diff --git a/proj/server/src/intercept.py b/proj/server/src/intercept.py
index 6124df6..8b51082 100755
--- a/proj/server/src/intercept.py
+++ b/proj/server/src/intercept.py
@@ -19,13 +19,13 @@ class ThreadedUnixStreamServer(ThreadingMixIn, UnixStreamServer):
 
 class Handler(StreamRequestHandler):
     pid: int
-    stack: list[tuple[str, tuple]]
+    stack: list[tuple[int, str, tuple]]
+    ret_addr: int
 
-    def before(self) -> None:
-        pass
-
-    def after(self) -> None:
-        pass
+    def before(self) -> None: pass
+    def after(self) -> None: pass
+    def before_fallback(self, func_name: str, *args) -> str: pass
+    def after_fallback(self, func_name: str, *args, **kwargs) -> None: pass
 
     def handle(self):
         first = self.rfile.readline()
@@ -93,7 +93,7 @@ class Handler(StreamRequestHandler):
         idx = len(m.group(0))
         if a.startswith('0x'):
             val = int(a[2:], 16)
-        elif a.startswith('0'):
+        elif a.startswith('0') and len(a) > 1:
             val = int(a[1:], 8)
         else:
             val = int(a, 10)
@@ -157,68 +157,131 @@ class Handler(StreamRequestHandler):
             call = data.decode('utf-8')
             print(f'[{self.pid}] {call}')
             func_name = call[:call.find('(')]
-            args, _ = Handler.parse_args(call[call.find('(') + 1:-1])
-            self.stack.append((func_name, args))
+            self.ret_addr = int(call[call.rfind(':') + 1:], 0)
+            args, _ = Handler.parse_args(call[call.find('(') + 1:call.rfind(':') - 1])
+            self.stack.append((self.ret_addr, func_name, args))
             try:
                 func = getattr(self, f'before_{func_name}')
-                if callable(func):
-                    command = func(*args) or 'ok'
-                else:
-                    command = 'ok'
+                if not callable(func):
+                    func = None
             except AttributeError:
-                command = 'ok'
+                func = None
+            try:
+                if func is None:
+                    raise NotImplementedError()
+                command = func(*args) or self.before_fallback(func_name, *args) or 'ok'
+            except NotImplementedError:
+                command = self.before_fallback(func_name, *args) or 'ok'
             print(f'[{self.pid}]  -> {command}')
             self.wfile.write(command.encode('utf-8') + b'\n')
         else:
             ret = data.decode('utf-8')
-            ret_value, _ = Handler.parse_arg(ret[7:])
-            func_name, args = self.stack.pop()
+            ret_value, _ = Handler.parse_arg(ret[7:].split(';')[0])
+            self.ret_addr, func_name, args = self.stack.pop()
             try:
                 func = getattr(self, f'after_{func_name}')
-                if callable(func):
-                    if ret_value is None:
-                        func(*args)
-                    else:
-                        func(*args, ret_value)
+                if not callable(func):
+                    func = None
             except AttributeError:
-                pass
+                func = None
+            try:
+                if func is None:
+                    raise NotImplementedError()
+                if ret_value is None:
+                    func(*args)
+                else:
+                    func(*args, ret_value)
+            except NotImplementedError:
+                if ret_value is None:
+                    self.after_fallback(func_name, *args)
+                else:
+                    self.after_fallback(func_name, *args, ret_value)
             print(f'[{self.pid}]  -> {ret}')
 
