diff --git a/CHANGELOG.md b/CHANGELOG.md
index d0f894ed0..bb3702950 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,15 +6,36 @@
 
 - The `sentry_options_set_logger` function now accepts a `userdata` parameter.
 - The `name` parameter of `sentry_options_add_attachment(w)` was removed, it will now be inferred from the filename of `path`.
-- The transport startup hook that is set via `sentry_transport_set_startup_func` now needs to return a `bool`, and a failure will propagate to `sentry_init`.
+- The transport startup hook that is set via `sentry_transport_set_startup_func` now needs to return an `int`, and a failure will propagate to `sentry_init`.
+- The return value of the transport shutdown hook set via `sentry_transport_set_shutdown_func` was also changed to return an `int`.
+- Both functions should return _0_ on success, and a non-zero error code on failure, as does `sentry_init`.
+- Similarly, the return value of `sentry_shutdown` was also changed to an `int`, and will return _0_ on success and a non-zero error code on unclean shutdown.
 
 ```c
 // before
 sentry_options_set_logger(options, my_custom_logger);
 sentry_options_add_attachment(options, "some-attachment", "/path/to/some-attachment.txt");
+
+void transport_startup(sentry_options_t *options, void*state) {
+}
+sentry_transport_set_startup_func(transport, transport_startup);
+bool transport_shutdown(uint64_t timeout, void*state) {
+  return true;
+}
+sentry_transport_set_shutdown_func(transport, transport_shutdown);
+
 // after
 sentry_options_set_logger(options, my_custom_logger, NULL);
 sentry_options_add_attachment(options, "/path/to/some-attachment.txt");
+
+int transport_startup(sentry_options_t *options, void*state) {
+  return 0;
+}
+sentry_transport_set_startup_func(transport, transport_startup);
+int transport_shutdown(uint64_t timeout, void*state) {
+  return 0;
+}
+sentry_transport_set_shutdown_func(transport, transport_shutdown);
 ```
 
 **Features**:
diff --git a/examples/example.c b/examples/example.c
index 929ae26a0..ccddca886 100644
--- a/examples/example.c
+++ b/examples/example.c
@@ -1,4 +1,5 @@
 #include "sentry.h"
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
diff --git a/include/sentry.h b/include/sentry.h
index cfe24aa77..3427bddb5 100644
--- a/include/sentry.h
+++ b/include/sentry.h
@@ -71,7 +71,6 @@ extern "C" {
 
 #include <inttypes.h>
 #include <stdarg.h>
-#include <stdbool.h>
 #include <stddef.h>
 
 /* context type dependencies */
@@ -561,22 +560,21 @@ SENTRY_API void sentry_transport_set_free_func(
  *
  * This hook is called from within `sentry_init` and will get a reference to the
  * options which can be used to initialize a transports internal state.
- * Returning `false` from this hook will signal failure and will bubble up to
- * `sentry_init`.
+ * It should return `0` on success. A failure will bubble up to `sentry_init`.
  */
 SENTRY_API void sentry_transport_set_startup_func(sentry_transport_t *transport,
-    bool (*startup_func)(const sentry_options_t *options, void *state));
+    int (*startup_func)(const sentry_options_t *options, void *state));
 
 /**
  * Sets the transport shutdown hook.
  *
- * This hook will receive a millisecond-resolution timeout; it should return
- * `true` in case all the pending envelopes have been sent within the timeout,
- * or `false` if the timeout was hit.
+ * This hook will receive a millisecond-resolution timeout.
+ * It should return `0` on success in case all the pending envelopes have been
+ * sent within the timeout, or `1` if the timeout was hit.
  */
 SENTRY_API void sentry_transport_set_shutdown_func(
     sentry_transport_t *transport,
-    bool (*shutdown_func)(uint64_t timeout, void *state));
+    int (*shutdown_func)(uint64_t timeout, void *state));
 
 /**
  * Generic way to free a transport.
@@ -885,13 +883,16 @@ SENTRY_API void sentry_options_set_system_crash_reporter_enabled(
  * they cannot be modified any more.
  * Depending on the configured transport and backend, this function might not be
  * fully thread-safe.
+ * Returns 0 on success.
  */
 SENTRY_API int sentry_init(sentry_options_t *options);
 
