diff --git a/libc/calls/close-nt.c b/libc/calls/close-nt.c
index 74475c8d3..180f03f4a 100644
--- a/libc/calls/close-nt.c
+++ b/libc/calls/close-nt.c
@@ -17,13 +17,14 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/internal.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/weaken.h"
 #include "libc/nt/enum/filetype.h"
 #include "libc/nt/files.h"
 #include "libc/nt/runtime.h"
+#include "libc/runtime/runtime.h"
 #include "libc/runtime/zipos.internal.h"
 #include "libc/sock/syscall_fd.internal.h"
 #include "libc/sysv/consts/o.h"
@@ -64,5 +65,7 @@ textwindows int sys_close_nt(int fd, int fildes) {
     default:
       break;
   }
+  if (f->shared && !f->isdup)
+    munmap(f->shared, sizeof(struct Cursor));
   return CloseHandle(f->handle) ? 0 : __winerr();
 }
diff --git a/libc/calls/dup-nt.c b/libc/calls/dup-nt.c
index 35a07a223..898a653d3 100644
--- a/libc/calls/dup-nt.c
+++ b/libc/calls/dup-nt.c
@@ -82,6 +82,8 @@ static textwindows int sys_dup_nt_impl(int oldfd, int newfd, int flags,
 
   g_fds.p[newfd] = g_fds.p[oldfd];
   g_fds.p[newfd].handle = handle;
+  g_fds.p[newfd].isdup = true;
+  g_fds.p[oldfd].isdup = true;  // TODO(jart): is it possible to avoid leak?
   if (flags & _O_CLOEXEC) {
     g_fds.p[newfd].flags |= _O_CLOEXEC;
   } else {
diff --git a/libc/calls/fcntl-nt.c b/libc/calls/fcntl-nt.c
index a94130412..5c746b487 100644
--- a/libc/calls/fcntl-nt.c
+++ b/libc/calls/fcntl-nt.c
@@ -20,13 +20,13 @@
 #include "libc/calls/calls.h"
 #include "libc/calls/createfileflags.internal.h"
 #include "libc/calls/internal.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/struct/flock.h"
 #include "libc/calls/struct/sigset.internal.h"
 #include "libc/calls/syscall-nt.internal.h"
 #include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/calls/wincrash.internal.h"
 #include "libc/errno.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/kprintf.h"
 #include "libc/intrin/weaken.h"
 #include "libc/limits.h"
@@ -151,7 +151,7 @@ static textwindows int sys_fcntl_nt_lock(struct Fd *f, int fd, int cmd,
     case SEEK_SET:
       break;
     case SEEK_CUR:
-      off = f->pointer + off;
+      off = f->shared->pointer + off;
       break;
     case SEEK_END: {
       int64_t size;
@@ -351,9 +351,14 @@ textwindows int sys_fcntl_nt(int fd, int cmd, uintptr_t arg) {
       }
       rc = 0;
     } else if (cmd == F_SETLK || cmd == F_SETLKW || cmd == F_GETLK) {
-      pthread_mutex_lock(&g_locks.mu);
-      rc = sys_fcntl_nt_lock(g_fds.p + fd, fd, cmd, arg);
-      pthread_mutex_unlock(&g_locks.mu);
+      struct Fd *f = g_fds.p + fd;
+      if (f->shared) {
+        pthread_mutex_lock(&g_locks.mu);
+        rc = sys_fcntl_nt_lock(f, fd, cmd, arg);
+        pthread_mutex_unlock(&g_locks.mu);
+      } else {
+        rc = ebadf();
+      }
     } else if (cmd == F_DUPFD || cmd == F_DUPFD_CLOEXEC) {
       rc = sys_fcntl_nt_dupfd(fd, cmd, arg);
     } else {
diff --git a/libc/calls/lseek-nt.c b/libc/calls/lseek-nt.c
index 11ef3471f..9ca3342e9 100644
--- a/libc/calls/lseek-nt.c
+++ b/libc/calls/lseek-nt.c
@@ -31,7 +31,7 @@ static textwindows int64_t GetPosition(struct Fd *f, int whence) {
     case SEEK_SET:
       return 0;
     case SEEK_CUR:
-      return f->pointer;
+      return f->shared->pointer;
     case SEEK_END: {
       struct NtByHandleFileInformation wst;
       if (!GetFileInformationByHandle(f->handle, &wst)) {
@@ -67,11 +67,14 @@ textwindows int64_t sys_lseek_nt(int fd, int64_t offset, int whence) {
   } else if (__isfdkind(fd, kFdFile)) {
     struct Fd *f = g_fds.p + fd;
     int filetype = GetFileType(f->handle);
-    if (filetype != kNtFileTypePipe && filetype != kNtFileTypeChar) {
+    if (filetype != kNtFileTypePipe &&  //
+        filetype != kNtFileTypeChar &&  //
+        f->shared) {
       int64_t res;
-      if ((res = Seek(f, offset, whence)) != -1) {
-        f->pointer = res;
-      }
+      __fd_lock(f);
+      if ((res = Seek(f, offset, whence)) != -1)
+        f->shared->pointer = res;
+      __fd_unlock(f);
       return res;
     } else {
       return espipe();
diff --git a/libc/calls/open-nt.c b/libc/calls/open-nt.c
index c7339171b..58d0cefc5 100644
--- a/libc/calls/open-nt.c
+++ b/libc/calls/open-nt.c
@@ -138,6 +138,7 @@ static textwindows int sys_open_nt_file(int dirfd, const char *file,
   int64_t handle;
   if ((handle = sys_open_nt_impl(dirfd, file, flags, mode,
                                  kNtFileFlagOverlapped)) != -1) {
+    g_fds.p[fd].shared = __cursor_new();
     g_fds.p[fd].handle = handle;
     g_fds.p[fd].kind = kFdFile;
     g_fds.p[fd].flags = flags;
@@ -170,14 +171,14 @@ static textwindows int sys_open_nt_no_handle(int fd, int flags, int mode,
 
