diff --git a/expected/wasm32-wasi-threads/defined-symbols.txt b/expected/wasm32-wasi-threads/defined-symbols.txt
index 75954e6ec..c913f1620 100644
--- a/expected/wasm32-wasi-threads/defined-symbols.txt
+++ b/expected/wasm32-wasi-threads/defined-symbols.txt
@@ -390,6 +390,7 @@ __wasilibc_nocwd_scandirat
 __wasilibc_nocwd_symlinkat
 __wasilibc_nocwd_utimensat
 __wasilibc_open_nomode
+__wasilibc_populate_preopens
 __wasilibc_pthread_self
 __wasilibc_register_preopened_fd
 __wasilibc_rename_newat
diff --git a/expected/wasm32-wasi/defined-symbols.txt b/expected/wasm32-wasi/defined-symbols.txt
index a69263216..6e743228b 100644
--- a/expected/wasm32-wasi/defined-symbols.txt
+++ b/expected/wasm32-wasi/defined-symbols.txt
@@ -329,6 +329,7 @@ __wasilibc_nocwd_scandirat
 __wasilibc_nocwd_symlinkat
 __wasilibc_nocwd_utimensat
 __wasilibc_open_nomode
+__wasilibc_populate_preopens
 __wasilibc_register_preopened_fd
 __wasilibc_rename_newat
 __wasilibc_rename_oldat
diff --git a/libc-bottom-half/cloudlibc/src/libc/unistd/close.c b/libc-bottom-half/cloudlibc/src/libc/unistd/close.c
deleted file mode 100644
index 2f5814bb6..000000000
--- a/libc-bottom-half/cloudlibc/src/libc/unistd/close.c
+++ /dev/null
@@ -1,16 +0,0 @@
-// Copyright (c) 2015-2016 Nuxi, https://nuxi.nl/
-//
-// SPDX-License-Identifier: BSD-2-Clause
-
-#include <wasi/api.h>
-#include <errno.h>
-#include <unistd.h>
-
-int close(int fildes) {
-  __wasi_errno_t error = __wasi_fd_close(fildes);
-  if (error != 0) {
-    errno = error;
-    return -1;
-  }
-  return 0;
-}
diff --git a/libc-bottom-half/headers/public/wasi/libc.h b/libc-bottom-half/headers/public/wasi/libc.h
index b50518bfc..18d8e9e3c 100644
--- a/libc-bottom-half/headers/public/wasi/libc.h
+++ b/libc-bottom-half/headers/public/wasi/libc.h
@@ -11,6 +11,12 @@ extern "C" {
 struct stat;
 struct timespec;
 
+/// Populate libc's preopen tables. This is normally done automatically
+/// just before it's needed, however if you call `__wasi_fd_renumber` or
+/// `__wasi_fd_close` directly, and you need the preopens to be accurate
+/// afterward, you should call this before doing so.
+void __wasilibc_populate_preopens(void);
+
 /// Register the given pre-opened file descriptor under the given path.
 ///
 /// This function does not take ownership of `prefix` (it makes its own copy).
diff --git a/libc-bottom-half/sources/__wasilibc_fd_renumber.c b/libc-bottom-half/sources/__wasilibc_fd_renumber.c
index aa9d8dcb1..47992e91f 100644
--- a/libc-bottom-half/sources/__wasilibc_fd_renumber.c
+++ b/libc-bottom-half/sources/__wasilibc_fd_renumber.c
@@ -4,6 +4,9 @@
 #include <unistd.h>
 
