Primitive types

Standard types

While the C standard allows for variation in the size of most integer types, Koffi enforces the same definition for most primitive types, listed below:

Koffi also accepts BigInt values when converting from JS to C integers. If the value exceeds the range of the C type, Koffi will convert the number to an undefined value. In the reverse direction, BigInt values are automatically used when needed for big 64-bit integers.

Koffi defines a few more types that can change size depending on the OS and the architecture:

Primitive types can be specified by name (in a string) or through koffi.types:

// These two lines do the same:
let struct1 = koffi.struct({ dummy: 'long' });
let struct2 = koffi.struct({ dummy: koffi.types.long });

Endian-sensitive integers

New in Koffi 2.1

Koffi defines a bunch of endian-sensitive types, which can be used when dealing with binary data (network payloads, binary file formats, etc.).

Struct types

Struct definition

Koffi converts JS objects to C structs, and vice-versa.

Unlike function declarations, as of now there is only one way to create a struct type, with the koffi.struct() function. This function takes two arguments: the first one is the name of the type, and the second one is an object containing the struct member names and types. You can omit the first argument to declare an anonymous struct.

The following example illustrates how to declare the same struct in C and in JS with Koffi:

typedef struct A {
    int a;
    char b;
    const char *c;
    struct {
        double d1;
        double d2;
    } d;
} A;

const A = koffi.struct('A', {
    a: 'int',
    b: 'char',
    c: 'const char *', // Koffi does not care about const, it is ignored
    d: koffi.struct({
        d1: 'double',
        d2: 'double'
    })
});

Koffi automatically follows the platform C ABI regarding alignment and padding. However, you can override these rules if needed with:

• Pack all members without padding with koffi.pack() (instead of koffi.struct())
• Change alignment of a specific member as shown below

// This struct is 3 bytes long
const PackedStruct = koffi.pack('PackedStruct', {
    a: 'int8_t',
    b: 'int16_t'
});

// This one is 10 bytes long, the second member has an alignment requirement of 8 bytes
const BigStruct = koffi.struct('BigStruct', {
    a: 'int8_t',
    b: [8, 'int16_t']
})

Once a struct is declared, you can use it by name (with a string, like you can do for primitive types) or through the value returned by the call to koffi.struct(). Only the latter is possible when declaring an anonymous struct.

// The following two function declarations are equivalent, and declare a function taking an A value and returning A
const Function1 = lib.func('A Function(A value)');
const Function2 = lib.func('Function', A, [A]);

Opaque types

Many C libraries use some kind of object-oriented API, with a pair of functions dedicated to create and delete objects. An obvious example of this can be found in stdio.h, with the opaque FILE * pointer. You can open and close files with fopen() and fclose(), and manipule the opaque pointer with other functions such as fread() or ftell().

In Koffi, you can manage this with opaque types. Declare the opaque type with koffi.opaque(name), and use a pointer to this type either as a return type or some kind of output parameter (with a double pointer).

The full example below implements an iterative string builder (concatenator) in C, and uses it from Javascript to output a mix of Hello World and FizzBuzz. The builder is hidden behind an opaque type, and is created and destroyed using a pair of C functions: ConcatNew (or ConcatNewOut) and ConcatFree.

// Build with: clang -fPIC -o handles.so -shared handles.c -Wall -O2

#include <stdlib.h>
#include <stdbool.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

typedef struct Fragment {
    struct Fragment *next;

    size_t len;
    char str[];
} Fragment;

typedef struct Concat {
    Fragment *first;
    Fragment *last;

    size_t total;
} Concat;

bool ConcatNewOut(Concat **out)
{
    Concat *c = malloc(sizeof(*c));
    if (!c) {
        fprintf(stderr, "Failed to allocate memory: %s\n", strerror(errno));
        return false;
    }

    c->first = NULL;
    c->last = NULL;
    c->total = 0;

    *out = c;
    return true;
}

Concat *ConcatNew()
{
    Concat *c = NULL;
    ConcatNewOut(&c);
    return c;
}

void ConcatFree(Concat *c)
{
    if (!c)
        return;

    Fragment *f = c->first;

    while (f) {
        Fragment *next = f->next;
        free(f);
        f = next;
    }

    free(c);
}

bool ConcatAppend(Concat *c, const char *frag)
{
    size_t len = strlen(frag);

    Fragment *f = malloc(sizeof(*f) + len + 1);
    if (!f) {
        fprintf(stderr, "Failed to allocate memory: %s\n", strerror(errno));
        return false;
    }

    f->next = NULL;
    if (c->last) {
        c->last->next = f;
    } else {
        c->first = f;
    }
    c->last = f;
    c->total += len;

    f->len = len;
    memcpy(f->str, frag, len);
    f->str[len] = 0;

    return true;
}

const char *ConcatBuild(Concat *c)
{
    Fragment *r = malloc(sizeof(*r) + c->total + 1);
    if (!r) {
        fprintf(stderr, "Failed to allocate memory: %s\n", strerror(errno));
        return NULL;
    }

    r->next = NULL;
    r->len = 0;

