diff --git a/Userland/Libraries/LibC/bits/pthread_integration.h b/Userland/Libraries/LibC/bits/pthread_integration.h
index f671cdf064a..bbc7e820dea 100644
--- a/Userland/Libraries/LibC/bits/pthread_integration.h
+++ b/Userland/Libraries/LibC/bits/pthread_integration.h
@@ -31,9 +31,9 @@ int __pthread_key_delete(pthread_key_t);
 void* __pthread_getspecific(pthread_key_t);
 int __pthread_setspecific(pthread_key_t, const void*);
 
-int __pthread_self();
+int __pthread_self(void);
 
-void __pthread_key_destroy_for_current_thread();
+void __pthread_key_destroy_for_current_thread(void);
 
 #define __PTHREAD_MUTEX_NORMAL 0
 #define __PTHREAD_MUTEX_RECURSIVE 1
diff --git a/Userland/Libraries/LibC/float.h b/Userland/Libraries/LibC/float.h
index e3843499c3b..575278e24a0 100644
--- a/Userland/Libraries/LibC/float.h
+++ b/Userland/Libraries/LibC/float.h
@@ -8,7 +8,7 @@
 #pragma once
 
 // Defined in fenv.cpp, but we must not include fenv.h, so here's its prototype.
-int fegetround();
+int fegetround(void);
 
 #define FLT_RADIX 2
 #define DECIMAL_DIG 21
diff --git a/Userland/Libraries/LibC/grp.h b/Userland/Libraries/LibC/grp.h
index 1acc7dee921..20df7747c7a 100644
--- a/Userland/Libraries/LibC/grp.h
+++ b/Userland/Libraries/LibC/grp.h
@@ -20,9 +20,9 @@ struct group {
     char** gr_mem;
 };
 
-struct group* getgrent();
-void setgrent();
-void endgrent();
+struct group* getgrent(void);
+void setgrent(void);
+void endgrent(void);
 struct group* getgrnam(const char* name);
 struct group* getgrgid(gid_t);
 int putgrent(const struct group*, FILE*);
diff --git a/Userland/Libraries/LibC/locale.h b/Userland/Libraries/LibC/locale.h
index 2c9cfb2d4aa..7d4069ecbab 100644
--- a/Userland/Libraries/LibC/locale.h
+++ b/Userland/Libraries/LibC/locale.h
@@ -54,7 +54,7 @@ struct lconv {
     char int_n_sign_posn;
 };
 
-struct lconv* localeconv();
+struct lconv* localeconv(void);
 char* setlocale(int category, const char* locale);
 
 __END_DECLS
diff --git a/Userland/Libraries/LibC/net/if.h b/Userland/Libraries/LibC/net/if.h
index dd6a4ca97a9..d96333a0135 100644
--- a/Userland/Libraries/LibC/net/if.h
+++ b/Userland/Libraries/LibC/net/if.h
@@ -12,7 +12,7 @@ __BEGIN_DECLS
 
 unsigned int if_nametoindex(char const* ifname);
 char* if_indextoname(unsigned int ifindex, char* ifname);
-struct if_nameindex* if_nameindex();
+struct if_nameindex* if_nameindex(void);
 void if_freenameindex(struct if_nameindex* ptr);
 
 __END_DECLS
diff --git a/Userland/Libraries/LibC/netdb.h b/Userland/Libraries/LibC/netdb.h
index 43ab958f1d9..409be7d437e 100644
--- a/Userland/Libraries/LibC/netdb.h
+++ b/Userland/Libraries/LibC/netdb.h
@@ -30,11 +30,11 @@ struct servent {
     char* s_proto;
 };
 
-struct servent* getservent();
+struct servent* getservent(void);
 struct servent* getservbyname(const char* name, const char* protocol);
 struct servent* getservbyport(int port, const char* protocol);
 void setservent(int stay_open);
-void endservent();
+void endservent(void);
 
 struct protoent {
     char* p_name;
@@ -42,10 +42,10 @@ struct protoent {
     int p_proto;
 };
 