 static textwindows int sys_open_nt_dup(int fd, int flags, int mode, int oldfd) {
   int64_t handle;
-  if (!__isfdopen(oldfd)) {
+  if (!__isfdopen(oldfd))
     return enoent();
-  }
   if (DuplicateHandle(GetCurrentProcess(), g_fds.p[oldfd].handle,
                       GetCurrentProcess(), &handle, 0, true,
                       kNtDuplicateSameAccess)) {
     g_fds.p[fd] = g_fds.p[oldfd];
     g_fds.p[fd].handle = handle;
+    g_fds.p[fd].isdup = true;
     g_fds.p[fd].mode = mode;
     if (!sys_fcntl_nt_setfl(fd, flags)) {
       return fd;
diff --git a/libc/calls/readwrite-nt.c b/libc/calls/readwrite-nt.c
index 6fbfc1075..5ad587ac3 100644
--- a/libc/calls/readwrite-nt.c
+++ b/libc/calls/readwrite-nt.c
@@ -19,9 +19,9 @@
 #include "libc/calls/createfileflags.internal.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/sig.internal.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/struct/sigset.h"
 #include "libc/calls/syscall_support-nt.internal.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/weaken.h"
 #include "libc/nt/enum/filetype.h"
 #include "libc/nt/errors.h"
@@ -51,20 +51,19 @@ sys_readwrite_nt(int fd, void *data, size_t size, ssize_t offset,
   uint32_t exchanged;
   struct Fd *f = g_fds.p + fd;
 
-  // win32 i/o apis generally take 32-bit values thus we implicitly
-  // truncate outrageously large sizes. linux actually does it too!
-  size = MIN(size, 0x7ffff000);
-
   // pread() and pwrite() perform an implicit lseek() operation, so
   // similar to the lseek() system call, they too raise ESPIPE when
   // operating on a non-seekable file.
   bool pwriting = offset != -1;
-  bool seekable =
-      (f->kind == kFdFile && GetFileType(handle) == kNtFileTypeDisk) ||
-      f->kind == kFdDevNull || f->kind == kFdDevRandom;
-  if (pwriting && !seekable) {
+  bool isdisk = f->kind == kFdFile && GetFileType(handle) == kNtFileTypeDisk;
+  bool seekable = isdisk || f->kind == kFdDevNull || f->kind == kFdDevRandom;
+  if (pwriting && !seekable)
     return espipe();
-  }
+
+  // determine if we need to lock a file descriptor across processes
+  bool locked = isdisk && !pwriting && f->shared;
+  if (locked)
+    __fd_lock(f);
 
