Porting code from other platforms to WebAssembly

In the form that's currently standardized and supported by browsers and other hosts, WebAssembly is a 32-bit architecture. You have to take this into account when porting code from other platforms, since Int type is a signed 32-bit integer, and UInt is an unsigned 32-bit integer when building with SwiftWasm. You'll need to audit codepaths that cast 64-bit integers to Int or UInt, and a good amount of cross-platform test coverage can help with that.

Additionally, there a differences in APIs exposed by the standard C library and Swift core libraries which we discuss in the next few subsections.

WASILibc module

When porting existing projects from other platforms to SwiftWasm you might stumble upon code that relies on importing a platform-specific C library module directly. It looks like import Glibc on Linux, or import Darwin on Apple platforms. Fortunately, most of the standard C library APIs are available when using SwiftWasm, you just need to use import WASILibc to get access to it. Most probably you want to preserve compatibility with other platforms, thus your imports would look like this:

#if canImport(Darwin)
import Darwin
#elseif canImport(Glibc)
import Glibc
#elseif canImport(WASILibc)
import WASILibc


WebAssembly and WASI provide a constrained environment, which currently does not directly support multi-threading, or filesystem access in the browser. Thus, you should not expect these APIs to work when importing WASILibc. Please be aware of these limitations when porting your code, which also has an impact on what can be supported in the Swift Foundation at the moment.

Swift Foundation and Dispatch

The Foundation core library is available in SwiftWasm, but in a limited capacity. The main reason is that the Dispatch core library is unavailable due to the lack of standardized multi-threading support in WebAssembly and SwiftWasm itself. Many Foundation APIs rely on the presence of Dispatch under the hood, specifically file system access and threading helpers. A few other types are unavailable in browsers or aren't standardized in WASI hosts, such as support for sockets and low-level networking, or support for time zone files, and they had to be disabled. These types are therefore absent in SwiftWasm Foundation:

  • FoundationNetworking types, such as URLSession and related APIs
  • FileManager
  • Host
  • Notification
  • NotificationQueue
  • NSKeyedArchiver
  • NSKeyedArchiverHelpers
  • NSKeyedCoderOldStyleArray
  • NSKeyedUnarchiver
  • NSNotification
  • NSSpecialValue
  • Port
  • PortMessage
  • Process
  • ProcessInfo
  • PropertyListEncoder
  • RunLoop
  • Stream
  • Thread
  • Timer
  • UserDefaults

Related functions and properties on other types are also absent or disabled. We would like to make them available in the future as soon as possible, and we invite you to contribute and help us in achieving this goal!