Make important improvements

- Fix preadv() and pwritev() for old distros
- Introduce _npassert() and _unassert() macros
- Prove that file locks work properly on Windows
- Support fcntl(F_DUPFD_CLOEXEC) on more systems

libc/assert.h

@@ -7,16 +7,29 @@ extern bool __assert_disable;
 void __assert_fail(const char *, const char *, int) hidden relegated;
 
 #ifdef NDEBUG
-#define assert(EXPR) ((void)0)
+#define assert(x) ((void)0)
 #else
-#define assert(EXPR) \
-  ((void)((EXPR) || (__assert_fail(#EXPR, __FILE__, __LINE__), 0)))
+#define assert(x) ((void)((x) || (__assert_fail(#x, __FILE__, __LINE__), 0)))
 #endif
 
 #ifndef __cplusplus
 #define static_assert _Static_assert
 #endif
 
+#define _unassert(x)                 \
+  do {                               \
+    if (__builtin_expect(!(x), 0)) { \
+      unreachable;                   \
+    }                                \
+  } while (0)
+
+#define _npassert(x)                 \
+  do {                               \
+    if (__builtin_expect(!(x), 0)) { \
+      notpossible;                   \
+    }                                \
+  } while (0)
+
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
 #endif /* COSMOPOLITAN_LIBC_ASSERT_H_ */