 /**
  * Shuts down the sentry client and forces transports to flush out.
+ *
+ * Returns 0 on success.
  */
-SENTRY_API void sentry_shutdown(void);
+SENTRY_API int sentry_shutdown(void);
 
 /**
  * Clears the internal module cache.
diff --git a/src/backends/sentry_backend_breakpad.cpp b/src/backends/sentry_backend_breakpad.cpp
index 8fd5d96b8..38137a48b 100644
--- a/src/backends/sentry_backend_breakpad.cpp
+++ b/src/backends/sentry_backend_breakpad.cpp
@@ -176,7 +176,7 @@ sentry__breakpad_backend_callback(
     return succeeded;
 }
 
-static bool
+static int
 sentry__breakpad_backend_startup(
     sentry_backend_t *backend, const sentry_options_t *options)
 {
diff --git a/src/backends/sentry_backend_crashpad.cpp b/src/backends/sentry_backend_crashpad.cpp
index c8d8d9c9e..eaebc20bb 100644
--- a/src/backends/sentry_backend_crashpad.cpp
+++ b/src/backends/sentry_backend_crashpad.cpp
@@ -95,7 +95,7 @@ sentry__crashpad_backend_shutdown(sentry_backend_t *backend)
     data->db = nullptr;
 }
 
-static bool
+static int
 sentry__crashpad_backend_startup(
     sentry_backend_t *backend, const sentry_options_t *options)
 {
@@ -130,7 +130,7 @@ sentry__crashpad_backend_startup(
         || !sentry__path_is_file(absolute_handler_path)) {
         SENTRY_WARN("unable to start crashpad backend, invalid handler_path");
         sentry__path_free(absolute_handler_path);
-        return false;
+        return 1;
     }
 
     SENTRY_TRACEF("starting crashpad backend with handler "
@@ -191,7 +191,7 @@ sentry__crashpad_backend_startup(
         // not calling `shutdown`
         delete data->db;
         data->db = nullptr;
-        return false;
+        return 1;
     }
 
     if (!options->system_crash_reporter_enabled) {
@@ -202,7 +202,7 @@ sentry__crashpad_backend_startup(
         crashpad_info->set_system_crash_reporter_forwarding(
             crashpad::TriState::kDisabled);
     }
-    return true;
+    return 0;
 }
 
 static void
diff --git a/src/backends/sentry_backend_inproc.c b/src/backends/sentry_backend_inproc.c
index 816cedd1e..9f048cdc0 100644
--- a/src/backends/sentry_backend_inproc.c
+++ b/src/backends/sentry_backend_inproc.c
@@ -71,7 +71,7 @@ invoke_signal_handler(int signum, siginfo_t *info, void *user_context)
     }
 }
 
-static bool
+static int
 startup_inproc_backend(
     sentry_backend_t *UNUSED(backend), const sentry_options_t *UNUSED(options))
 {
@@ -81,14 +81,14 @@ startup_inproc_backend(
         if (sigaction(
                 SIGNAL_DEFINITIONS[i].signum, NULL, &g_previous_handlers[i])
             == -1) {
-            return false;
+            return 1;
         }
     }
 
     // install our own signal handler
     g_signal_stack.ss_sp = sentry_malloc(SIGNAL_STACK_SIZE);
     if (!g_signal_stack.ss_sp) {
-        return false;
+        return 1;
     }
     g_signal_stack.ss_size = SIGNAL_STACK_SIZE;
     g_signal_stack.ss_flags = 0;
@@ -100,7 +100,7 @@ startup_inproc_backend(
     for (size_t i = 0; i < SIGNAL_COUNT; ++i) {
         sigaction(SIGNAL_DEFINITIONS[i].signum, &g_sigaction, NULL);
     }
-    return true;
+    return 0;
 }
 
 static void
@@ -148,13 +148,13 @@ static const struct signal_slot SIGNAL_DEFINITIONS[SIGNAL_COUNT] = {
 
 static LONG WINAPI handle_exception(EXCEPTION_POINTERS *);
 
