[SPCA] finish linking

semester3/spca/code-examples/00_c/04_toolchain/01_dynamic_linking.c

@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <dlfcn.h>  // contains addvec
+
+int x[2] = {1, 2}; int y[2] = {3, 4}; int z[2];
+
+int main(int argc, char *argv[])
+{
+    void *handle;
+    void (*addvec)(int *, int *, int *, int);       // Declaration
+    char *error;
+
+    handle = dlopen("./libvector.so", RTLD_LAZY);   // Load .so, makes addvec usable
+    if (!handle) {
+        fprintf(stderr, "%s\n", dlerror());
+        exit(1);
+    }
+
+    addvec = dlsym(handle, "addvec");               // get a pointer to advec
+    if ((error = dlerror()) != NULL) {
+        fprintf(stderr, "%s\n", error);
+        exit(1);
+    }
+
+    addvec(x, y, z, 2);                             // Now callable like any other function
+    printf("z = [%d %d]\n", z[0], z[1]);
+
+    if (dlclose(handle) < 0) {                      // Unload shared library
+        fprintf(stderr, "%s\n", dlerror());
+        exit(1);
+    }
+    return 0;
+}

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

@@ -53,3 +53,31 @@ in \texttt{C}, function symbols can explicitly be declared weak using:
 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)
+
+\textbf{Command line order matters} for this, since the Linker will scan \texttt{.o} and \texttt{.a} files in this order. In general, libraries should therefore be linked \textit{last}.
+
+\newpage
+\subsubsection{Packaging Libraries}
+
+Using just the Linker, there are only 2 inconvenient ways to package libraries:
+
+\begin{enumerate}
+    \item All functions into 1 file $\mapsto$ linking unnecessarily big objects.
+    \item One function per file $\mapsto$ Requires linking a lot of files, annoying for programmer.
+\end{enumerate}
+
+\textbf{Static Libraries} solve this: The linker looks for functions inside the static library, and only links matching archive \textit{members} into the executable. However, these come with issues too:
+
+\begin{enumerate}
+    \item Duplication in stored executables (e.g. \texttt{libc.a} functions)
+    \item Duplication in running executables
+    \item Any fix in a library requires importing applications to explicitly relink
+\end{enumerate}
+
+\textbf{Shared Libraries} solve this: These are linked at load-time or during run-time. Another advantage is that \textit{multiple} processes can use the same shared library simultaneously. This is how, for example, \texttt{libc} is packaged.
+
+During runtime, shared libraries can be loaded using \texttt{dlopen}:
+
+\inputcodewithfilename{gas}{code-examples/00_c/04_toolchain/}{01_dynamic_linking.c}
+
+\newpage

semester3/spca/spca-summary.tex

@@ -1,6 +1,6 @@
 \documentclass{article}
 
-\input{C:/projects/latex/dist/full.tex}
+\input{~/projects/latex/dist/full.tex}
 
 \renewcommand{\authorTitle}{Robin Bacher, Janis Hutz\\\url{https://github.com/janishutz/eth-summaries}}
 \renewcommand{\authorHeaders}{Robin Bacher, Janis Hutz}