diff --git a/libc/include/setjmp.h b/libc/include/setjmp.h
index 195f251..67d3c2f 100644
--- a/libc/include/setjmp.h
+++ b/libc/include/setjmp.h
@@ -65,18 +65,18 @@
 
 __BEGIN_DECLS
 
-int _setjmp(jmp_buf __env);
-void _longjmp(jmp_buf __env, int __value);
+int _setjmp(jmp_buf __env) __returns_twice;
+__noreturn void _longjmp(jmp_buf __env, int __value);
 
-int setjmp(jmp_buf __env);
-void longjmp(jmp_buf __env, int __value);
+int setjmp(jmp_buf __env) __returns_twice;
+__noreturn void longjmp(jmp_buf __env, int __value);
 
 #define setjmp(__env) setjmp(__env)
 
-int sigsetjmp(sigjmp_buf __env, int __save_signal_mask)
-    __INTRODUCED_IN_ARM(9) __INTRODUCED_IN_MIPS(12) __INTRODUCED_IN_X86(12);
-void siglongjmp(sigjmp_buf __env, int __value)
-    __INTRODUCED_IN_ARM(9) __INTRODUCED_IN_MIPS(12) __INTRODUCED_IN_X86(12);
+int sigsetjmp(sigjmp_buf __env, int __save_signal_mask) __returns_twice __INTRODUCED_IN_ARM(9)
+    __INTRODUCED_IN_MIPS(12) __INTRODUCED_IN_X86(12);
+__noreturn void siglongjmp(sigjmp_buf __env, int __value) __INTRODUCED_IN_ARM(9)
+    __INTRODUCED_IN_MIPS(12) __INTRODUCED_IN_X86(12);
 
 __END_DECLS
 
diff --git a/libc/include/sys/cdefs.h b/libc/include/sys/cdefs.h
index a919a79..ca9374e 100644
--- a/libc/include/sys/cdefs.h
+++ b/libc/include/sys/cdefs.h
@@ -86,6 +86,7 @@
 #define __noreturn __attribute__((__noreturn__))
 #define __mallocfunc  __attribute__((__malloc__))
 #define __packed __attribute__((__packed__))
+#define __returns_twice __attribute__((__returns_twice__))
 #define __unused __attribute__((__unused__))
 #define __used __attribute__((__used__))
 
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 90173aa..9000407 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -78,7 +78,7 @@
 __noreturn void _exit(int __status);
 
 pid_t  fork(void);
-pid_t  vfork(void);
+pid_t  vfork(void) __returns_twice;
 pid_t  getpid(void);
 pid_t  gettid(void) __attribute_const__;
 pid_t  getpgid(pid_t __pid);