   // when a file is opened in overlapped mode win32 requires that we
   // take over full responsibility for managing our own file pointer
@@ -72,8 +71,8 @@ sys_readwrite_nt(int fd, void *data, size_t size, ssize_t offset,
   // the sense that it behaves so differently from linux, that using
   // win32 i/o required more compatibilty toil than doing it by hand
   if (!pwriting) {
-    if (seekable) {
-      offset = f->pointer;
+    if (seekable && f->shared) {
+      offset = f->shared->pointer;
     } else {
       offset = 0;
     }
@@ -82,8 +81,11 @@ sys_readwrite_nt(int fd, void *data, size_t size, ssize_t offset,
 RestartOperation:
   bool eagained = false;
   // check for signals and cancelation
-  if (_check_cancel() == -1)
+  if (_check_cancel() == -1) {
+    if (locked)
+      __fd_unlock(f);
     return -1;  // ECANCELED
+  }
   if (_weaken(__sig_get) && (sig = _weaken(__sig_get)(waitmask))) {
     goto HandleInterrupt;
   }
@@ -114,16 +116,16 @@ RestartOperation:
     }
     ok = true;
   }
-  if (ok) {
+  if (ok)
     ok = GetOverlappedResult(handle, &overlap, &exchanged, true);
-  }
   CloseHandle(overlap.hEvent);
 
   // if i/o succeeded then return its result
   if (ok) {
-    if (!pwriting && seekable) {
-      f->pointer = offset + exchanged;
-    }
+    if (!pwriting && seekable && f->shared)
+      f->shared->pointer = offset + exchanged;
+    if (locked)
+      __fd_unlock(f);
     return exchanged;
   }
 
@@ -131,23 +133,32 @@ RestartOperation:
   if (GetLastError() == kNtErrorOperationAborted) {
     // raise EAGAIN if it's due to O_NONBLOCK mmode
     if (eagained) {
+      if (locked)
+        __fd_unlock(f);
       return eagain();
     }
     // otherwise it must be due to a kill() via __sig_cancel()
     if (_weaken(__sig_relay) && (sig = _weaken(__sig_get)(waitmask))) {
     HandleInterrupt:
+      if (locked)
+        __fd_unlock(f);
       int handler_was_called = _weaken(__sig_relay)(sig, SI_KERNEL, waitmask);
       if (_check_cancel() == -1)
         return -1;  // possible if we SIGTHR'd
+      if (locked)
+        __fd_lock(f);
       // read() is @restartable unless non-SA_RESTART hands were called
-      if (!(handler_was_called & SIG_HANDLED_NO_RESTART)) {
+      if (!(handler_was_called & SIG_HANDLED_NO_RESTART))
         goto RestartOperation;
-      }
     }
+    if (locked)
+      __fd_unlock(f);
     return eintr();
   }
 
   // read() and write() have generally different error-handling paths
+  if (locked)
+    __fd_unlock(f);
   return -2;
 }
 
diff --git a/libc/intrin/fds.c b/libc/intrin/fds.c
index f70abfb92..6f830dfbb 100644
--- a/libc/intrin/fds.c
+++ b/libc/intrin/fds.c
@@ -16,13 +16,13 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/intrin/fds.h"
 #include "libc/calls/internal.h"
 #include "libc/calls/state.internal.h"
 #include "libc/calls/ttydefaults.h"
 #include "libc/dce.h"
 #include "libc/intrin/atomic.h"
 #include "libc/intrin/extend.h"
-#include "libc/intrin/fds.h"
 #include "libc/intrin/kprintf.h"
 #include "libc/intrin/nomultics.h"
 #include "libc/intrin/pushpop.h"
@@ -40,6 +40,7 @@
 #include "libc/sock/sock.h"
 #include "libc/sysv/consts/map.h"
 #include "libc/sysv/consts/o.h"
+#include "libc/sysv/consts/prot.h"
 #include "libc/thread/thread.h"
 