-    def before_malloc(self, size: int) -> str: pass
+    def before_malloc(self, size: int) -> str:
+        raise NotImplementedError()
     def after_malloc(self, size: int,
-                     ret_value: int, errno: str = None) -> None: pass
-    def before_calloc(self, nmemb: int, size: int) -> str: pass
+                     ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_calloc(self, nmemb: int, size: int) -> str:
+        raise NotImplementedError()
     def after_calloc(self, nmemb: int, size: int,
-                     ret_value: int, errno: str = None) -> None: pass
-    def before_realloc(self, ptr: int, size: int) -> str: pass
+                     ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_realloc(self, ptr: int, size: int) -> str:
+        raise NotImplementedError()
     def after_realloc(self, ptr: int, size: int,
-                      ret_value: int, errno: str = None) -> None: pass
-    def before_reallocarray(self, ptr: int, nmemb: int, size: int) -> str: pass
+                      ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_reallocarray(self, ptr: int, nmemb: int, size: int) -> str:
+        raise NotImplementedError()
     def after_reallocarray(self, ptr: int, nmemb: int, size: int,
-                           ret_value: int, errno: str = None) -> None: pass
-    def before_free(self, ptr: int) -> str: pass
-    def after_free(self, ptr: int) -> None: pass
-    def before_getopt(self, argc: int, argv: Pointer[list[Pointer[bytes]]], optstring: Pointer[bytes]) -> str: pass
+                           ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_free(self, ptr: int) -> str:
+        raise NotImplementedError()
+    def after_free(self, ptr: int) -> None:
+        raise NotImplementedError()
+    def before_getopt(self, argc: int, argv: Pointer[list[Pointer[bytes]]], optstring: Pointer[bytes]) -> str:
+        raise NotImplementedError()
     def after_getopt(self, argc: int, argv: Pointer[list[Pointer[bytes]]], optstring: Pointer[bytes],
-                     ret_value: int) -> None: pass
-    def before_close(self, fildes: int) -> str: pass
+                     ret_value: int) -> None:
+        raise NotImplementedError()
+    def before_close(self, fildes: int) -> str:
+        raise NotImplementedError()
     def after_close(self, fildes: int,
-                    ret_value: int, errno: str = None) -> None: pass
-    def before_sem_init(self, sem: int, pshared: int, value: int) -> str: pass
+                    ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_init(self, sem: int, pshared: int, value: int) -> str:
+        raise NotImplementedError()
     def after_sem_init(self, sem: int, pshared: int, value: int,
-                       ret_value: int, errno: str = None) -> None: pass
-    def before_sem_open(self, name: str, oflag: Flags, mode: Optional[int], value: Optional[int]) -> str: pass
+                       ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_open(self, name: str, oflag: Flags, mode: Optional[int], value: Optional[int]) -> str:
+        raise NotImplementedError()
     def after_sem_open(self, name: str, oflag: Flags, mode: Optional[int], value: Optional[int],
-                       ret_value: int, errno: str = None) -> None: pass
-    def before_sem_post(self, sem: int) -> str: pass
+                       ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_post(self, sem: int) -> str:
+        raise NotImplementedError()
     def after_sem_post(self, sem: int,
-                       ret_value: int, errno: str = None) -> None: pass
-    def before_sem_wait(self, sem: int) -> str: pass
+                       ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_wait(self, sem: int) -> str:
+        raise NotImplementedError()
     def after_sem_wait(self, sem: int,
-                       ret_value: int, errno: str = None) -> None: pass
-    def before_sem_timedwait(self, sem: int, abs_timeout: Pointer[StructTimeSpec]): pass
+                       ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_trywait(self, sem: int) -> str:
+        raise NotImplementedError()
+    def after_sem_trywait(self, sem: int,
+                          ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_timedwait(self, sem: int, abs_timeout: Pointer[StructTimeSpec]) -> str:
+        raise NotImplementedError()
     def after_sem_timedwait(self, sem: int, abs_timeout: Pointer[StructTimeSpec],
-                            ret_value: int, errno: str = None): pass
+                            ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_getvalue(self, sem: int, value_ptr: int) -> str:
+        raise NotImplementedError()
+    def after_sem_getvalue(self, sem:int, value_ptr: int,
+                           ret_value: int, errno: str = None, value: int = None) -> None:
+        raise NotImplementedError()
+    def before_sem_close(self, sem: int) -> str:
+        raise NotImplementedError()
+    def after_sem_close(self, sem: int,
+                        ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_unlink(self, name: Pointer[bytes]) -> str:
+        raise NotImplementedError()
+    def after_sem_unlink(self, name: Pointer[bytes],
+                         ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
+    def before_sem_destroy(self, sem: int) -> str:
+        raise NotImplementedError()
+    def after_sem_destroy(self, sem: int,
+                          ret_value: int, errno: str = None) -> None:
+        raise NotImplementedError()
 