-static bool
+static int
 startup_inproc_backend(
     sentry_backend_t *UNUSED(backend), const sentry_options_t *UNUSED(options))
 {
     g_previous_handler = SetUnhandledExceptionFilter(&handle_exception);
     SetErrorMode(SEM_FAILCRITICALERRORS);
-    return true;
+    return 0;
 }
 
 static void
diff --git a/src/sentry_backend.h b/src/sentry_backend.h
index a2b3e9837..de395d67e 100644
--- a/src/sentry_backend.h
+++ b/src/sentry_backend.h
@@ -13,7 +13,7 @@
  */
 struct sentry_backend_s;
 typedef struct sentry_backend_s {
-    bool (*startup_func)(
+    int (*startup_func)(
         struct sentry_backend_s *, const sentry_options_t *options);
     void (*shutdown_func)(struct sentry_backend_s *);
     void (*free_func)(struct sentry_backend_s *);
diff --git a/src/sentry_boot.h b/src/sentry_boot.h
index b861950dc..3636f7806 100644
--- a/src/sentry_boot.h
+++ b/src/sentry_boot.h
@@ -34,5 +34,6 @@
 #endif
 
 #include <sentry.h>
+#include <stdbool.h>
 
 #endif
diff --git a/src/sentry_core.c b/src/sentry_core.c
index 669433e64..24ec5c19e 100644
--- a/src/sentry_core.c
+++ b/src/sentry_core.c
@@ -97,7 +97,7 @@ sentry_init(sentry_options_t *options)
     load_user_consent(options);
 
     if (transport) {
-        if (!sentry__transport_startup(transport, options)) {
+        if (sentry__transport_startup(transport, options) != 0) {
             SENTRY_WARN("failed to initialize transport");
             goto fail;
         }
@@ -107,7 +107,7 @@ sentry_init(sentry_options_t *options)
     sentry_backend_t *backend = options->backend;
     if (backend && backend->startup_func) {
         SENTRY_TRACE("starting backend");
-        if (!backend->startup_func(backend, options)) {
+        if (backend->startup_func(backend, options) != 0) {
             SENTRY_WARN("failed to initialize backend");
             goto fail;
         }
@@ -147,7 +147,7 @@ sentry_init(sentry_options_t *options)
     return 1;
 }
 
-void
+int
 sentry_shutdown(void)
 {
     sentry_end_session();
@@ -156,21 +156,22 @@ sentry_shutdown(void)
     sentry_options_t *options = g_options;
     sentry__mutex_unlock(&g_options_mutex);
 
+    size_t dumped_envelopes = 0;
     if (options) {
-        size_t dumped_envelopes = 0;
+        if (options->backend && options->backend->shutdown_func) {
+            SENTRY_TRACE("shutting down backend");
+            options->backend->shutdown_func(options->backend);
+        }
         if (options->transport) {
             // TODO: make this configurable
-            if (!sentry__transport_shutdown(
-                    options->transport, SENTRY_DEFAULT_SHUTDOWN_TIMEOUT)) {
+            if (sentry__transport_shutdown(
+                    options->transport, SENTRY_DEFAULT_SHUTDOWN_TIMEOUT)
+                != 0) {
                 SENTRY_WARN("transport did not shut down cleanly");
             }
             dumped_envelopes = sentry__transport_dump_queue(
                 options->transport, options->run);
         }
-        if (options->backend && options->backend->shutdown_func) {
-            SENTRY_TRACE("shutting down backend");
-            options->backend->shutdown_func(options->backend);
-        }
         if (!dumped_envelopes) {
             sentry__run_clean(options->run);
         }
@@ -182,6 +183,7 @@ sentry_shutdown(void)
     sentry__mutex_unlock(&g_options_mutex);
     sentry__scope_cleanup();
     sentry__modulefinder_cleanup();
+    return (int)dumped_envelopes;
 }
 
 void
diff --git a/src/sentry_sync.c b/src/sentry_sync.c
index e5fe6338d..6e154e961 100644
--- a/src/sentry_sync.c
+++ b/src/sentry_sync.c
@@ -194,7 +194,7 @@ worker_thread(void *data)
     return 0;
 }
 
