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Commit 206d305

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Add code for int main with no return.
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‎009-int-main-no-return/Makefile‎

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# generate c++17 ast, llvm ir, assembly, and debuggable main executable
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all:
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clang++ -std=c++17 -Xclang -ast-dump -fsyntax-only main.cpp > main.ast && \
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clang++ -std=c++17 -S -emit-llvm main.cpp -o main.ll && \
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clang++ -std=c++17 -S main.cpp -o main.s && \
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clang++ -std=c++17 -g main.cpp -o main
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clean:
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rm -rf main.dSYM main main.ast main.ll main.s

‎009-int-main-no-return/README.md‎

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# No Return in Int Main
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## TL;DR
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- I would always write `return 0;` inside the `int main()` function because I thought it was required
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- This may have been a force of habit from learning C in a computer architecture course in college
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- Turns out that `return 0;` is option, and not required in `int main()`
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- The following code (with no return statement in the main function) will compile and run correctly
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- In order to investigate why, we need to understand the compiler. (In this case Clang.)
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```cpp
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int main() {
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int a = 123;
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}
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```
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## Compiler Steps
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The compiler steps:
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1. C++ (source code)
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2. AST (clang frontend)
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3. IR (intermediate representation) -> happens in this stage
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4. Assembly
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5. Binary/Executable
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The compiler steps in-depth (thanks ChatGPT):
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1. C++ Source Code → your .cpp text file.
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2. AST (Abstract Syntax Tree) → Clang frontend parses the source into a tree of functions, variables, statements, etc.
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3. IR (LLVM Intermediate Representation) → platform-independent SSA-like code for optimizations.
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4. Assembly → CPU-specific instructions (.s).
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5. Binary / Executable → linked machine code you can run (a.out or your program).
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## Why?
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Why is `return 0;` optional in `int main()`?
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- Many C compilers would `return 0;` by default
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- GCC, MSVC did this because OS expected return value
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- It would be undefined behavior in C++ otherwise
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- This is one of biggest concerns for C++ in this day
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- So much so that US govt asking companies to use safe languages
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- Was standardized in C++11
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- But C++ compilers basically did this before that
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## References
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- ChatGPT and Gemini (if these count)
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TranslationUnitDecl
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|- TypedefDecl __int128_t '__int128' (implicit)
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| `- BuiltinType '__int128'
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|- TypedefDecl __uint128_t 'unsigned __int128' (implicit)
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| `- BuiltinType 'unsigned __int128'
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|- TypedefDecl __NSConstantString '__NSConstantString_tag' (implicit)
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| `- RecordType '__NSConstantString_tag'
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| `- CXXRecord '__NSConstantString_tag'
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|- TypedefDecl __builtin_ms_va_list 'char *' (implicit)
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| `- PointerType 'char *'
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| `- BuiltinType 'char'
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|- TypedefDecl __builtin_va_list '__va_list_tag[1]' (implicit)
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| `- ConstantArrayType '__va_list_tag[1]' size=1
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| `- RecordType '__va_list_tag'
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| `- CXXRecord '__va_list_tag'
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`- FunctionDecl main 'int ()'
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`- CompoundStmt
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`- DeclStmt
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`- VarDecl a 'int' cinit
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`- IntegerLiteral 123

‎009-int-main-no-return/main.cpp‎

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// #include <iostream>
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int main() {
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int a = 123;
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// std::cout << a << std::endl;
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return 0;
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}
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// int main() {
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// // return 0;
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// }

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