 #define OPEN_MAX 16
@@ -156,12 +157,29 @@ textstartup void __init_fds(int argc, char **argv, char **envp) {
         f->kind = kind;
         f->flags = flags;
         f->mode = mode;
-        f->pointer = pointer;
         f->type = type;
         f->family = family;
         f->protocol = protocol;
         atomic_store_explicit(&fds->f, fd + 1, memory_order_relaxed);
+
+        //
+        // - v1 abi: This field was originally the file pointer.
+        //
+        // - v2 abi: This field is the negated shared memory address.
+        //
+        if (f->kind == kFdFile) {
+          if (pointer < 0) {
+            f->shared = (struct Cursor *)(uintptr_t)-pointer;
+          } else if ((f->shared = __cursor_new())) {
+            f->shared->pointer = pointer;
+          }
+        }
       }
     }
+    for (int i = 0; i < 3; ++i) {
+      struct Fd *f = fds->p + i;
+      if (f->kind == kFdFile && !f->shared)
+        f->shared = __cursor_new();
+    }
   }
 }
diff --git a/libc/intrin/fds.h b/libc/intrin/fds.h
index e3ff36fc5..dc6ac70f4 100644
--- a/libc/intrin/fds.h
+++ b/libc/intrin/fds.h
@@ -1,5 +1,7 @@
 #ifndef COSMOPOLITAN_LIBC_CALLS_STRUCT_FD_INTERNAL_H_
 #define COSMOPOLITAN_LIBC_CALLS_STRUCT_FD_INTERNAL_H_
+#include "libc/atomic.h"
+#include "libc/thread/thread.h"
 COSMOPOLITAN_C_START_
 
 #define kFdEmpty     0
@@ -13,19 +15,25 @@ COSMOPOLITAN_C_START_
 #define kFdDevNull   9
 #define kFdDevRandom 10
 
+struct Cursor {
+  pthread_mutex_t lock;
+  long pointer;
+};
+
 struct Fd {
   char kind;
+  bool isdup;
   bool isbound;
   unsigned flags;
   unsigned mode;
   long handle;
-  long pointer;
   int family;
   int type;
   int protocol;
   unsigned rcvtimeo; /* millis; 0 means wait forever */
   unsigned sndtimeo; /* millis; 0 means wait forever */
   void *connect_op;
+  struct Cursor *shared;
 };
 
 struct Fds {
@@ -34,5 +42,9 @@ struct Fds {
   struct Fd *p, *e;
 };
 
+void __fd_lock(struct Fd *);
+void __fd_unlock(struct Fd *);
+struct Cursor *__cursor_new(void);
+
 COSMOPOLITAN_C_END_
 #endif /* COSMOPOLITAN_LIBC_CALLS_STRUCT_FD_INTERNAL_H_ */
diff --git a/libc/intrin/fds_lock.c b/libc/intrin/fds_lock.c
index c32367d85..dd3d28491 100644
--- a/libc/intrin/fds_lock.c
+++ b/libc/intrin/fds_lock.c
@@ -17,6 +17,8 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/state.internal.h"
+#include "libc/intrin/fds.h"
+#include "libc/runtime/runtime.h"
 #include "libc/thread/thread.h"
 
