libc: Implement _exit()

Apparently, I implemented _Exit in stdlib.h but forgot to add _exit to unistd.h...

libc/include/bits/attrs.h

@@ -3,7 +3,8 @@
 #ifndef _BITS_ATTRS_H
 #define _BITS_ATTRS_H
 
-#if !defined(_STDLIB_H) && !defined(_STRING_H) && !defined(_ASSERT_H) && !defined(_SETJMP_H) && !defined(_SYS_TIME_H)
+#if !defined(_STDLIB_H) && !defined(_UNISTD_H) && !defined(_STRING_H) && !defined(_ASSERT_H) && !defined(_SETJMP_H) && \
+    !defined(_SYS_TIME_H)
 #error "Never include bits/attrs.h directly; use one of the standard library headers."
 #endif
 

libc/include/unistd.h

@@ -7,6 +7,7 @@
 #include <stddef.h>
 
 #include <bits/access.h>
+#include <bits/attrs.h>
 #include <bits/seek.h>
 #include <stdint.h>
 #include <sys/types.h>
@@ -56,6 +57,9 @@ extern "C"
     /* Set the current process' effective group ID. */
     int setegid(gid_t gid);
 
+    /* Set the current process or a child process's process group. */
+    int setpgid(pid_t pid, pid_t pgid);
+
     /* Change the owner and group of a file. */
     int chown(const char* path, uid_t uid, gid_t gid);
 
@@ -86,6 +90,9 @@ extern "C"
     /* Call the operating system kernel for a specific service. */
     long syscall(long num, ...);
 
+    /* Exit the process immediately, without performing any cleanup actions. */
+    __noreturn void _exit(int status);
+
     /* Sleep for X microseconds. */
     int usleep(useconds_t us);
 

libc/src/unistd.cpp

@@ -246,6 +246,12 @@ extern "C"
         return rc;
     }
 
+    __noreturn void _exit(int status)
+    {
+        syscall(SYS_exit, status);
+        __builtin_unreachable();
+    }
+
     int usleep(useconds_t us)
     {
         long rc = syscall(SYS_usleep, us);