Update 'Obtaining the Oodle library on Linux'

Obtaining-the-Oodle-library-on-Linux.md

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+The Unreal Engine (currently v5.0.3) includes a version of the Oodle data compression library compiled for Linux (at `Engine/Source/Runtime/OodleDataCompression/Sdks/2.9.5/lib/Linux`). However, this archive was compiled for static linking, and all symbols were marked as hidden.
+
+To be able to create a shared library from that, follow the steps described in [StackOverflow - Keep all exported symbols when creating a shared library from a static library](https://stackoverflow.com/questions/54664759/keep-all-exported-symbols-when-creating-a-shared-library-from-a-static-library):
+
+**elf_unhide.py**:
+```python
+#!/usr/bin/python
+# unhide.py - Replace hidden with default visibility on global symbols defined
+#   in an ELF object file
+
+import argparse, sys, lief
+from lief.ELF import SYMBOL_BINDINGS, SYMBOL_VISIBILITY, SYMBOL_TYPES
+
+def warn(msg):
+    sys.stderr.write("WARNING: " + msg + "\n")
+
+def unhide(objfile_in, objfile_out = None, namedsyms=None):
+    if not objfile_out:
+        objfile_out = objfile_in
+    binary = lief.parse(objfile_in)
+    allsyms = { sym.name for sym in binary.symbols }
+    selectedsyms = set([])
+    nasyms = { sym.name for sym in binary.symbols if \
+                            sym.type == SYMBOL_TYPES.NOTYPE or \
+                            sym.binding != SYMBOL_BINDINGS.GLOBAL or \
+                            sym.visibility != SYMBOL_VISIBILITY.HIDDEN }
+    if namedsyms:
+        namedsyms = set(namedsyms)
+        nosyms = namedsyms - allsyms
+        for nosym in nosyms:
+            warn("No symbol " + nosym + " in " + objfile_in + ": ignored")
+        for sym in namedsyms & nasyms:
+            warn("Input symbol " + sym + \
+                " is not a hidden global symbol defined in " + objfile_in + \
+                ": ignored")
+        selectedsyms = namedsyms - nosyms
+    else:
+        selectedsyms = allsyms
+
+    selectedsyms -= nasyms
+    unhidden = 0;
+    for sym in binary.symbols:
+        if sym.name in selectedsyms:
+            sym.visibility = SYMBOL_VISIBILITY.DEFAULT
+            unhidden += 1
+            print("Unhidden: " + sym.name)
+    print("{} symbols were unhidden".format(unhidden))
+    binary.write(objfile_out)
+
+def get_args():
+    parser = argparse.ArgumentParser(
+        description="Replace hidden with default visibility on " + \
+            "global symbols defined in an ELF object file.")
+    parser.add_argument("ELFIN",help="ELF object file to read")
+    parser.add_argument("-s","--symbol",metavar="SYMBOL",action="append",
+        help="Unhide SYMBOL. " + \
+            "If unspecified, unhide all hidden global symbols defined in ELFIN")
+    parser.add_argument("--symfile",
+        help="File of whitespace-delimited symbols to unhide")
+    parser.add_argument("-o","--out",metavar="ELFOUT",
+        help="ELF object file to write. If unspecified, rewrite ELFIN")
+    return parser.parse_args()
+
+
+def main():
+    args = get_args()
+    objfile_in = args.ELFIN
+    objfile_out = args.out
+    symlist = args.symbol
+    if not symlist:
+        symlist = []
+    symfile = args.symfile
+    if symfile:
+        with open(symfile,"r") as fh:
+            symlist += [word for line in fh for word in line.split()]
+    unhide(objfile_in,objfile_out,symlist)
+
+main()
+```
+
+**elf_unhide.sh**:
+```bash
+#!/bin/bash
+
+TEMP=$(mktemp -d)
+
+OLD_ARCHIVE="$(realpath "$1")"
+NEW_ARCHIVE="$(realpath "$2")"
+
+ar --output $TEMP xv $OLD_ARCHIVE
+
+find $TEMP -type f -exec ./elf_unhide.py --symfile symbols.txt "{}" \;
+
+ar rcs $NEW_ARCHIVE $TEMP/*.o
+
+rm -r $TEMP
+echo "$(basefile $NEW_ARCHIVE) made"
+```
+
+**Create the shared library**:
+```shell
+./elf_unhide.sh liboo2corelinux64.a liboo2corelinux64_out.a
+g++ -shared -o liboo2corelinux64.so -Wl,--whole-archive ./liboo2corelinux64_out.a -Wl,--no-whole-archive
+```