dyld overview

I just try to overview the dyld for that developers who has difficult to read dyld opensource. Let’s start.

Background

For iOS app developers you should know that when we write an app,we need normally depend on a lots of dynamic frameworks.Those framework do not packeted in our app, they are in system directory some where.How our code can find them, use them? This is the work that dyld should to do.

User space

When we tap an app icon,then an app is started.App is a process.It has owned user space .

What is user space? If you don’t know clearly,you can just consider it as data address. Assume code int i =3; int *p = &iin c langurage ,p is the address of var i.

The code where we write app to,and the dynamic framework we denpening on,They are all in the user space.Like figure 1-1

Figure 1-1

You can set DYLD_PRINT_SEGMENTS environment variables to see dyld logs in case deepen the impression. Also user image listlldb instrument to debug.

Symbol bind

Now,we have knowed all the addresses of dynamic framework we depend on.Then we can bind symbol.

What is symbol bind?For example, assume code Class cls = [NSObjcet class];,we use the +[NSObject class]method.When we write code, we donnot know the the address of +[NSObject class]method.We can use a placeholer for the method, like figure 1-2.

Figure 1-2.

But now we know the actual address,the we can fill in it.Assume the address of method +[NSObject class] is 0x7fffad1bf140,After fill in,like figure 1-3

Figure 1-3

This is so-called bind.

So far,the dyld’s main work is done.

One more thing

If you read the dyld opensource,you can’t go around the dyld_shared_cache，imageLoader,what are they? I just apply a figure 1-4,leave the rest to you.

Figure 1-4.