 void __fds_lock(void) {
@@ -26,3 +28,23 @@ void __fds_lock(void) {
 void __fds_unlock(void) {
   pthread_mutex_unlock(&__fds_lock_obj);
 }
+
+void __fd_lock(struct Fd *f) {
+  pthread_mutex_lock(&f->shared->lock);
+}
+
+void __fd_unlock(struct Fd *f) {
+  pthread_mutex_unlock(&f->shared->lock);
+}
+
+struct Cursor *__cursor_new(void) {
+  struct Cursor *c;
+  if ((c = _mapshared(sizeof(struct Cursor)))) {
+    pthread_mutexattr_t attr;
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
+    pthread_mutex_init(&c->lock, &attr);
+    pthread_mutexattr_destroy(&attr);
+  }
+  return c;
+}
diff --git a/libc/runtime/mapshared.c b/libc/intrin/mapshared.c
similarity index 100%
rename from libc/runtime/mapshared.c
rename to libc/intrin/mapshared.c
diff --git a/libc/proc/describefds.c b/libc/proc/describefds.c
index d2cee918c..e54615a2f 100644
--- a/libc/proc/describefds.c
+++ b/libc/proc/describefds.c
@@ -17,9 +17,9 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
-#include "libc/intrin/fds.h"
 #include "libc/calls/syscall_support-nt.internal.h"
 #include "libc/fmt/itoa.h"
+#include "libc/intrin/fds.h"
 #include "libc/intrin/strace.h"
 #include "libc/mem/mem.h"
 #include "libc/nt/files.h"
@@ -151,7 +151,12 @@ textwindows char *__describe_fds(const struct Fd *fds, size_t fdslen,
     *p++ = '_';
     p = FormatInt64(p, f->mode);
     *p++ = '_';
-    p = FormatInt64(p, f->pointer);
+    //
+    // - v1 abi: This field was originally the file pointer.
+    //
+    // - v2 abi: This field is the negated shared memory address.
+    //
+    p = FormatInt64(p, -(uintptr_t)f->shared);
     *p++ = '_';
     p = FormatInt64(p, f->type);
     *p++ = '_';
diff --git a/libc/sock/sendfile.c b/libc/sock/sendfile.c
index 26d7cef55..249793784 100644
--- a/libc/sock/sendfile.c
+++ b/libc/sock/sendfile.c
@@ -62,9 +62,10 @@ static dontinline textwindows ssize_t sys_sendfile_nt(
     int outfd, int infd, int64_t *opt_in_out_inoffset, uint32_t uptobytes) {
   ssize_t rc;
   uint32_t flags = 0;
+  bool locked = false;
   int64_t ih, oh, eof, offset;
   struct NtByHandleFileInformation wst;
-  if (!__isfdkind(infd, kFdFile))
+  if (!__isfdkind(infd, kFdFile) || !g_fds.p[infd].shared)
     return ebadf();
   if (!__isfdkind(outfd, kFdSocket))
     return ebadf();
@@ -73,7 +74,9 @@ static dontinline textwindows ssize_t sys_sendfile_nt(
   if (opt_in_out_inoffset) {
     offset = *opt_in_out_inoffset;
   } else {
-    offset = g_fds.p[infd].pointer;
+    locked = true;
+    __fd_lock(&g_fds.p[infd]);
+    offset = g_fds.p[infd].shared->pointer;
   }
   if (GetFileInformationByHandle(ih, &wst)) {
     // TransmitFile() returns EINVAL if `uptobytes` goes past EOF.
@@ -82,9 +85,10 @@ static dontinline textwindows ssize_t sys_sendfile_nt(
       uptobytes = eof - offset;
     }
   } else {
+    if (locked)
+      __fd_unlock(&g_fds.p[infd]);
     return ebadf();
   }
-  BLOCK_SIGNALS;
   struct NtOverlapped ov = {.hEvent = WSACreateEvent(), .Pointer = offset};
   cosmo_once(&g_transmitfile.once, transmitfile_init);
   if (g_transmitfile.lpTransmitFile(oh, ih, uptobytes, 0, &ov, 0, 0) ||
@@ -95,7 +99,7 @@ static dontinline textwindows ssize_t sys_sendfile_nt(
       if (opt_in_out_inoffset) {
         *opt_in_out_inoffset = offset + rc;
       } else {
-        g_fds.p[infd].pointer = offset + rc;
+        g_fds.p[infd].shared->pointer = offset + rc;
       }
     } else {
       rc = __winsockerr();
@@ -103,8 +107,9 @@ static dontinline textwindows ssize_t sys_sendfile_nt(
   } else {
     rc = __winsockerr();
   }
+  if (locked)
+    __fd_unlock(&g_fds.p[infd]);
   WSACloseEvent(ov.hEvent);
-  ALLOW_SIGNALS;
   return rc;
 }
 
