diff --git a/bindings/python/src/boost_python.hpp b/bindings/python/src/boost_python.hpp
index dd60b0132..90353a5b8 100644
--- a/bindings/python/src/boost_python.hpp
+++ b/bindings/python/src/boost_python.hpp
@@ -16,5 +16,9 @@
 #undef snprintf
 #endif
 
+#ifdef vsnprintf
+#undef vsnprintf
+#endif
+
 #endif
 
diff --git a/include/libtorrent/alert_types.hpp b/include/libtorrent/alert_types.hpp
index 12230c377..455c83688 100644
--- a/include/libtorrent/alert_types.hpp
+++ b/include/libtorrent/alert_types.hpp
@@ -51,20 +51,14 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/piece_block.hpp"
 #include "libtorrent/aux_/escape_string.hpp" // for convert_from_native
 #include "libtorrent/string_view.hpp"
+#include "libtorrent/stack_allocator.hpp"
 
 #include "libtorrent/aux_/disable_warnings_push.hpp"
-
 #include <boost/shared_array.hpp>
-
 #include "libtorrent/aux_/disable_warnings_pop.hpp"
 
 namespace libtorrent
 {
-
-	namespace aux {
-		struct stack_allocator;
-	}
-
 	// maps an operation id (from peer_error_alert and peer_disconnected_alert)
 	// to its name. See peer_connection for the constants
 	TORRENT_EXPORT char const* operation_name(int op);
@@ -99,7 +93,7 @@ namespace libtorrent
 	protected:
 		std::reference_wrapper<aux::stack_allocator const> m_alloc;
 	private:
-		int m_name_idx;
+		aux::allocation_slot m_name_idx;
 	};
 
 	// The peer alert is a base class for alerts that refer to a specific peer. It includes all
@@ -150,7 +144,7 @@ namespace libtorrent
 		std::string url;
 #endif
 	private:
-		int const m_url_idx;
+		aux::allocation_slot const m_url_idx;
 	};
 
 #define TORRENT_DEFINE_ALERT_IMPL(name, seq, prio) \
@@ -273,7 +267,7 @@ namespace libtorrent
 		// refers to the index of the file that was renamed,
 		int const index;
 	private:
-		int const m_name_idx;
+		aux::allocation_slot const m_name_idx;
 	};
 
 	// This is posted as a response to a torrent_handle::rename_file() call, if the rename
@@ -447,7 +441,7 @@ namespace libtorrent
 		char const* error_message() const;
 
 	private:
-		int const m_msg_idx;
+		aux::allocation_slot const m_msg_idx;
 	};
 
 	// This alert is triggered if the tracker reply contains a warning field.
@@ -473,7 +467,7 @@ namespace libtorrent
 		char const* warning_message() const;
 
 	private:
-		int const m_msg_idx;
+		aux::allocation_slot const m_msg_idx;
 	};
 
 	// This alert is generated when a scrape request succeeds.
@@ -524,7 +518,7 @@ namespace libtorrent
 		char const* error_message() const;
 
 	private:
-		int const m_msg_idx;
+		aux::allocation_slot const m_msg_idx;
 	};
 
 	// This alert is only for informational purpose. It is generated when a tracker announce
@@ -892,7 +886,7 @@ namespace libtorrent
 		char const* storage_path() const;
 
 	private:
-		int const m_path_idx;
+		aux::allocation_slot const m_path_idx;
 	};
 
 	// The ``storage_moved_failed_alert`` is generated when an attempt to move the storage,
@@ -923,7 +917,7 @@ namespace libtorrent
 		// terminated string naming which one, otherwise it's nullptr.
 		char const* operation;
 	private:
-		int const m_file_idx;
+		aux::allocation_slot const m_file_idx;
 	};
 
 	// This alert is generated when a request to delete the files of a torrent complete.
@@ -1088,8 +1082,8 @@ namespace libtorrent
 		char const* error_message() const;
 
 	private:
-		int const m_url_idx;
-		int const m_msg_idx;
+		aux::allocation_slot const m_url_idx;
+		aux::allocation_slot const m_msg_idx;
 	};
 
 	// If the storage fails to read or write files that it needs access to, this alert is
@@ -1123,7 +1117,7 @@ namespace libtorrent
 		std::string msg;
 #endif
 	private:
-		int const m_file_idx;
+		aux::allocation_slot const m_file_idx;
 	};
 