 
 class MemoryAllocationTester(Handler):
@@ -284,6 +347,82 @@ class MemoryAllocationTester(Handler):
             del self.allocated[ptr]
 
 
+class ReturnValueCheckTester(Handler):
+    pass
+
+
+class InterruptedCheckTester(Handler):
+    cycles: int = 50
+    counter: int = 0
+    last_func_name: Optional[str] = None
+    last_ret_addr: Optional[int] = None
+    tested_functions: dict[tuple[str, int], str]
+
+    @property
+    def while_testing(self) -> bool:
+        return self.counter % self.cycles != 0
+
+    def before(self) -> None:
+        self.tested_functions = {}
+
+    def after(self) -> None:
+        if self.while_testing:
+            self.error()
+        for (name, ret_addr), status in self.tested_functions.items():
+            print(f'{name} (0x{ret_addr:x}) -> {status}')
+
+    def error(self):
+        print(f'Error: Return value and errno EINTR not handled correctly in {self.last_func_name} (return address 0x{self.last_ret_addr:x})')
+        self.tested_functions[(self.last_func_name, self.last_ret_addr)] = 'failed'
+        self.counter = 0
+        self.last_func_name = None
+        self.last_ret_addr = None
+
+    def after_fallback(self, func_name: str, *args, **kwargs) -> None:
+        if self.while_testing and self.last_func_name != func_name:
+            self.error()
+
+    def before_sem_wait(self, sem: int) -> str:
+        if self.last_ret_addr and self.last_ret_addr != self.ret_addr:
+            self.error()
+        self.counter += 1
+        if self.while_testing:
+            self.last_ret_addr = self.ret_addr
+            self.last_func_name = 'sem_wait'
+            self.tested_functions[(self.last_func_name, self.last_ret_addr)] = 'running'
+            return 'fail EINTR'
+        else:
+            self.tested_functions[(self.last_func_name, self.last_ret_addr)] = 'passed'
+            self.last_ret_addr = None
+            self.last_func_name = None
+            return 'return 0'
+
+    def before_sem_trywait(self, sem: int) -> str:
+        self.counter += 1
+        if self.while_testing:
+            self.last_ret_addr = self.ret_addr
+            self.last_func_name = 'sem_trywait'
+            return 'fail EINTR'
+        else:
+            self.last_ret_addr = None
+            self.last_func_name = None
+            return 'return 0'
+
+    def before_sem_timedwait(self, sem: int, abs_timeout: Pointer[StructTimeSpec]) -> str:
+        self.counter += 1
+        if self.while_testing:
+            self.last_ret_addr = self.ret_addr
+            self.last_func_name = 'sem_timedwait'
+            return 'fail EINTR'
+        else:
+            self.last_ret_addr = None
+            self.last_func_name = None
+            return 'return 0'
+
+    def before_sem_post(self, sem: int) -> str:
+        return 'return 0'
+
+
 def intercept(socket: str, handler: type[Handler]) -> None:
     try:
         with ThreadedUnixStreamServer(socket, handler) as server:
diff --git a/proj/server/src/server.py b/proj/server/src/server.py
index 260c825..221d111 100755
--- a/proj/server/src/server.py
+++ b/proj/server/src/server.py
@@ -39,7 +39,7 @@ def main() -> None:
     parser = argparse.ArgumentParser()
     parser.add_argument('socket', metavar='FILE')
     args = parser.parse_args()
-    intercept.intercept(args.socket, intercept.MemoryAllocationTester)
+    intercept.intercept(args.socket, intercept.InterruptedCheckTester)
 
