libc: Implement atexit() and _exit()

exit() now calls registered handlers before calling _exit().

And initialize_libc() can now register a handler to close stdout and stderr on program termination!! :)

libs/libc/include/stdlib.h

@@ -12,11 +12,11 @@ extern "C"
     /* Aborts the program. */
     noreturn void abort();
 
-    /* Exits the program with the specified status code. */
+    /* Normally exits the program with the specified status code. */
     noreturn void exit(int status);
 
-    /* Not implemented. */
-    int atexit(void (*)(void));
+    /* Registers a handler function to be run at normal program termination. */
+    int atexit(void (*handler)(void));
 
     /* Not implemented.*/
     int atoi(const char*);

libs/libc/include/unistd.h

@@ -1,6 +1,7 @@
 #ifndef _UNISTD_H
 #define _UNISTD_H
 
+#include <bits/macros.h>
 #include <bits/seek.h>
 #include <sys/types.h>
 
@@ -20,6 +21,9 @@ extern "C"
     /* Not implemented. */
     pid_t fork(void);
 
+    /* Terminates the program with the status code status. */
+    noreturn void _exit(int status);
+
     /* Calls the kernel for a specific service, determined by number. */
     long syscall(long number, ...);
 

libs/libc/src/atexit.cpp

@@ -0,0 +1,25 @@
+#include <stdlib.h>
+#include <unistd.h>
+
+#define ATEXIT_MAX_FUNCS 32
+
+typedef void (*atexit_func_t)(void);
+
+atexit_func_t atexit_functions[ATEXIT_MAX_FUNCS];
+int atexit_function_count = 0;
+
+extern "C"
+{
+    int atexit(atexit_func_t handler)
+    {
+        if (atexit_function_count >= ATEXIT_MAX_FUNCS) return -1;
+        atexit_functions[atexit_function_count++] = handler;
+        return 0;
+    }
+
+    noreturn void exit(int status)
+    {
+        for (int i = 0; i < atexit_function_count; i++) { atexit_functions[i](); }
+        _exit(status);
+    }
+}

libs/libc/src/init.cpp

@@ -4,6 +4,12 @@
 #include <stdlib.h>
 #include <unistd.h>
 
+static void terminate_libc()
+{
+    fclose(stdout);
+    fclose(stderr);
+}
+
 extern "C" void initialize_libc()
 {
     if (lseek(0, 0, SEEK_CUR) < 0)
@@ -24,4 +30,5 @@ extern "C" void initialize_libc()
         errno = 0;
     }
     else { stderr = fdopen(1, "rw"); }
+    atexit(terminate_libc);
 }

libs/libc/src/stdlib.cpp

@@ -7,18 +7,7 @@ extern "C"
 {
     noreturn void abort()
     {
-        exit(-1);
-    }
-
-    noreturn void exit(int status)
-    {
-        syscall(SYS_exit, status);
-        __builtin_unreachable();
-    }
-
-    int atexit(void (*)(void))
-    {
-        NOT_IMPLEMENTED("atexit");
+        _exit(-1);
     }
 
     int atoi(const char*)

libs/libc/src/unistd.cpp

@@ -95,4 +95,10 @@ extern "C"
     {
         return syscall(SYS_seek, fd, offset, whence);
     }
+
+    noreturn void _exit(int status)
+    {
+        syscall(SYS_exit, status);
+        __builtin_unreachable();
+    }
 }