diff --git a/libc/assert.h b/libc/assert.h
index ce6933b2a..e6ef79237 100644
--- a/libc/assert.h
+++ b/libc/assert.h
@@ -17,6 +17,20 @@ void __assert_fail(const char *, const char *, int) hidden relegated;
 #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_ */
diff --git a/libc/calls/preadv.c b/libc/calls/preadv.c
index 25ec37f4c..349c00988 100644
--- a/libc/calls/preadv.c
+++ b/libc/calls/preadv.c
@@ -16,12 +16,15 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/assert.h"
 #include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/state.internal.h"
 #include "libc/calls/strace.internal.h"
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/iovec.internal.h"
+#include "libc/calls/struct/sigset.h"
+#include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
@@ -33,74 +36,89 @@
 #include "libc/intrin/weaken.h"
 #include "libc/macros.internal.h"
 #include "libc/sysv/consts/iov.h"
+#include "libc/sysv/consts/sig.h"
 #include "libc/sysv/errfuns.h"
 #include "libc/zipos/zipos.internal.h"
 
 static ssize_t Preadv(int fd, struct iovec *iov, int iovlen, int64_t off) {
-  static bool once, demodernize;
-  int i, err;
-  ssize_t rc;
-  size_t got, toto;
+  int e, i;
+  size_t got;
+  bool masked;
+  ssize_t rc, toto;
+  sigset_t mask, oldmask;
+
+  if (fd < 0) {
+    return ebadf();
+  }
+
+  if (iovlen < 0) {
+    return einval();
+  }
+
+  if (IsAsan() && !__asan_is_valid_iov(iov, iovlen)) {
+    return efault();
+  }
 
-  if (fd < 0) return einval();
-  if (iovlen < 0) return einval();
-  if (IsAsan() && !__asan_is_valid_iov(iov, iovlen)) return efault();
   if (fd < g_fds.n && g_fds.p[fd].kind == kFdZip) {
     return weaken(__zipos_read)(
         (struct ZiposHandle *)(intptr_t)g_fds.p[fd].handle, iov, iovlen, off);
-  } else if (IsWindows()) {
+  }
+
+  if (IsMetal()) {
+    return espipe();  // must be serial or console if not zipos
+  }
+
+  if (IsWindows()) {
     if (fd < g_fds.n) {
       return sys_read_nt(g_fds.p + fd, iov, iovlen, off);
     } else {
       return ebadf();
     }
-  } else if (IsMetal()) {
-    return enosys();
   }
 
-  if (iovlen == 1) {
-    return sys_pread(fd, iov[0].iov_base, iov[0].iov_len, off, off);
-  }
-
-  /*
-   * NT, 2018-era XNU, and 2007-era Linux don't support this system call
-   */
-  if (!__vforked && !once) {
-    err = errno;
-    rc = sys_preadv(fd, iov, iovlen, off, off);
-    if (rc == -1 && errno == ENOSYS) {
-      errno = err;
-      once = true;
-      demodernize = true;
-      STRACE("demodernizing %s() due to %s", "preadv", "ENOSYS");
-    } else {
-      once = true;
-      return rc;
-    }
-  }
-
-  if (!demodernize) {
-    return sys_preadv(fd, iov, iovlen, off, off);
+  while (iovlen && !iov->iov_len) {
+    --iovlen;
+    ++iov;
   }
 
   if (!iovlen) {
-    return sys_pread(fd, NULL, 0, off, off);
+    return sys_pread(fd, 0, 0, off, off);
   }
 
+  if (iovlen == 1) {
+    return sys_pread(fd, iov->iov_base, iov->iov_len, off, off);
+  }
+
+  e = errno;
+  rc = sys_preadv(fd, iov, iovlen, off, off);
+  if (rc != -1 || errno != ENOSYS) return rc;
+  errno = e;
+
   for (toto = i = 0; i < iovlen; ++i) {
     rc = sys_pread(fd, iov[i].iov_base, iov[i].iov_len, off, off);
     if (rc == -1) {
-      if (toto && (errno == EINTR || errno == EAGAIN)) {
-        return toto;
-      } else {
-        return -1;
+      if (!toto) {
+        toto = -1;
       }
+      break;
     }
+
     got = rc;
     toto += got;
+    off += got;
     if (got != iov[i].iov_len) {
       break;
     }
+
+    if (!masked) {
+      sigfillset(&mask);
+      _npassert(!sys_sigprocmask(SIG_SETMASK, &mask, &oldmask));
+      masked = true;
+    }
+  }
+
+  if (masked) {
+    _npassert(!sys_sigprocmask(SIG_SETMASK, &oldmask, 0));
   }
 