 	// This alert is generated when the metadata has been completely received and the info-hash
@@ -1305,7 +1299,7 @@ namespace libtorrent
 
 	private:
 		std::reference_wrapper<aux::stack_allocator const> m_alloc;
-		int const m_interface_idx;
+		aux::allocation_slot const m_interface_idx;
 	};
 
 	// This alert is posted when the listen port succeeds to be opened on a
@@ -1452,7 +1446,7 @@ namespace libtorrent
 		// TODO: 2 should the alert baseclass have this object instead?
 		std::reference_wrapper<aux::stack_allocator const> m_alloc;
 
-		int const m_log_idx;
+		aux::allocation_slot const m_log_idx;
 	};
 
 	// This alert is generated when a fastresume file has been passed to
@@ -1489,7 +1483,7 @@ namespace libtorrent
 		std::string msg;
 #endif
 	private:
-		int const m_path_idx;
+		aux::allocation_slot const m_path_idx;
 	};
 
 	// This alert is posted when an incoming peer connection, or a peer that's about to be added
@@ -1690,7 +1684,7 @@ namespace libtorrent
 		char const* tracker_id() const;
 
 	private:
-		int const m_tracker_idx;
+		aux::allocation_slot const m_tracker_idx;
 	};
 
 	// This alert is posted when the initial DHT bootstrap is done.
@@ -1729,7 +1723,7 @@ namespace libtorrent
 		char const* filename() const;
 
 	private:
-		int const m_file_idx;
+		aux::allocation_slot const m_file_idx;
 	};
 
 	// This is always posted for SSL torrents. This is a reminder to the client that
@@ -2120,7 +2114,7 @@ namespace libtorrent
 
 	private:
 		std::reference_wrapper<aux::stack_allocator const> m_alloc;
-		int const m_str_idx;
+		aux::allocation_slot const m_str_idx;
 	};
 
 	// This alert is posted by torrent wide events. It's meant to be used for
@@ -2148,7 +2142,7 @@ namespace libtorrent
 #endif
 
 	private:
-		int const m_str_idx;
+		aux::allocation_slot const m_str_idx;
 	};
 
 	// This alert is posted by events specific to a peer. It's meant to be used
@@ -2195,7 +2189,7 @@ namespace libtorrent
 #endif
 
 	private:
-		int const m_str_idx;
+		aux::allocation_slot const m_str_idx;
 	};
 
 	// posted if the local service discovery socket fails to start properly.
@@ -2342,7 +2336,7 @@ namespace libtorrent
 
 	private:
 		std::reference_wrapper<aux::stack_allocator const> m_alloc;
-		int const m_msg_idx;
+		aux::allocation_slot const m_msg_idx;
 	};
 
 	// This alert is posted every time a DHT message is sent or received. It is
@@ -2380,7 +2374,7 @@ namespace libtorrent
 
 	private:
 		std::reference_wrapper<aux::stack_allocator> m_alloc;
-		int const m_msg_idx;
+		aux::allocation_slot const m_msg_idx;
 		int const m_size;
 	};
 
@@ -2408,7 +2402,7 @@ namespace libtorrent
 	private:
 		std::reference_wrapper<aux::stack_allocator> m_alloc;
 		int const m_num_peers;
-		int m_peers_idx;
+		aux::allocation_slot m_peers_idx;
 	};
 
 	// This is posted exactly once for every call to session_handle::dht_direct_request.
@@ -2438,7 +2432,7 @@ namespace libtorrent
 
 	private:
 		std::reference_wrapper<aux::stack_allocator> m_alloc;
-		int const m_response_idx;
+		aux::allocation_slot const m_response_idx;
 		int const m_response_size;
 	};
 
@@ -2486,7 +2480,7 @@ namespace libtorrent
 		std::vector<piece_block> blocks() const;
 
 	private:
-		int const m_array_idx;
+		aux::allocation_slot const m_array_idx;
 		int const m_num_blocks;
 	};
 
@@ -2508,7 +2502,7 @@ namespace libtorrent
 
 	private:
 		std::reference_wrapper<aux::stack_allocator> m_alloc;
-		int const m_msg_idx;
+		aux::allocation_slot const m_msg_idx;
 	};
 