@@ -186,7 +191,9 @@ ssize_t sendfile(int outfd, int infd, int64_t *opt_in_out_inoffset,
   } else if (IsFreebsd() || IsXnu()) {
     rc = sys_sendfile_bsd(outfd, infd, opt_in_out_inoffset, uptobytes);
   } else if (IsWindows()) {
+    BLOCK_SIGNALS;
     rc = sys_sendfile_nt(outfd, infd, opt_in_out_inoffset, uptobytes);
+    ALLOW_SIGNALS;
   } else {
     rc = enosys();
   }
diff --git a/test/libc/intrin/munmap_test.c b/test/libc/intrin/munmap_test.c
index 9bf2ef344..eb61bb282 100644
--- a/test/libc/intrin/munmap_test.c
+++ b/test/libc/intrin/munmap_test.c
@@ -185,8 +185,8 @@ TEST(munmap, tinyFile_preciseUnmapSize) {
 TEST(munmap, tinyFile_mapThriceUnmapOnce) {
   char *p;
   ASSERT_NE(MAP_FAILED, (p = mmap(0, gransz*5, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)));
-  ASSERT_SYS(0, 0, munmap(p, gransz*5));
   ASSERT_SYS(0, 3, open("doge", O_RDWR | O_CREAT | O_TRUNC, 0644));
+  ASSERT_SYS(0, 0, munmap(p, gransz*5));
   ASSERT_SYS (0, 5, write(3, "hello", 5));
   ASSERT_EQ(p+gransz*0, mmap(p+gransz*0, gransz, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, -1, 0));
   ASSERT_EQ(p+gransz*1, mmap(p+gransz*1, 5, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0));
diff --git a/test/math/bf16_test.c b/test/math/bf16_test.c
index 532cebda5..05df1f63f 100644
--- a/test/math/bf16_test.c
+++ b/test/math/bf16_test.c
@@ -1,22 +1,19 @@
-/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
-│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
-╞══════════════════════════════════════════════════════════════════════════════╡
-│ Copyright 2024 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/math.h"
+// Copyright 2024 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 <math.h>
 
 #define CHECK(x) \
   if (!(x))      \
diff --git a/test/posix/file_offset_shared_test.c b/test/posix/file_offset_shared_test.c
new file mode 100644
index 000000000..5124de186
--- /dev/null
+++ b/test/posix/file_offset_shared_test.c
@@ -0,0 +1,107 @@
+// Copyright 2024 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 <stdatomic.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+// test that file offset is shared across multiple processes
+
+atomic_int *phase;
+
+int main() {
+
+  if ((phase = mmap(0, sizeof(atomic_int), PROT_READ | PROT_WRITE,
+                    MAP_SHARED | MAP_ANONYMOUS, -1, 0)) == MAP_FAILED)
+    return 1;
+
+  int fd;
+  char path[] = "/tmp/fd_test.XXXXXX";
+  if ((fd = mkstemp(path)) == -1)
+    return 2;
+  if (lseek(fd, 0, SEEK_CUR) != 0)
+    return 22;
+
+  if (write(fd, "0", 1) != 1)
+    return 3;
+  if (lseek(fd, 0, SEEK_CUR) != 1)
+    return 33;
+
+  int pid;
+  if ((pid = fork()) == -1)
+    return 4;
+
+  if (!pid) {
+    if (write(fd, "1", 1) != 1)
+      _Exit(100);
+    if (lseek(fd, 0, SEEK_CUR) != 2)
+      _Exit(101);
+
+    *phase = 1;
+    for (;;)
+      if (*phase == 2)
+        break;
+
+    if (write(fd, "3", 1) != 1)
+      _Exit(102);
+    if (lseek(fd, 0, SEEK_CUR) != 4)
+      _Exit(103);
+    *phase = 3;
+    _Exit(0);
+  }
+
+  for (;;)
+    if (*phase == 1)
+      break;
+
+  if (write(fd, "2", 1) != 1)
+    return 5;
+  if (lseek(fd, 0, SEEK_CUR) != 3)
+    return 55;
+
+  *phase = 2;
+  for (;;)
+    if (*phase == 3)
+      break;
+
+  if (write(fd, "4", 1) != 1)
+    return 6;
+  if (lseek(fd, 0, SEEK_CUR) != 5)
+    return 66;
+
+  int ws;
+  if (wait(&ws) == -1)
+    return 7;
+  if (!WIFEXITED(ws))
+    return 8;
+  if (WEXITSTATUS(ws))
+    return WEXITSTATUS(ws);
+
+  char buf[16] = {0};
+  if (pread(fd, buf, 15, 0) != 5)
+    return 12;
+  if (lseek(fd, 0, SEEK_CUR) != 5)
+    return 77;
+
+  if (close(fd))
+    return 13;
+
+  if (unlink(path))
+    return 14;
+
+  if (strcmp(buf, "01234"))
+    return 15;
+}