   return toto;
diff --git a/libc/calls/pwritev.c b/libc/calls/pwritev.c
index 120da1dba..eda149625 100644
--- a/libc/calls/pwritev.c
+++ b/libc/calls/pwritev.c
@@ -16,11 +16,13 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/assert.h"
 #include "libc/calls/calls.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/strace.internal.h"
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/iovec.internal.h"
+#include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/syscall-sysv.internal.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
@@ -31,75 +33,90 @@
 #include "libc/intrin/weaken.h"
 #include "libc/macros.internal.h"
 #include "libc/sysv/consts/iov.h"
+#include "libc/sysv/consts/sig.h"
 #include "libc/sysv/errfuns.h"
 #include "libc/zipos/zipos.internal.h"
 
 static ssize_t Pwritev(int fd, const struct iovec *iov, int iovlen,
                        int64_t off) {
-  static bool once, demodernize;
-  int i, err;
-  ssize_t rc;
-  size_t sent, toto;
+  int i, e;
+  bool masked;
+  size_t sent;
+  ssize_t rc, toto;
+  sigset_t mask, oldmask;
+
+  if (fd < 0) {
+    return ebadf();
+  }
+
+  if (iovlen < 0) {
+    return einval();
+  }
+
+  if (IsAsan() && !__asan_is_valid_iov(iov, iovlen)) {
+    return efault();
+  }
 
-  if (fd < 0) return einval();
-  if (iovlen < 0) return einval();
-  if (IsAsan() && !__asan_is_valid_iov(iov, iovlen)) return efault();
   if (fd < g_fds.n && g_fds.p[fd].kind == kFdZip) {
     return weaken(__zipos_write)(
         (struct ZiposHandle *)(intptr_t)g_fds.p[fd].handle, iov, iovlen, off);
-  } else if (IsWindows()) {
+  }
+
+  if (IsWindows()) {
     if (fd < g_fds.n) {
       return sys_write_nt(fd, iov, iovlen, off);
     } else {
       return ebadf();
     }
-  } else if (IsMetal()) {
-    return enosys();
   }
 
-  if (iovlen == 1) {
-    return sys_pwrite(fd, iov[0].iov_base, iov[0].iov_len, off, off);
+  if (IsMetal()) {
+    return espipe();  // must be serial or console if not zipos
   }
 
-  /*
-   * NT, 2018-era XNU, and 2007-era Linux don't support this system call
-   */
-  if (!once) {
-    err = errno;
-    rc = sys_pwritev(fd, iov, iovlen, off, off);
-    if (rc == -1 && errno == ENOSYS) {
-      errno = err;
-      once = true;
-      demodernize = true;
-      STRACE("demodernizing %s() due to %s", "pwritev", "ENOSYS");
-    } else {
-      once = true;
-      return rc;
-    }
-  }
-
-  if (!demodernize) {
-    return sys_pwritev(fd, iov, iovlen, off, off);
+  while (iovlen && !iov->iov_len) {
+    --iovlen;
+    ++iov;
   }
 
   if (!iovlen) {
-    return sys_pwrite(fd, NULL, 0, off, off);
+    return sys_pwrite(fd, 0, 0, off, off);
   }
 
+  if (iovlen == 1) {
+    return sys_pwrite(fd, iov->iov_base, iov->iov_len, off, off);
+  }
+
+  e = errno;
+  rc = sys_pwritev(fd, iov, iovlen, off, off);
+  if (rc != -1 || errno != ENOSYS) return rc;
+  errno = e;
+
   for (toto = i = 0; i < iovlen; ++i) {
     rc = sys_pwrite(fd, iov[i].iov_base, iov[i].iov_len, off, off);
     if (rc == -1) {
-      if (toto && (errno == EINTR || errno == EAGAIN)) {
-        return toto;
-      } else {
-        return -1;
+      if (!toto) {
+        toto = -1;
       }
+      break;
     }
+
     sent = rc;
     toto += sent;
+    off += sent;
     if (sent != iov[i].iov_len) {
       break;
     }
+
+    if (!masked) {
+      sigfillset(&mask);
+      _npassert(!sys_sigprocmask(SIG_SETMASK, &mask, &oldmask));
+      masked = true;
+    }
+  }
+
+  if (masked) {
+    _npassert(!sys_sigprocmask(SIG_SETMASK, &oldmask, 0));
   }
 