-void endprotoent();
+void endprotoent(void);
 struct protoent* getprotobyname(const char* name);
 struct protoent* getprotobynumber(int proto);
-struct protoent* getprotoent();
+struct protoent* getprotoent(void);
 void setprotoent(int stay_open);
 
 extern int h_errno;
diff --git a/Userland/Libraries/LibC/pwd.h b/Userland/Libraries/LibC/pwd.h
index e78ad50ab39..a7d8acd24c9 100644
--- a/Userland/Libraries/LibC/pwd.h
+++ b/Userland/Libraries/LibC/pwd.h
@@ -22,9 +22,9 @@ struct passwd {
     char* pw_shell;
 };
 
-struct passwd* getpwent();
-void setpwent();
-void endpwent();
+struct passwd* getpwent(void);
+void setpwent(void);
+void endpwent(void);
 struct passwd* getpwnam(const char* name);
 struct passwd* getpwuid(uid_t);
 int putpwent(const struct passwd* p, FILE* stream);
diff --git a/Userland/Libraries/LibC/sched.h b/Userland/Libraries/LibC/sched.h
index 613dbc56022..db7ddc2caed 100644
--- a/Userland/Libraries/LibC/sched.h
+++ b/Userland/Libraries/LibC/sched.h
@@ -11,7 +11,7 @@
 
 __BEGIN_DECLS
 
-int sched_yield();
+int sched_yield(void);
 
 struct sched_param {
     int sched_priority;
diff --git a/Userland/Libraries/LibC/shadow.h b/Userland/Libraries/LibC/shadow.h
index ac58c0af40a..2853db4b624 100644
--- a/Userland/Libraries/LibC/shadow.h
+++ b/Userland/Libraries/LibC/shadow.h
@@ -27,9 +27,9 @@ struct spwd {
 };
 
 #ifndef AK_OS_MACOS
-struct spwd* getspent();
-void setspent();
-void endspent();
+struct spwd* getspent(void);
+void setspent(void);
+void endspent(void);
 struct spwd* getspnam(const char* name);
 int putspent(struct spwd* p, FILE* stream);
 
diff --git a/Userland/Libraries/LibC/stdio.h b/Userland/Libraries/LibC/stdio.h
index b8879e4fb3f..c1bf665d196 100644
--- a/Userland/Libraries/LibC/stdio.h
+++ b/Userland/Libraries/LibC/stdio.h
@@ -49,7 +49,7 @@ int fgetc(FILE*);
 int fgetc_unlocked(FILE*);
 int getc(FILE*);
 int getc_unlocked(FILE* stream);
-int getchar();
+int getchar(void);
 ssize_t getdelim(char**, size_t*, int, FILE*);
 ssize_t getline(char**, size_t*, FILE*);
 int ungetc(int c, FILE*);
@@ -94,7 +94,7 @@ int setvbuf(FILE*, char* buf, int mode, size_t);
 void setbuf(FILE*, char* buf);
 void setlinebuf(FILE*);
 int rename(const char* oldpath, const char* newpath);
-FILE* tmpfile();
+FILE* tmpfile(void);
 char* tmpnam(char*);
 FILE* popen(const char* command, const char* type);
 int pclose(FILE*);
diff --git a/Userland/Libraries/LibC/stdlib.h b/Userland/Libraries/LibC/stdlib.h
index 68caf7a3cbb..abab2a44cbc 100644
--- a/Userland/Libraries/LibC/stdlib.h
+++ b/Userland/Libraries/LibC/stdlib.h
@@ -16,7 +16,7 @@ __BEGIN_DECLS
 #define EXIT_SUCCESS 0
 #define EXIT_FAILURE 1
 
-__attribute__((noreturn)) void _abort();
+__attribute__((noreturn)) void _abort(void);
 
