Extract actual function acquisition

The pointers to the actual function implementations are now fetched inside the `actual` namespace declared in the `actual_function.h` header.

Fixed source of _noreturn_ related warning during compilation by adding the appropriate flag. Sadly this means that we can not use `std::function` in this context as it doesn't seem to carry these _c-like_ flags.

1 files changed, 60 insertions, 0 deletions

diff --git a/src/actual_function.h b/src/actual_function.h
new file mode 100644
index 0000000..0ffe0c2
--- /dev/null
+++ b/src/actual_function.h
@@ -0,0 +1,60 @@
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
+#ifndef CHANGE_SRC_ACTUAL_FUNCTION_H_
+#define CHANGE_SRC_ACTUAL_FUNCTION_H_
+
+#include <dlfcn.h>
+#include <sys/mman.h>
+
+#include <memory>
+#include <cstring>
+
+namespace {
+	template <class Result, typename... Arguments>
+	std::function<Result(Arguments...)> get_actual_function(
+		const std::string& symbol_name) {
+
+		Result (*real_function)(Arguments...) = nullptr;
+		const void* symbol_address = dlsym(RTLD_NEXT, symbol_name.c_str());
+
+		std::memcpy(&real_function, &symbol_address, sizeof(symbol_address));
+
+		return std::function<Result(Arguments...)>(real_function);
+	}
+}
+
+namespace actual {
+	static auto write = get_actual_function<
+		ssize_t,
+		int, const void*, size_t
+	>("write");
+
+	static auto rename = get_actual_function<
+		int,
+		const char*, const char*
+	>("rename");
+
+	static auto rmdir = get_actual_function<
+		int,
+		const char*
+	>("rmdir");
+
+	static auto unlink = get_actual_function<
+		int,
+		const char*
+	>("unlink");
+
+	static auto unlinkat = get_actual_function<
+		int,
+		int, const char*, int
+	>("unlinkat");
+
+	static auto mmap = get_actual_function<
+		void*,
+		void*, size_t, int, int, int, off_t
+	>("mmap");
+}
+
+#endif  // CHANGE_SRC_ACTUAL_FUNCTION_H_