# Miscellaneous

## Types

### Introspection

*New in Koffi 2.0: `koffi.resolve()`, new in Koffi 2.2: `koffi.offsetof()`*

```{note}
The value returned by `introspect()` has **changed in version 2.0 and in version 2.2**.

In Koffi 1.x, it could only be used with struct types and returned the object passed to koffi.struct() with the member names and types.

Starting in Koffi 2.2, each record member is exposed as an object containing the name, the type and the offset within the record.

Consult the [migration guide](migration.md) for more information.
```

Use `koffi.introspect(type)` to get detailed information about a type: name, primitive, size, alignment, members (record types), reference type (array, pointer) and length (array).

```js
const FoobarType = koffi.struct('FoobarType', {
    a: 'int',
    b: 'char *',
    c: 'double'
});

console.log(koffi.introspect(FoobarType));

// Expected result on 64-bit machines:
// {
//     name: 'FoobarType',
//     primitive: 'Record',
//     size: 24,
//     alignment: 8,
//     members: {
//         a: { name: 'a', type: [External: 4b28a60], offset: 0 },
//         b: { name: 'b', type: [External: 4b292e0], offset: 8 },
//         c: { name: 'c', type: [External: 4b29260], offset: 16 }
//     }
// }
```

Koffi also exposes a few more utility functions to get a subset of this information:

- `koffi.sizeof(type)` to get the size of a type
- `koffi.alignof(type)` to get the alignment of a type
- `koffi.offsetof(type, member_name)` to get the offset of a record member
- `koffi.resolve(type)` to get the resolved type object from a type string

Just like before, you can refer to primitive types by their name or through `koffi.types`:

```js
// These two lines do the same:
console.log(koffi.sizeof('long'));
console.log(koffi.sizeof(koffi.types.long));
```

### Aliases

*New in Koffi 2.0*

You can alias a type with `koffi.alias(name, type)`. Aliased types are completely equivalent.

## Settings

### Memory usage

For synchronous/normal calls, Koffi uses two preallocated memory blocks:

- One to construct the C stack and assign registers, subsequently used by the platform-specific assembly code (1 MiB by default)
- One to allocate strings and big objects/structs (2 MiB by default)

Unless very big strings or objects (at least more than one page of memory) are used, Koffi does not directly allocate any extra memory during calls or callbacks. However, please note that the JS engine (V8) might.

The size (in bytes) of these preallocated blocks can be changed. Use `koffi.config()` to get an object with the settings, and `koffi.config(obj)` to apply new settings.

```js
let config = koffi.config();
console.log(config);
```

The same is true for asynchronous calls. When an asynchronous call is made, Koffi will allocate new blocks unless there is an unused (resident) set of blocks still available. Once the asynchronous call is finished, these blocks are freed if there are more than `resident_async_pools` sets of blocks left around.

There cannot be more than `max_async_calls` running at the same time.

### Default settings

Setting              | Default | Description
-------------------- | ------- | -----------------------------------------------
sync_stack_size      | 1 MiB   | Stack size for synchronous calls
sync_heap_size       | 2 MiB   | Heap size for synchronous calls
async_stack_size     | 256 kiB | Stack size for asynchronous calls
async_heap_size      | 512 kiB | Heap size for asynchronous calls
resident_async_pools | 2       | Number of resident pools for asynchronous calls
max_async_calls      | 64      | Maximum number of ongoing asynchronous calls
max_type_size        | 64 MiB  | Maximum size of Koffi types (for arrays and structs)

## Usage statistics

*New in Koffi 2.3.2*

You can use `koffi.stats()` to get a few statistics related to Koffi.

## POSIX error codes

*New in Koffi 2.3.14*

You can use `koffi.errno()` to the current errno value, and `koffi.errno(value)` to change it.

The standard POSIX error codes are available in `koffi.os.errno`, as in the example below:

```js
const assert = require('assert');

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

const lib = koffi.load('libc.so.6');

const close = lib.func('int close(int fd)');

close(-1);
assert.equal(koffi.errno(), koffi.os.errno.EBADF);

console.log('close() with invalid FD is POSIX compliant!');
```