-void
+int
 sentry__bgworker_start(sentry_bgworker_t *bgw)
 {
     SENTRY_TRACE("starting background worker thread");
@@ -204,7 +204,9 @@ sentry__bgworker_start(sentry_bgworker_t *bgw)
     if (sentry__thread_spawn(&bgw->thread_id, &worker_thread, bgw) != 0) {
         sentry__atomic_fetch_and_add(&bgw->running, -1);
         sentry__bgworker_decref(bgw);
+        return 1;
     }
+    return 0;
 }
 
 static void
diff --git a/src/sentry_sync.h b/src/sentry_sync.h
index 3c8e9d724..b76429942 100644
--- a/src/sentry_sync.h
+++ b/src/sentry_sync.h
@@ -300,8 +300,9 @@ void sentry__bgworker_decref(sentry_bgworker_t *bgw);
 
 /**
  * Start a new background worker thread associated with `bgw`.
+ * Returns 0 on success.
  */
-void sentry__bgworker_start(sentry_bgworker_t *bgw);
+int sentry__bgworker_start(sentry_bgworker_t *bgw);
 
 /**
  * This will try to shut down the background worker thread, with a `timeout`.
diff --git a/src/sentry_transport.c b/src/sentry_transport.c
index de798026e..953c4da72 100644
--- a/src/sentry_transport.c
+++ b/src/sentry_transport.c
@@ -10,12 +10,12 @@
 
 typedef struct sentry_transport_s {
     void (*send_envelope_func)(sentry_envelope_t *envelope, void *state);
-    bool (*startup_func)(const sentry_options_t *options, void *state);
-    bool (*shutdown_func)(uint64_t timeout, void *state);
+    int (*startup_func)(const sentry_options_t *options, void *state);
+    int (*shutdown_func)(uint64_t timeout, void *state);
     void (*free_func)(void *state);
     size_t (*dump_func)(sentry_run_t *run, void *state);
     void *state;
-    bool was_started;
+    bool running;
 } sentry_transport_t;
 
 sentry_transport_t *
@@ -45,14 +45,14 @@ sentry_transport_set_free_func(
 
 void
 sentry_transport_set_startup_func(sentry_transport_t *transport,
-    bool (*startup_func)(const sentry_options_t *options, void *state))
+    int (*startup_func)(const sentry_options_t *options, void *state))
 {
     transport->startup_func = startup_func;
 }
 
 void
 sentry_transport_set_shutdown_func(sentry_transport_t *transport,
-    bool (*shutdown_func)(uint64_t timeout, void *state))
+    int (*shutdown_func)(uint64_t timeout, void *state))
 {
     transport->shutdown_func = shutdown_func;
 }
@@ -65,27 +65,28 @@ sentry__transport_send_envelope(
     transport->send_envelope_func(envelope, transport->state);
 }
 
-bool
+int
 sentry__transport_startup(
     sentry_transport_t *transport, const sentry_options_t *options)
 {
     if (transport->startup_func) {
         SENTRY_TRACE("starting transport");
-        transport->was_started
-            = transport->startup_func(options, transport->state);
-        return transport->was_started;
+        int rv = transport->startup_func(options, transport->state);
+        transport->running = rv == 0;
+        return rv;
     }
-    return true;
+    return 0;
 }
 
-bool
+int
 sentry__transport_shutdown(sentry_transport_t *transport, uint64_t timeout)
 {
-    if (transport->shutdown_func && transport->was_started) {
+    if (transport->shutdown_func && transport->running) {
         SENTRY_TRACE("shutting down transport");
+        transport->running = false;
         return transport->shutdown_func(timeout, transport->state);
     }
-    return true;
+    return 0;
 }
 
 void
diff --git a/src/sentry_transport.h b/src/sentry_transport.h
index 2e7d6780b..286fdad48 100644
--- a/src/sentry_transport.h
+++ b/src/sentry_transport.h
@@ -24,15 +24,18 @@ void sentry__transport_send_envelope(
 
 /**
  * Calls the transports startup hook.
+ *
+ * Returns 0 on success.
  */
-bool sentry__transport_startup(
+int sentry__transport_startup(
     sentry_transport_t *transport, const sentry_options_t *options);
 
 /**
  * Instructs the transport to shut down.
+ *
+ * Returns 0 on success.
  */
-bool sentry__transport_shutdown(
-    sentry_transport_t *transport, uint64_t timeout);
+int sentry__transport_shutdown(sentry_transport_t *transport, uint64_t timeout);
 
