Using dlsym to load both functions and constants

Using Swift
1

I was successfully using dlsym to dynamically load and invoke functions at runtime. Now I need to load constants and for some reason unsafeBitCast fails on me, whereas assumingMemoryBound works. However, using assumingMemoryBound with a function somehow messes with it and it crashes during the invocation.

unsafeBitCast fails with constant:

import Foundation
internal typealias F = @convention(c) (_ alloc: CFAllocator?, _ cStr: UnsafePointer<UInt8>, _ encoding: CFStringEncoding) -> CFString?
func load<T>(_ name: String) -> T {
    unsafeBitCast(dlsym(dlopen(nil, RTLD_NOW), name), to: T.self)
}
print(load("CFStringCreateWithCString") as F) // (Function)
print((load("CFStringCreateWithCString") as F)(nil, [102, 111, 111, 0], CFStringBuiltInEncodings.UTF8.rawValue)!) // foo
print(load("kCFErrorLocalizedFailureKey") as CFString) // EXC_BAD_ACCESS

assumingMemoryBound fails with invocation:

import Foundation
internal typealias F = @convention(c) (_ alloc: CFAllocator?, _ cStr: UnsafePointer<UInt8>, _ encoding: CFStringEncoding) -> CFString?
func load<T>(_ name: String) -> T {
    dlsym(dlopen(nil, RTLD_NOW), name).assumingMemoryBound(to: T.self).pointee
}
print(load("kCFErrorLocalizedFailureKey") as CFString) // NSLocalizedFailure
print(load("CFStringCreateWithCString") as F) // (Function)
print((load("CFStringCreateWithCString") as F)(nil, [102, 111, 111, 0], CFStringBuiltInEncodings.UTF8.rawValue)!) // EXC_BAD_ACCESS

Can anybody explain what's going on? Is there a way to load all kinds of symbols with one declaration?

2

@eskimo, @NeoNacho, @mayoff, you guys posted some great answers on these matter. May I kindly ask for your input?

3

A function symbol generally resolves to the entry point of the function, which in C would be a function pointer type, or in Swift a @convention(c) function type. A data symbol generally resolves to the address of the data, not the data itself, so you probably want to resolve kCFErrorLocalizedFailureKey as an UnsafePointer<CFString> and load the CFString from the pointer.

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4

Argh! Brilliant! Thanks you very much!

You say generally. Is that the rule or might there be exceptions? Are there any other things to keep in mind when dlsym'ing things?

5

Well, there are also "absolute symbols" whose value can directly encode a pointer-sized integer value, but you're unlikely to encounter them in practice.

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