 __attribute__((malloc)) __attribute__((alloc_size(1))) void* malloc(size_t);
 __attribute__((malloc)) __attribute__((alloc_size(1, 2))) void* calloc(size_t nmemb, size_t);
@@ -33,7 +33,7 @@ int putenv(char*);
 int unsetenv(const char*);
 int clearenv(void);
 int setenv(const char* name, const char* value, int overwrite);
-const char* getprogname();
+const char* getprogname(void);
 void setprogname(const char*);
 int atoi(const char*);
 long atol(const char*);
@@ -47,9 +47,9 @@ unsigned long long strtoull(const char*, char** endptr, int base);
 unsigned long strtoul(const char*, char** endptr, int base);
 void qsort(void* base, size_t nmemb, size_t size, int (*compar)(const void*, const void*));
 void qsort_r(void* base, size_t nmemb, size_t size, int (*compar)(const void*, const void*, void*), void* arg);
-int atexit(void (*function)());
+int atexit(void (*function)(void));
 __attribute__((noreturn)) void exit(int status);
-__attribute__((noreturn)) void abort();
+__attribute__((noreturn)) void abort(void);
 char* ptsname(int fd);
 int ptsname_r(int fd, char* buffer, size_t);
 int abs(int);
@@ -70,10 +70,10 @@ char* realpath(const char* pathname, char* buffer);
 __attribute__((noreturn)) void _Exit(int status);
 
 #define RAND_MAX 32767
-int rand();
+int rand(void);
 void srand(unsigned seed);
 
-long int random();
+long int random(void);
 void srandom(unsigned seed);
 
 uint32_t arc4random(void);
diff --git a/Userland/Libraries/LibC/sys/auxv.h b/Userland/Libraries/LibC/sys/auxv.h
index 28da5da4e78..750383126cd 100644
--- a/Userland/Libraries/LibC/sys/auxv.h
+++ b/Userland/Libraries/LibC/sys/auxv.h
@@ -43,7 +43,7 @@ typedef struct
     union {
         long a_val;
         void* a_ptr;
-        void (*a_fnc)(); /* In spec, not used */
+        void (*a_fnc)(void); /* In spec, not used */
     } a_un;
 } auxv_t;
 
diff --git a/Userland/Libraries/LibC/sys/internals.h b/Userland/Libraries/LibC/sys/internals.h
index 4a275b28014..2b1b9a795de 100644
--- a/Userland/Libraries/LibC/sys/internals.h
+++ b/Userland/Libraries/LibC/sys/internals.h
@@ -12,11 +12,11 @@ __BEGIN_DECLS
 
 typedef void (*AtExitFunction)(void*);
 
-extern void __libc_init();
-extern void __malloc_init();
-extern void __stdio_init();
-extern void __begin_atexit_locking();
-extern void _init();
+extern void __libc_init(void);
+extern void __malloc_init(void);
+extern void __stdio_init(void);
+extern void __begin_atexit_locking(void);
+extern void _init(void);
 extern bool __environ_is_malloced;
 extern bool __stdio_is_initialized;
 extern bool __heap_is_stable;
@@ -24,8 +24,8 @@ extern void* __auxiliary_vector;
 
 int __cxa_atexit(AtExitFunction exit_function, void* parameter, void* dso_handle);
 void __cxa_finalize(void* dso_handle);
-__attribute__((noreturn)) void __cxa_pure_virtual() __attribute__((weak));
-__attribute__((noreturn)) void __stack_chk_fail();
-__attribute__((noreturn)) void __stack_chk_fail_local();
+__attribute__((noreturn)) void __cxa_pure_virtual(void) __attribute__((weak));
+__attribute__((noreturn)) void __stack_chk_fail(void);
+__attribute__((noreturn)) void __stack_chk_fail_local(void);
 
