithmetic on the Ptr type in Julia (e.g. using +) does not behave the same as C’s pointer arithmetic. Adding an integer to a Ptr in Julia always moves the pointer by some number of bytes, not elements. This way, the address values obtained from pointer arithmetic do not depend on the element types of pointers.

Passing Pointers for Modifying Inputs

Because C doesn’t support multiple return values, often C functions will take pointers to data that the function will modify. To accomplish this within a ccall you need to encapsulate the value inside an array of the appropriate type. When you pass the array as an argument with a Ptr type, julia will automatically pass a C pointer to the encapsulated data:

width = Cint[0]
range = Cfloat[0]
ccall(:foo, Void, (Ptr{Cint}, Ptr{Cfloat}), width, range)

This is used extensively in Julia’s LAPACK interface, where an integer info is passed to LAPACK by reference, and on return, includes the success code.

Garbage Collection Safety

When passing data to a ccall, it is best to avoid using the pointer() function. Instead define a convert method and pass the variables directly to the ccall. ccall automatically arranges that all of its arguments will be preserved from garbage collection until the call returns. If a C API will store a reference to memory allocated by Julia, after the ccall returns, you must arrange that the object remains visible to the garbage collector. The suggested way to handle this is to make a global variable of type Array{Any,1} to hold these values, until C interface notifies you that it is finished with them.

Whenever you have created a pointer to Julia data, you must ensure the original data exists until you are done with using the pointer. Many methods in Julia such as unsafe_load() and bytestring() make copies of data instead of taking ownership of the buffer, so that it is safe to free (or alter) the original data without affecting Julia. A notable exception is pointer_to_array() which, for performance reasons, shares (or can be told to take ownership of) the underlying buffer.

The garbage collector does not guarantee any order of finalization. That is, if a contained a reference to b and both a and b are due for garbage collection, there is no guarantee that b would be finalized after a. If proper finalization of a depends on b being valid, it must be handled in other ways.

Non-constant Function Specifications

A (name, library) function specification must be a constant expression. However, it is possible to use computed values as function names by staging through eval as follows:

@eval ccall(($(string("a","b")),"lib"), ...

This expression constructs a name using string, then substitutes this name into a new ccall expression, which is then evaluated. Keep in mind that eval only operates at the top level, so within this expression local variables will not be available (unless their values are substituted with $). For this reason, eval is typically only used to form top-level definitions, for example when wrapping libraries that contain many similar functions.

Indirect Calls

The first argument to ccall can also be an expression evaluated at run time. In this case, the expression must evaluate to a Ptr, which will be used as the address of the native function to call. This behavior occurs when the first ccall argument contains references to non-constants, such as local variables or function arguments.

Calling Convention

The second argument to ccall can optionally be a calling convention specifier (immediately preceding return type). Without any specifier, the platform-default C calling convention is used. Other supported conventions are: stdcall, cdecl, fastcall, and thiscall. For example (from base/libc.jl):

hn = Array(Uint8, 256)
err=ccall(:gethostname, stdcall, Int32, (Ptr{Uint8}, Uint32), hn, length(hn))

For more information, please see the LLVM Language Reference.

Accessing Global Variables

Global variables exported by native libraries can be accessed by name using the cglobal function. The arguments to cglobal are a symbol specification identical to that used by ccall, and a type describing the value stored in the variable:

julia> cglobal((:errno,:libc), Int32)
Ptr{Int32} @0x00007f418d0816b8

The result is a pointer giving the address of the value. The value can be manipulated through this pointer using unsafe_load and unsafe_store.

Passing Julia Callback Functions to C

It is possible to pass Julia functions to native functions that accept function pointer arguments. A classic example is the standard C library qsort function, declared 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:

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

Notice that we have to be careful about the return type: qsort expects a function returning a C int, so we must be sure to return Cint via a call to convert.

In order to pass this function to C, we obtain its address using the function cfunction:

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

cfunction accepts three arguments: the Julia function (mycompare), the return type (Cint), and a tuple of the argument types, in this case to sort an array of Cdouble (Float64) elements.

The final call to qsort looks like this:

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 into a Ptr{Cdouble}, how to compute the size of 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).


Some C libraries execute their callbacks from a different thread, and since Julia isn’t thread-safe you’ll need to take some extra precautions. In particular, you’ll need to set up a two-layered system: the C callback should only schedule (via Julia’s event loop) the execution of your “real” callback. To do this, you pass a function of one argument (the AsyncWork object for which the event was triggered, which you’ll probably just ignore) to SingleAsyncWork:

cb = Base.SingleAsyncWork(data -> my_real_callback(args))

The callback you pass to C should only execute a ccall to :uv_async_send, passing cb.handle as the argument.

More About Callbacks

For more details on how to pass callbacks to C libraries, see this blog post.


Limited support for C++ is provided by the Cpp and Clang packages.

Handling Platform Variations

When dealing with platform libraries, it is often necessary to provide special cases for various platforms. The variable OS_NAME can be used to write these special cases. Additionally, there are several macros intended to make this easier: @windows, @unix, @linux, and @osx. Note that linux and osx are mutually exclusive subsets of unix. Their usage takes the form of a ternary conditional operator, as demonstrated in the following examples.

Simple blocks:

ccall( (@windows? :_fopen : :fopen), ...)

Complex blocks:

@linux? (
       : begin

Chaining (parentheses optional, but recommended for readability):

@windows? :a : (@osx? :b : :c)