 #undef TORRENT_DEFINE_ALERT_IMPL
diff --git a/include/libtorrent/stack_allocator.hpp b/include/libtorrent/stack_allocator.hpp
index 647ec1ad2..84ea3bfd8 100644
--- a/include/libtorrent/stack_allocator.hpp
+++ b/include/libtorrent/stack_allocator.hpp
@@ -31,15 +31,32 @@ POSSIBILITY OF SUCH DAMAGE.
 */
 
 #ifndef TORRENT_STACK_ALLOCATOR
+#define TORRENT_STACK_ALLOCATOR
 
 #include "libtorrent/assert.hpp"
 #include "libtorrent/span.hpp"
 #include "libtorrent/string_view.hpp"
 
+#include <cstdio> // for vsnprintf
 #include <cstring>
 
 namespace libtorrent { namespace aux
 {
+	struct allocation_slot
+	{
+		allocation_slot() : m_idx(-1) {}
+		allocation_slot(allocation_slot const&) = default;
+		allocation_slot(allocation_slot&&) = default;
+		allocation_slot& operator=(allocation_slot const&) = default;
+		allocation_slot& operator=(allocation_slot&&) = default;
+		bool operator==(allocation_slot const& s) const { return m_idx == s.m_idx; }
+		bool operator!=(allocation_slot const& s) const { return m_idx != s.m_idx; }
+		friend struct stack_allocator;
+	private:
+		explicit allocation_slot(int idx) : m_idx(idx) {}
+		int val() const { return m_idx; }
+		int m_idx;
+	};
 
 	struct stack_allocator
 	{
@@ -49,25 +66,25 @@ namespace libtorrent { namespace aux
 		stack_allocator(stack_allocator const&) = delete;
 		stack_allocator& operator=(stack_allocator const&) = delete;
 
-		int copy_string(string_view str)
+		allocation_slot copy_string(string_view str)
 		{
 			int const ret = int(m_storage.size());
 			m_storage.resize(ret + str.size() + 1);
 			std::memcpy(&m_storage[ret], str.data(), str.size());
 			m_storage[ret + str.length()] = '\0';
-			return ret;
+			return allocation_slot(ret);
 		}
 
-		int copy_string(char const* str)
+		allocation_slot copy_string(char const* str)
 		{
 			int const ret = int(m_storage.size());
 			int const len = int(std::strlen(str));
 			m_storage.resize(ret + len + 1);
 			std::strcpy(&m_storage[ret], str);
-			return ret;
+			return allocation_slot(ret);
 		}
 
-		int format_string(char const* fmt, va_list v)
+		allocation_slot format_string(char const* fmt, va_list v)
 		{
 			int const ret = int(m_storage.size());
 			m_storage.resize(ret + 512);
@@ -89,39 +106,39 @@ namespace libtorrent { namespace aux
 
 			// +1 is to include the 0-terminator
 			m_storage.resize(ret + (len > 512 ? 512 : len) + 1);
-			return ret;
+			return allocation_slot(ret);
 		}
 
-		int copy_buffer(span<char const> buf)
+		allocation_slot copy_buffer(span<char const> buf)
 		{
 			int const ret = int(m_storage.size());
 			int const size = int(buf.size());
-			if (size < 1) return -1;
+			if (size < 1) return allocation_slot();
 			m_storage.resize(ret + size);
 			std::memcpy(&m_storage[ret], buf.data(), size);
-			return ret;
+			return allocation_slot(ret);
 		}
 
-		int allocate(int const bytes)
+		allocation_slot allocate(int const bytes)
 		{
-			if (bytes < 1) return -1;
+			if (bytes < 1) return allocation_slot();
 			int const ret = int(m_storage.size());
 			m_storage.resize(ret + bytes);
-			return ret;
+			return allocation_slot(ret);
 		}
 
-		char* ptr(int const idx)
+		char* ptr(allocation_slot const idx)
 		{
-			if(idx < 0) return NULL;
-			TORRENT_ASSERT(idx < int(m_storage.size()));
-			return &m_storage[idx];
+			if(idx.val() < 0) return nullptr;
+			TORRENT_ASSERT(idx.val() < int(m_storage.size()));
+			return &m_storage[idx.val()];
 		}
 
