[SPCA] start linking

semester3/spca/parts/02_toolchain/01_linking.tex

@@ -0,0 +1,55 @@
+\subsection{Linking}
+
+Linking is the final step in the compilation pipeline: separately compiled object files are combined into an executable.
+
+The advantages of using Linkers are clear:
+\begin{enumerate}
+    \item \textbf{Separate Compilation}: Changing one source file requires only recompiling that file.
+    \item \textbf{Space Optimization}: Executable code only contains functions (e.g. from libraries) that are actually used.
+\end{enumerate}
+
+\subsubsection{Symbol Resolution}
+
+The first step during Linking is Symbol Resolution.
+
+In the context of Linking, all variables and functions are considered \textit{Symbols}. Compilers store all symbol definitions in a \textit{Symbol Table}.
+The linker associates symbol references with \textit{exactly one} definition.
+
+\inlinedef \textbf{Symbol types}
+
+\begin{itemize}
+    \item \textbf{Global Symbols} can be referenced by other modules (e.g. \texttt{non-static} in \texttt{C})
+    \item \textbf{External Symbols} are referenced globals defined elsewhere
+    \item \textbf{Local Symbols} are defined and referenced exclusively in one module (e.g. \texttt{static} in \texttt{C})
+\end{itemize}
+
+Note: Local linker symbols and local program variables are \textit{not} the same.
+
+\inlinedef \textbf{Symbol strength}
+
+Duplicate symbols either lead to linking errors (\texttt{-fno-common}, the default) or compile (\texttt{-fcommon})
+
+\begin{itemize}
+    \item \textbf{Strong Symbols} are procedure names and initialized globals
+    \item \textbf{Weak Symbols} are uninitialized globals (on \texttt{-fcommon})
+\end{itemize}
+
+in \texttt{C}, function symbols can explicitly be declared weak using:
+\begin{minted}{C}
+    #pragma weak func
+    __attribute__((weak))__ func()
+\end{minted}
+
+\content{Duplicate Handling} The linker uses these definitions to handle duplicates:
+
+\begin{enumerate}
+    \item Given multiple strong symbols are illegal
+    \item Given a strong symbol and multiple weak symbols, pick the strong symbol
+    \item Given multiple weak symbols, choose an \textit{arbitrary} one
+\end{enumerate}
+
+\subsubsection{Relocation}
+
+The second step during Linking is Relocation.
+
+Code and data sections of separate sources are combined, and symbols are relocated from relative locations (in \texttt{.o} files) to absolute locations (in \texttt{.exe} files)

semester3/spca/parts/02_toolchain/02_file_types.tex

@@ -0,0 +1,64 @@
+\subsection{File types}
+
+The most common file types used during compilation:
+
+\begin{itemize}
+    \item \textbf{Source Code File} (\texttt{.c}) Uncompiled source code in \texttt{C}.
+    \item \textbf{Relocatable Object File} (\texttt{.o}) Code \& Data in a format ready for linking.
+    \item \textbf{Shared Object File} (\texttt{.so}) Special object file, can be loaded \& linked dynamically: at load or run time.
+    \item \textbf{Exectuable File} Code \& Data in a format that can be directly copied into memory \& run.
+    \item \textbf{Archive files} (\texttt{.a}) concatenate related \texttt{.o} files into one \texttt{.a}, with an index.
+\end{itemize}
+
+\content{Alternate names} On Windows, \texttt{.dll} files are used instead of \texttt{.so} and are called \textit{Dynamic Link Libraries}.
+
+\subsubsection{Executable and Linkable Format (ELF)}
+
+The standard unified format for all object files (\texttt{.exe}, \texttt{.o}, \texttt{.so}) in use since UNIX.
+
+\begin{center}
+    \begin{tabular}{l|l}
+        \textbf{Section}        & \textbf{Content} \\
+        \hline
+        ELF header              & contains basic information: Word size, byte ordering, file type, machine type \\
+        Segment header table    & page size, virtual address memory segments, segment sizes \\
+        \texttt{.text}          & actual code \\
+        \texttt{.rodata}        & (Read-only data) for example, jump tables \\
+        \texttt{.data}          & initialized global variables \\
+        \texttt{.bss}           & uninitialized global variables \\
+        \texttt{.symtab}        & symbol table, procedure and static variable names, section names \& locations. \\
+        \texttt{.rel.text}      & relocation info for \texttt{.text}, e.g. addresses of instructions that need modifying \\
+        \texttt{.rel.data}      & relocation info for \texttt{.data}, e.g. addresses of pointers that need modifying \\
+        \texttt{.debug}         & info for symbolic debugging (\texttt{gcc -g})
+    \end{tabular}
+\end{center}
+
+% Compact table with only the segment names
+
+% \begin{center}
+%     \begin{tabular}{|c|}
+%         \hline
+%         ELF header \\
+%         \hline 
+%         Segment header table \\
+%         \hline
+%         \texttt{.text} \\
+%         \hline 
+%         \texttt{.rodata} \\
+%         \hline
+%         \texttt{.data} \\
+%         \hline
+%         \texttt{.bss} \\
+%         \hline
+%         \texttt{.symtab} \\
+%         \hline
+%         \texttt{.rel.txt} \\
+%         \hline
+%         \texttt{.rel.data} \\
+%         \hline
+%         \texttt{.debug} \\
+%         \hline
+%         Section header table \\
+%         \hline
+%     \end{tabular}
+% \end{center}

semester3/spca/spca-summary.tex

@@ -125,6 +125,8 @@ If there are changes and you'd like to update this summary, please open a pull r
 \newsection
 \section{The gcc toolchain}
 \input{parts/02_toolchain/00_intro.tex}
+\input{parts/02_toolchain/01_linking.tex}
+\input{parts/02_toolchain/02_file_types.tex}
 
 
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