 __END_DECLS
diff --git a/Userland/Libraries/LibC/time.h b/Userland/Libraries/LibC/time.h
index 4d41ffec0b6..d5316c4ee44 100644
--- a/Userland/Libraries/LibC/time.h
+++ b/Userland/Libraries/LibC/time.h
@@ -37,11 +37,11 @@ time_t timegm(struct tm*);
 time_t time(time_t*);
 char* ctime(const time_t*);
 char* ctime_r(const time_t* tm, char* buf);
-void tzset();
+void tzset(void);
 char* asctime(const struct tm*);
 char* asctime_r(const struct tm*, char* buf);
 
-clock_t clock();
+clock_t clock(void);
 
 int clock_gettime(clockid_t, struct timespec*);
 int clock_settime(clockid_t, struct timespec*);
diff --git a/Userland/Libraries/LibC/unistd.h b/Userland/Libraries/LibC/unistd.h
index b12cc237af7..141ae5e5171 100644
--- a/Userland/Libraries/LibC/unistd.h
+++ b/Userland/Libraries/LibC/unistd.h
@@ -32,13 +32,13 @@ extern char** environ;
 
 int get_process_name(char* buffer, int buffer_size);
 int set_process_name(const char* name, size_t name_length);
-void dump_backtrace();
+void dump_backtrace(void);
 int fsync(int fd);
-int sysbeep();
-int gettid();
-int getpagesize();
-pid_t fork();
-pid_t vfork();
+int sysbeep(void);
+int gettid(void);
+int getpagesize(void);
+pid_t fork(void);
+pid_t vfork(void);
 int execv(const char* path, char* const argv[]);
 int execve(const char* filename, char* const argv[], char* const envp[]);
 int execvpe(const char* filename, char* const argv[], char* const envp[]);
@@ -46,19 +46,19 @@ int execvp(const char* filename, char* const argv[]);
 int execl(const char* filename, const char* arg, ...);
 int execle(const char* filename, const char* arg, ...);
 int execlp(const char* filename, const char* arg, ...);
-void sync();
+void sync(void);
 __attribute__((noreturn)) void _exit(int status);
 pid_t getsid(pid_t);
-pid_t setsid();
+pid_t setsid(void);
 int setpgid(pid_t pid, pid_t pgid);
 pid_t getpgid(pid_t);
-pid_t getpgrp();
-uid_t geteuid();
-gid_t getegid();
-uid_t getuid();
-gid_t getgid();
-pid_t getpid();
-pid_t getppid();
+pid_t getpgrp(void);
+uid_t geteuid(void);
+gid_t getegid(void);
+uid_t getuid(void);
+gid_t getgid(void);
+pid_t getpid(void);
+pid_t getppid(void);
 int getresuid(uid_t*, uid_t*, uid_t*);
 int getresgid(gid_t*, gid_t*, gid_t*);
 int getgroups(int size, gid_t list[]);
@@ -103,7 +103,7 @@ int isatty(int fd);
 int mknod(const char* pathname, mode_t, dev_t);
 long fpathconf(int fd, int name);
 long pathconf(const char* path, int name);
-char* getlogin();
+char* getlogin(void);
 int lchown(const char* pathname, uid_t uid, gid_t gid);
 int chown(const char* pathname, uid_t, gid_t);
 int fchown(int fd, uid_t, gid_t);
@@ -115,7 +115,7 @@ int umount(const char* mountpoint);
 int pledge(const char* promises, const char* execpromises);
 int unveil(const char* path, const char* permissions);
 char* getpass(const char* prompt);
-int pause();
+int pause(void);
 int chroot(const char*);
 
 enum {
diff --git a/Userland/Libraries/LibDl/dlfcn.h b/Userland/Libraries/LibDl/dlfcn.h
index 880dce74ac2..b8ab808793a 100644
--- a/Userland/Libraries/LibDl/dlfcn.h
+++ b/Userland/Libraries/LibDl/dlfcn.h
@@ -24,7 +24,7 @@ typedef struct __Dl_info {
 } Dl_info;
 
 int dlclose(void*);
-char* dlerror();
+char* dlerror(void);
 void* dlopen(const char*, int);
 void* dlsym(void*, const char*);
 int dladdr(void*, Dl_info*);