-		char const* ptr(int const idx) const
+		char const* ptr(allocation_slot const idx) const
 		{
-			if(idx < 0) return NULL;
-			TORRENT_ASSERT(idx < int(m_storage.size()));
-			return &m_storage[idx];
+			if(idx.val() < 0) return nullptr;
+			TORRENT_ASSERT(idx.val() < int(m_storage.size()));
+			return &m_storage[idx.val()];
 		}
 
 		void swap(stack_allocator& rhs)
diff --git a/src/alert.cpp b/src/alert.cpp
index 97c551401..81ab930d9 100644
--- a/src/alert.cpp
+++ b/src/alert.cpp
@@ -366,7 +366,7 @@ namespace libtorrent {
 		, msg(convert_from_native(e.message()))
 #endif
 		, error(e)
-		, m_msg_idx(-1)
+		, m_msg_idx()
 	{
 		TORRENT_ASSERT(!u.empty());
 	}
@@ -388,7 +388,7 @@ namespace libtorrent {
 #ifndef TORRENT_NO_DEPRECATE
 		return msg.c_str();
 #else
-		if (m_msg_idx == -1) return "";
+		if (m_msg_idx == aux::allocation_slot()) return "";
 		else return m_alloc.get().ptr(m_msg_idx);
 #endif
 	}
@@ -1817,7 +1817,7 @@ namespace libtorrent {
 #endif
 		, error(e)
 		, m_url_idx(alloc.copy_string(u))
-		, m_msg_idx(-1)
+		, m_msg_idx()
 	{}
 
 	url_seed_alert::url_seed_alert(aux::stack_allocator& alloc, torrent_handle const& h
@@ -1847,7 +1847,7 @@ namespace libtorrent {
 #ifndef TORRENT_NO_DEPRECATE
 		return msg.c_str();
 #else
-		if (m_msg_idx == -1) return "";
+		if (m_msg_idx == aux::allocation_slot()) return "";
 		return m_alloc.get().ptr(m_msg_idx);
 #endif
 	}
@@ -2042,7 +2042,8 @@ namespace libtorrent {
 		, addr(addr_)
 #endif
 		, m_alloc(alloc)
-		, m_response_idx(-1), m_response_size(0)
+		, m_response_idx()
+		, m_response_size(0)
 	{}
 
 	std::string dht_direct_response_alert::message() const
diff --git a/src/disk_io_thread.cpp b/src/disk_io_thread.cpp
index c63a1abc2..b1a81b58a 100644
--- a/src/disk_io_thread.cpp
+++ b/src/disk_io_thread.cpp
@@ -60,6 +60,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #if DEBUG_DISK_THREAD
 #include <cstdarg>
 #include <sstream>
+#include <cstdio> // for vsnprintf
 #define DLOG(...) debug_log(__VA_ARGS__)
 #else
 #define DLOG(...) do {} while(false)
@@ -94,7 +95,7 @@ namespace libtorrent
 		va_start(v, fmt);
 
 		char usr[2048];
-		int len = vsnprintf(usr, sizeof(usr), fmt, v);
+		int len = std::vsnprintf(usr, sizeof(usr), fmt, v);
 
 		static bool prepend_time = true;
 		if (!prepend_time)
diff --git a/src/lsd.cpp b/src/lsd.cpp
index 8050d65f5..3236f8287 100644
--- a/src/lsd.cpp
+++ b/src/lsd.cpp
@@ -33,6 +33,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <cstdlib>
 #include <cstdarg>
 #include <functional>
+#include <cstdio> // for vsnprintf
 
 #include "libtorrent/lsd.hpp"
 #include "libtorrent/io.hpp"
@@ -94,7 +95,7 @@ void lsd::debug_log(char const* fmt, ...) const
 	va_start(v, fmt);
 
 	char buf[1024];
-	vsnprintf(buf, sizeof(buf), fmt, v);
+	std::vsnprintf(buf, sizeof(buf), fmt, v);
 	va_end(v);
 	m_callback.log_lsd(buf);
 }
diff --git a/test/test_dht.cpp b/test/test_dht.cpp
index 397bc5b71..e2cc7fb05 100644
--- a/test/test_dht.cpp
+++ b/test/test_dht.cpp
@@ -57,6 +57,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <cstdarg>
 #include <tuple>
 #include <iostream>
+#include <cstdio> // for vsnprintf
 
 #include "setup_transfer.hpp"
 
