iOS Development: Unveiling the Mysteries of Compilation and Linking
As iOS developers, we often take the compilation and linking process for granted. We click the “Build” button in Xcode, and voilà! Our app is ready for deployment. But have you ever wondered what happens behind the scenes? In this article, we’ll delve into the intricacies of compilation and linking, exploring the steps involved and the tools used to transform our source code into a executable binary.
The Compilation Flow
The compilation flow is a multi-step process that can be broken down into four main stages: pre-compilation, compilation, assembly, and linking.
Pre-Compilation
The first stage is pre-compilation, where the source code file hello.c and associated header files (e.g., stdio.h) are pre-compiled into a .i document using the cpp pre-compiler. This process deals with source code file directives, such as #include, #define, and comments.
$ cpp hello.c > hello.i
The pre-compiled file (hello.i) does not contain any macro definitions, as all macros have been expanded and inserted into the file. This makes it difficult to determine whether a macro definition in a header file is correct or not.
Compilation
The compilation stage involves the core of the compilation process, which includes lexical analysis, syntax analysis, semantic analysis, and optimization. This process is equivalent to the following command:
$ gcc -S hello.i -o hello.s
The compilation process produces an assembly code file (hello.s) that contains machine-specific instructions.
Assembly
The assembly process involves translating the assembly code into machine code using an assembler. This process is relatively simple, as it involves translating assembly instructions into machine code without complex syntax, semantics, or instruction optimization.
$ as hello.s -o hello.o
Linking
The linking stage involves combining multiple object files into a single executable file. This process is typically performed using a linker, such as ld. The linker resolves external references and generates a single executable file.
$ ld hello.o -o hello
iOS Compiler
The iOS compiler, Clang, uses the LLVM three-phase design to optimize compilation. The front-end tasks include grammar analysis, semantic analysis, and generating intermediate code (IR). The back-end tasks involve code generation and optimization.
iOS Program - Detailed Compilation
The compilation process for an iOS program involves several stages:
- Write auxiliary files, including a script file structure correspondence table item and a project file dependencies written correspondence table file.
- Run the default script, which may include pre-set scripts to run.
- Compile the file, which involves carrying out the compilation process for each file, producing a Mach-O executable file.
- Link the file, which involves combining multiple executable files into one file.
- Copy the resource file, which involves copying resource files in the project target package.
- Compile the storyboard file, which involves compiling the storyboard file into machine code.
- Link the storyboard file, which involves combining the compiled storyboard file with other files.
- Compile the asset file, which involves compiling images into machine code.
- Run the Cocoapods script, which involves copying precompiled libraries and related resources to the package before compiling the project.
- The generated app packet.
- The Swift standard library is copied to the package.
- The package is signed.
- The compilation process is complete.
Conclusion
In conclusion, the compilation and linking process is a complex process that involves several stages, including pre-compilation, compilation, assembly, and linking. Understanding these stages can help iOS developers optimize their compilation process and improve their app’s performance. By exploring the intricacies of compilation and linking, we can gain a deeper understanding of the iOS development process and improve our skills as developers.