 /**
  * This will create a new platform specific HTTP transport.
diff --git a/src/transports/sentry_transport_curl.c b/src/transports/sentry_transport_curl.c
index 6f0a2d6af..5740c1adf 100644
--- a/src/transports/sentry_transport_curl.c
+++ b/src/transports/sentry_transport_curl.c
@@ -55,7 +55,7 @@ sentry__curl_bgworker_state_free(void *_state)
     sentry_free(state);
 }
 
-static bool
+static int
 sentry__curl_transport_start(
     const sentry_options_t *options, void *transport_state)
 {
@@ -64,7 +64,7 @@ sentry__curl_transport_start(
         CURLcode rv = curl_global_init(CURL_GLOBAL_ALL);
         if (rv != CURLE_OK) {
             SENTRY_WARNF("`curl_global_init` failed with code `%d`", (int)rv);
-            return false;
+            return 1;
         }
     }
 
@@ -80,17 +80,16 @@ sentry__curl_transport_start(
         // In this case we don’t start the worker at all, which means we can
         // still dump all unsent envelopes to disk on shutdown.
         SENTRY_WARN("`curl_easy_init` failed");
-        return false;
+        return 1;
     }
-    sentry__bgworker_start(bgworker);
-    return true;
+    return sentry__bgworker_start(bgworker);
 }
 
-static bool
+static int
 sentry__curl_transport_shutdown(uint64_t timeout, void *transport_state)
 {
     sentry_bgworker_t *bgworker = (sentry_bgworker_t *)transport_state;
-    return !sentry__bgworker_shutdown(bgworker, timeout);
+    return sentry__bgworker_shutdown(bgworker, timeout);
 }
 
 static size_t
diff --git a/src/transports/sentry_transport_winhttp.c b/src/transports/sentry_transport_winhttp.c
index 6b267866c..4ebc0e0b6 100644
--- a/src/transports/sentry_transport_winhttp.c
+++ b/src/transports/sentry_transport_winhttp.c
@@ -42,15 +42,19 @@ static void
 sentry__winhttp_bgworker_state_free(void *_state)
 {
     winhttp_bgworker_state_t *state = _state;
-    WinHttpCloseHandle(state->connect);
-    WinHttpCloseHandle(state->session);
+    if (state->connect) {
+        WinHttpCloseHandle(state->connect);
+    }
+    if (state->session) {
+        WinHttpCloseHandle(state->session);
+    }
     sentry__rate_limiter_free(state->ratelimiter);
     sentry_free(state->user_agent);
     sentry_free(state->proxy);
     sentry_free(state);
 }
 
-static bool
+static int
 sentry__winhttp_transport_start(
     const sentry_options_t *opts, void *transport_state)
 {
@@ -92,17 +96,16 @@ sentry__winhttp_transport_start(
     }
     if (!state->session) {
         SENTRY_WARN("`WinHttpOpen` failed");
-        return false;
+        return 1;
     }
-    sentry__bgworker_start(bgworker);
-    return true;
+    return sentry__bgworker_start(bgworker);
 }
 
-static bool
+static int
 sentry__winhttp_transport_shutdown(uint64_t timeout, void *transport_state)
 {
     sentry_bgworker_t *bgworker = (sentry_bgworker_t *)transport_state;
-    return !sentry__bgworker_shutdown(bgworker, timeout);
+    return sentry__bgworker_shutdown(bgworker, timeout);
 }
 
 static void
@@ -230,7 +233,9 @@ sentry__winhttp_send_task(void *_envelope, void *_state)
     SENTRY_TRACEF("request handled in %llums", now - started);
 
 exit:
-    WinHttpCloseHandle(request);
+    if (request) {
+        WinHttpCloseHandle(request);
+    }
     sentry_free(url);
     sentry_free(headers);
     sentry__prepared_http_request_free(req);
diff --git a/tests/unit/test_failures.c b/tests/unit/test_failures.c
index 3b65b375d..96483b316 100644
--- a/tests/unit/test_failures.c
+++ b/tests/unit/test_failures.c
@@ -2,11 +2,11 @@
 #include "sentry_testsupport.h"
 #include <sentry.h>
 
-static bool
+static int
 transport_startup_fail(
     const sentry_options_t *UNUSED(options), void *UNUSED(state))
 {
-    return false;
+    return 1;
 }
 
 static void