 
 if __name__ == '__main__':
diff --git a/proj/test1/src/intercept.c b/proj/test1/src/intercept.c
index dfae993..cf55db6 100644
--- a/proj/test1/src/intercept.c
+++ b/proj/test1/src/intercept.c
@@ -499,7 +499,7 @@ static void init(void) {
 
 void *__sym(malloc)(size_t size) {
     init();
-    msg("malloc(%li)", size);
+    msg("malloc(%li): %p", size, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -515,7 +515,7 @@ void *__sym(malloc)(size_t size) {
 
 void *__sym(calloc)(size_t nmemb, size_t size) {
     init();
-    msg("calloc(%li, %li)", nmemb, size);
+    msg("calloc(%li, %li): %p", nmemb, size, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -531,7 +531,7 @@ void *__sym(calloc)(size_t nmemb, size_t size) {
 
 void *__sym(realloc)(void *ptr, size_t size) {
     init();
-    msg("realloc(%p, %li)", ptr, size);
+    msg("realloc(%p, %li): %p", ptr, size, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -547,7 +547,7 @@ void *__sym(realloc)(void *ptr, size_t size) {
 
 void *__sym(reallocarray)(void *ptr, size_t nmemb, size_t size) {
     init();
-    msg("reallocarray(%p, %li)", ptr, size);
+    msg("reallocarray(%p, %li): %p", ptr, size, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -563,7 +563,7 @@ void *__sym(reallocarray)(void *ptr, size_t nmemb, size_t size) {
 
 void __sym(free)(void *ptr) {
     init();
-    msg("free(%p)", ptr);
+    msg("free(%p): %p", ptr, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -577,7 +577,7 @@ void __sym(free)(void *ptr) {
 
 int __sym(getopt)(const int argc, char *const argv[], const char *shortopts) {
     init();
-    msg("getopt(%i, %as, %es)", argc, argv, argc, shortopts);
+    msg("getopt(%i, %as, %es): %p", argc, argv, argc, shortopts, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -595,7 +595,7 @@ int __sym(getopt)(const int argc, char *const argv[], const char *shortopts) {
 
 int __sym(close)(int fildes) {
     init();
-    msg("close(%i)", fildes);
+    msg("close(%i): %p", fildes, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -611,7 +611,7 @@ int __sym(close)(int fildes) {
 
 int __sym(sem_init)(sem_t *sem, int pshared, unsigned int value) {
     init();
-    msg("sem_init(%p, %i, %u)", sem, pshared, value);
+    msg("sem_init(%p, %i, %u): %p", sem, pshared, value, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -640,9 +640,9 @@ sem_t *__sym(sem_open)(const char *name, int oflag, ...) {
         mode_arg = va_arg(args, mode_t);
         value = va_arg(args, unsigned int);
         va_end(args);
-        msg("sem_open(%es, 0%o:|%s, 0%03o, %u)", name, oflag, ostr, mode_arg, value);
+        msg("sem_open(%es, 0%o:|%s, 0%03o, %u): %p", name, oflag, ostr, mode_arg, value, __builtin_return_address(0));
     } else {
-        msg("sem_open(%es, 0%o:|%s)", name, oflag, ostr);
+        msg("sem_open(%es, 0%o:|%s): %p", name, oflag, ostr, __builtin_return_address(0));
     }
 
     char overwrite[BUFFER_SIZE];
@@ -714,7 +714,7 @@ sem_t *__sym(sem_open)(const char *name, int oflag, ...) {
 
 int __sym(sem_post)(sem_t *sem) {
     init();
-    msg("sem_post(%p)", sem);
+    msg("sem_post(%p): %p", sem, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -730,7 +730,7 @@ int __sym(sem_post)(sem_t *sem) {
 