   return toto;
diff --git a/test/libc/calls/preadv_test.c b/test/libc/calls/preadv_test.c
new file mode 100644
index 000000000..334918e93
--- /dev/null
+++ b/test/libc/calls/preadv_test.c
@@ -0,0 +1,88 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2022 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/calls.h"
+#include "libc/calls/struct/iovec.h"
+#include "libc/errno.h"
+#include "libc/sysv/consts/o.h"
+#include "libc/testlib/testlib.h"
+
+char testlib_enable_tmp_setup_teardown;
+
+TEST(preadv, ebadf) {
+  EXPECT_SYS(EBADF, -1, preadv(-1, 0, 0, 0));
+  EXPECT_SYS(EBADF, -1, pwritev(-1, 0, 0, 0));
+}
+
+TEST(preadv, testOneIovecWorks_itDelegatesToPread) {
+  char b1[2] = "hi";
+  struct iovec v[1] = {{b1, 2}};
+  ASSERT_SYS(0, 3, open("foo", O_RDWR | O_CREAT | O_TRUNC, 0644));
+  EXPECT_SYS(0, 2, pwritev(3, v, 1, 0));
+  b1[0] = 0;
+  b1[1] = 0;
+  EXPECT_SYS(0, 0, preadv(3, 0, 0, 0));
+  EXPECT_SYS(0, 0, pwritev(3, 0, 0, 0));
+  EXPECT_SYS(0, 2, preadv(3, v, 1, 0));
+  ASSERT_EQ('h', b1[0]);
+  ASSERT_EQ('i', b1[1]);
+  b1[0] = 0;
+  b1[1] = 0x55;
+  EXPECT_SYS(0, 1, preadv(3, v, 1, 1));
+  ASSERT_EQ('i', b1[0]);
+  ASSERT_EQ(0x55, b1[1]);
+  ASSERT_SYS(0, 0, close(3));
+}
+
+TEST(preadv, testTwoIovecWorks_itMustDoItsThing) {
+  char b1[1] = "h";
+  char b2[1] = "i";
+  struct iovec v[2] = {{b1, 1}, {b2, 1}};
+  ASSERT_SYS(0, 3, open("foo", O_RDWR | O_CREAT | O_TRUNC, 0644));
+  EXPECT_SYS(0, 2, pwritev(3, v, 2, 0));
+  b1[0] = 0;
+  b2[0] = 0;
+  EXPECT_SYS(0, 2, preadv(3, v, 2, 0));
+  ASSERT_EQ('h', b1[0]);
+  ASSERT_EQ('i', b2[0]);
+  b1[0] = 0;
+  b2[0] = 0x55;
+  EXPECT_SYS(0, 1, preadv(3, v, 2, 1));
+  ASSERT_EQ('i', b1[0]);
+  ASSERT_EQ(0x55, b2[0]);
+  ASSERT_SYS(0, 0, close(3));
+}
+
+TEST(pwritev, writePastEof_zeroExtendsFile_alsoDoesntChangeFilePointer) {
+  char b[6] = {0x55, 0x55, 0x55, 0x55, 0x55, 0x55};
+  char s[2] = "hi";
+  struct iovec v[1] = {{s, 2}};
+  ASSERT_SYS(0, 3, open("foo", O_RDWR | O_CREAT | O_TRUNC, 0644));
+  ASSERT_SYS(EINVAL, -1, pwritev(3, v, 1, -999));
+  ASSERT_SYS(0, 2, pwritev(3, v, 1, 0));
+  ASSERT_SYS(0, 2, pwritev(3, v, 1, 4));
+  ASSERT_SYS(0, 2, preadv(3, v, 1, 0));
+  ASSERT_SYS(0, 6, read(3, b, 6));
+  ASSERT_EQ('h', b[0]);
+  ASSERT_EQ('i', b[1]);
+  ASSERT_EQ(0, b[2]);
+  ASSERT_EQ(0, b[3]);
+  ASSERT_EQ('h', b[4]);
+  ASSERT_EQ('i', b[5]);
+  ASSERT_SYS(0, 0, close(3));
+}