@@ -489,7 +490,7 @@ struct obs : dht::dht_observer
 		va_list v;
 		va_start(v, fmt);
 		char buf[1024];
-		vsnprintf(buf, sizeof(buf), fmt, v);
+		std::vsnprintf(buf, sizeof(buf), fmt, v);
 		va_end(v);
 		std::printf("%s\n", buf);
 		m_log.push_back(buf);
diff --git a/test/test_fast_extension.cpp b/test/test_fast_extension.cpp
index c61d5f342..dc4a9865c 100644
--- a/test/test_fast_extension.cpp
+++ b/test/test_fast_extension.cpp
@@ -48,6 +48,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <functional>
 #include <iostream>
 #include <cstdarg>
+#include <cstdio> // for vsnprintf
 
 using namespace libtorrent;
 using namespace std::placeholders;
@@ -59,7 +60,7 @@ void log(char const* fmt, ...)
 	va_start(v, fmt);
 
 	char buf[1024];
-	vsnprintf(buf, sizeof(buf), fmt, v);
+	std::vsnprintf(buf, sizeof(buf), fmt, v);
 	va_end(v);
 
 	std::printf("\x1b[1m\x1b[36m%s: %s\x1b[0m\n"
diff --git a/test/test_stack_allocator.cpp b/test/test_stack_allocator.cpp
index e5a7dd470..23b509ddc 100644
--- a/test/test_stack_allocator.cpp
+++ b/test/test_stack_allocator.cpp
@@ -34,17 +34,17 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/stack_allocator.hpp"
 
 using libtorrent::aux::stack_allocator;
+using libtorrent::aux::allocation_slot;
 
 TORRENT_TEST(copy_string)
 {
 	stack_allocator a;
-	int const idx1 = a.copy_string("testing");
+	allocation_slot const idx1 = a.copy_string("testing");
 
 	// attempt to trigger a reallocation
 	a.allocate(100000);
 
-	int const idx2 = a.copy_string(std::string("foobar"));
-
+	allocation_slot const idx2 = a.copy_string(std::string("foobar"));
 
 	TEST_CHECK(strcmp(a.ptr(idx1), "testing") == 0);
 	TEST_CHECK(strcmp(a.ptr(idx2), "foobar") == 0);
@@ -53,7 +53,7 @@ TORRENT_TEST(copy_string)
 TORRENT_TEST(copy_buffer)
 {
 	stack_allocator a;
-	int const idx1 = a.copy_buffer(libtorrent::span<char const>("testing"));
+	allocation_slot const idx1 = a.copy_buffer(libtorrent::span<char const>("testing"));
 
 	// attempt to trigger a reallocation
 	a.allocate(100000);
@@ -61,18 +61,17 @@ TORRENT_TEST(copy_buffer)
 	TEST_CHECK(strcmp(a.ptr(idx1), "testing") == 0);
 
 	// attempt zero size allocation
-	int const idx2 = a.copy_buffer({});
-	TEST_CHECK(idx2 == -1);
+	allocation_slot const idx2 = a.copy_buffer({});
 
 	// attempt to get a pointer after zero allocation
 	char* ptr = a.ptr(idx2);
-	TEST_CHECK(ptr == NULL);
+	TEST_CHECK(ptr == nullptr);
 }
 
 TORRENT_TEST(allocate)
 {
 	stack_allocator a;
-	int const idx1 = a.allocate(100);
+	allocation_slot const idx1 = a.allocate(100);
 	char* ptr = a.ptr(idx1);
 	for (int i = 0; i < 100; ++i)
 		ptr[i] = char(i % 256);
@@ -85,12 +84,11 @@ TORRENT_TEST(allocate)
 		TEST_CHECK(ptr[i] == char(i % 256));
 
 	// attempt zero size allocation
-	int const idx2 = a.allocate(0);
-	TEST_CHECK(idx2 == -1);
+	allocation_slot const idx2 = a.allocate(0);
 
 	// attempt to get a pointer after zero allocation
 	ptr = a.ptr(idx2);
-	TEST_CHECK(ptr == NULL);
+	TEST_CHECK(ptr == nullptr);
 }
 
 TORRENT_TEST(swap)
@@ -98,8 +96,8 @@ TORRENT_TEST(swap)
 	stack_allocator a1;
 	stack_allocator a2;
 
-	int const idx1 = a1.copy_string("testing");
-	int const idx2 = a2.copy_string("foobar");
+	allocation_slot const idx1 = a1.copy_string("testing");
+	allocation_slot const idx2 = a2.copy_string("foobar");
 
 	a1.swap(a2);