 int __sym(sem_wait)(sem_t *sem) {
     init();
-    msg("sem_wait(%p)", sem);
+    msg("sem_wait(%p): %p", sem, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -746,7 +746,7 @@ int __sym(sem_wait)(sem_t *sem) {
 
 int __sym(sem_trywait)(sem_t *sem) {
     init();
-    msg("sem_trywait(%p)", sem);
+    msg("sem_trywait(%p): %p", sem, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -762,7 +762,7 @@ int __sym(sem_trywait)(sem_t *sem) {
 
 int __sym(sem_timedwait)(sem_t *restrict sem, const struct timespec *restrict abs_timeout) {
     init();
-    msg("sem_timedwait(%p, %p:{tv_sec: %li, tv_nsec: %li})", sem, abs_timeout, abs_timeout->tv_sec, abs_timeout->tv_nsec);
+    msg("sem_timedwait(%p, %p:{tv_sec: %li, tv_nsec: %li}): %p", sem, abs_timeout, abs_timeout->tv_sec, abs_timeout->tv_nsec, __builtin_return_address(0));
     struct timespec overwrite;
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
@@ -807,7 +807,7 @@ int __sym(sem_timedwait)(sem_t *restrict sem, const struct timespec *restrict ab
 
 int __sym(sem_getvalue)(sem_t *restrict sem, int *restrict value) {
     init();
-    msg("sem_getvalue(%p, %p)", sem, value);
+    msg("sem_getvalue(%p, %p): %p", sem, value, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -823,7 +823,7 @@ int __sym(sem_getvalue)(sem_t *restrict sem, int *restrict value) {
 
 int __sym(sem_close)(sem_t *sem) {
     init();
-    msg("sem_close(%p)", sem);
+    msg("sem_close(%p): %p", sem, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
@@ -839,7 +839,7 @@ int __sym(sem_close)(sem_t *sem) {
 
 int __sym(sem_unlink)(const char *name) {
     init();
-    msg("sem_unlink(%es)", name);
+    msg("sem_unlink(%es): %p", name, __builtin_return_address(0));
     char overwrite[BUFFER_SIZE];
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
@@ -870,7 +870,7 @@ int __sym(sem_unlink)(const char *name) {
 
 int __sym(sem_destroy)(sem_t *sem) {
     init();
-    msg("sem_destroy(%p)", sem);
+    msg("sem_destroy(%p): %p", sem, __builtin_return_address(0));
     if (mode >= 4) {
         char buf[BUFFER_SIZE];
         rcv(buf, sizeof(buf));
diff --git a/proj/test1/src/main.c b/proj/test1/src/main.c
index 5c6395e..c4e15ba 100644
--- a/proj/test1/src/main.c
+++ b/proj/test1/src/main.c
@@ -5,6 +5,7 @@
 #include <stdlib.h>
 #include <errno.h>
 #include <string.h>
+#include <semaphore.h>
 
 static void usage(const char *prog_name) {
     fprintf(stderr, "usage: %s [-a]\n", prog_name);
@@ -22,6 +23,29 @@ void do_something(void) {
     free(mem);
 }
 
+void do_sem(void) {
+
+    sem_t sem;
+    sem_init(&sem, 0, 1);
+
+    sem_post(&sem);
+
+    while (sem_wait(&sem) == -1) {
+        if (errno != EINTR) {
+            // TODO fail
+        }
+    }
+
+    do_something();
+
+    sem_wait(&sem);
+
+    do_something();
+
+    sem_destroy(&sem);
+
+}
+
 int main(const int argc, char *const argv[]) {
     for (int ch; (ch = getopt(argc, argv, "a")) != -1;) {
         switch (ch) {
@@ -37,4 +61,5 @@ int main(const int argc, char *const argv[]) {
     do_something();
     do_something();
     do_something();
+    do_sem();
 }