    Fragment *f = c->first;

    while (f) {
        Fragment *next = f->next;

        memcpy(r->str + r->len, f->str, f->len);
        r->len += f->len;

        free(f);
        f = next;
    }
    r->str[r->len] = 0;

    c->first = r;
    c->last = r;

    return r->str;
}

// ES6 syntax: import koffi from 'koffi';
const koffi = require('koffi');

const lib = koffi.load('./handles.so');

const Concat = koffi.opaque('Concat');
const ConcatNewOut = lib.func('bool ConcatNewOut(_Out_ Concat **out)');
const ConcatNew = lib.func('Concat *ConcatNew()');
const ConcatFree = lib.func('void ConcatFree(Concat *c)');
const ConcatAppend = lib.func('bool ConcatAppend(Concat *c, const char *frag)');
const ConcatBuild = lib.func('const char *ConcatBuild(Concat *c)');

let c = ConcatNew();
if (!c) {
    // This is stupid, it does the same, but try both versions (return value, output parameter)
    let ptr = [null];
    if (!ConcatNewOut(ptr))
        throw new Error('Allocation failure');
    c = ptr[0];
}

try {
    if (!ConcatAppend(c, 'Hello... '))
        throw new Error('Allocation failure');
    if (!ConcatAppend(c, 'World!\n'))
        throw new Error('Allocation failure');

    for (let i = 1; i <= 30; i++) {
        let frag;
        if (i % 15 == 0) {
            frag = 'FizzBuzz';
        } else if (i % 5 == 0) {
            frag = 'Buzz';
        } else if (i % 3 == 0) {
            frag = 'Fizz';
        } else {
            frag = String(i);
        }

        if (!ConcatAppend(c, frag))
            throw new Error('Allocation failure');
        if (!ConcatAppend(c, ' '))
            throw new Error('Allocation failure');
    }

    let str = ConcatBuild(c);
    if (str == null)
        throw new Error('Allocation failure');
    console.log(str);
} finally {
    ConcatFree(c);
}

Array types

Fixed-size C arrays

Changed in Koffi 2.7.1

Fixed-size arrays are declared with koffi.array(type, length). Just like in C, they cannot be passed as functions parameters (they degenerate to pointers), or returned by value. You can however embed them in struct types.

Koffi applies the following conversion rules when passing arrays to/from C:

• JS to C: Koffi can take a normal Array (e.g. [1, 2]) or a TypedArray of the correct type (e.g. Uint8Array for an array of uint8_t numbers)
• C to JS (return value, output parameters, callbacks): Koffi will use a TypedArray if possible. But you can change this behavior when you create the array type with the optional hint argument: koffi.array('uint8_t', 64, 'Array'). For non-number types, such as arrays of strings or structs, Koffi creates normal arrays.

See the example below:

// ES6 syntax: import koffi from 'koffi';
const koffi = require('koffi');

// Those two structs are exactly the same, only the array conversion hint is different
const Foo1 = koffi.struct('Foo1', {
    i: 'int',
    a16: koffi.array('int16_t', 2)
});
const Foo2 = koffi.struct('Foo2', {
    i: 'int',
    a16: koffi.array('int16_t', 2, 'Array')
});

// Uses an hypothetical C function that just returns the struct passed as a parameter
const ReturnFoo1 = lib.func('Foo1 ReturnFoo(Foo1 p)');
const ReturnFoo2 = lib.func('Foo2 ReturnFoo(Foo2 p)');

console.log(ReturnFoo1({ i: 5, a16: [6, 8] })) // Prints { i: 5, a16: Int16Array(2) [6, 8] }
console.log(ReturnFoo2({ i: 5, a16: [6, 8] })) // Prints { i: 5, a16: [6, 8] }

You can also declare arrays with the C-like short syntax in type declarations, as shown below:

const StructType = koffi.struct('StructType', {
    f8: 'float [8]',
    self4: 'StructType *[4]'
});

Fixed-size string buffers

Changed in Koffi 2.9.0

Koffi can also convert JS strings to fixed-sized arrays in the following cases:

• char arrays are filled with the UTF-8 encoded string, truncated if needed. The buffer is always NUL-terminated.
• char16 (or char16_t) arrays are filled with the UTF-16 encoded string, truncated if needed. The buffer is always NUL-terminated.
• char32 (or char32_t) arrays are filled with the UTF-32 encoded string, truncated if needed. The buffer is always NUL-terminated.

The reverse case is also true, Koffi can convert a C fixed-size buffer to a JS string. This happens by default for char, char16_t and char32_t arrays, but you can also explicitly ask for this with the String array hint (e.g. koffi.array('char', 8, 'String')).

Dynamic arrays (pointers)

In C, dynamically-sized arrays are usually passed around as pointers. Read more about array pointers in the relevant section.

Union types

The declaration and use of union types will be explained in a later section, they are only briefly mentioned here if you need them.