 int __wasilibc_fd_renumber(int fd, int newfd) {
+    // Scan the preopen fds before making any changes.
+    __wasilibc_populate_preopens();
+
     __wasi_errno_t error = __wasi_fd_renumber(fd, newfd);
     if (error != 0) {
         errno = error;
@@ -11,3 +14,22 @@ int __wasilibc_fd_renumber(int fd, int newfd) {
     }
     return 0;
 }
+
+int close(int fd) {
+    // Scan the preopen fds before making any changes.
+    __wasilibc_populate_preopens();
+
+    __wasi_errno_t error = __wasi_fd_close(fd);
+    if (error != 0) {
+        errno = error;
+        return -1;
+    }
+
+    return 0;
+}
+
+weak void __wasilibc_populate_preopens(void) {
+    // This version does nothing. It may be overridden by a version which does
+    // something if `__wasilibc_find_abspath` or `__wasilibc_find_relpath` are
+    // used.
+}
diff --git a/libc-bottom-half/sources/preopens.c b/libc-bottom-half/sources/preopens.c
index 7293c8c49..b495433fa 100644
--- a/libc-bottom-half/sources/preopens.c
+++ b/libc-bottom-half/sources/preopens.c
@@ -25,6 +25,7 @@ typedef struct preopen {
 } preopen;
 
 /// A simple growable array of `preopen`.
+static _Atomic _Bool preopens_populated = false;
 static preopen *preopens;
 static size_t num_preopens;
 static size_t preopen_capacity;
@@ -100,12 +101,9 @@ static const char *strip_prefixes(const char *path) {
     return path;
 }
 
-/// Register the given preopened file descriptor under the given path.
-///
-/// This function takes ownership of `prefix`.
-static int internal_register_preopened_fd(__wasi_fd_t fd, const char *relprefix) {
-    LOCK(lock);
-
+/// Similar to `internal_register_preopened_fd_unlocked` but does not
+/// take a lock.
+static int internal_register_preopened_fd_unlocked(__wasi_fd_t fd, const char *relprefix) {
     // Check preconditions.
     assert_invariants();
     assert(fd != AT_FDCWD);
@@ -113,22 +111,32 @@ static int internal_register_preopened_fd(__wasi_fd_t fd, const char *relprefix)
     assert(relprefix != NULL);
 
     if (num_preopens == preopen_capacity && resize() != 0) {
-        UNLOCK(lock);
         return -1;
     }
 
     char *prefix = strdup(strip_prefixes(relprefix));
     if (prefix == NULL) {
-        UNLOCK(lock);
         return -1;
     }
     preopens[num_preopens++] = (preopen) { prefix, fd, };
 
     assert_invariants();
-    UNLOCK(lock);
     return 0;
 }
 
+/// Register the given preopened file descriptor under the given path.
+///
+/// This function takes ownership of `prefix`.
+static int internal_register_preopened_fd(__wasi_fd_t fd, const char *relprefix) {
+    LOCK(lock);
+
+    int r = internal_register_preopened_fd_unlocked(fd, relprefix);
+
+    UNLOCK(lock);
+
+    return r;
+}
+
 /// Are the `prefix_len` bytes pointed to by `prefix` a prefix of `path`?
 static bool prefix_matches(const char *prefix, size_t prefix_len, const char *path) {
     // Allow an empty string as a prefix of any relative path.
@@ -152,6 +160,8 @@ static bool prefix_matches(const char *prefix, size_t prefix_len, const char *pa
 
 // See the documentation in libc.h
 int __wasilibc_register_preopened_fd(int fd, const char *prefix) {
+    __wasilibc_populate_preopens();
+
     return internal_register_preopened_fd((__wasi_fd_t)fd, prefix);
 }
 
@@ -172,6 +182,8 @@ int __wasilibc_find_relpath(const char *path,
 int __wasilibc_find_abspath(const char *path,
                             const char **abs_prefix,
                             const char **relative_path) {
+    __wasilibc_populate_preopens();
+
     // Strip leading `/` characters, the prefixes we're mataching won't have
     // them.
     while (*path == '/')
@@ -219,13 +231,20 @@ int __wasilibc_find_abspath(const char *path,
     return fd;
 }
 
-/// This is referenced by weak reference from crt1.c and lives in the same
-/// source file as `__wasilibc_find_relpath` so that it's linked in when it's
-/// needed.
-// Concerning the 51 -- see the comment by the constructor priority in
-// libc-bottom-half/sources/environ.c.
-__attribute__((constructor(51)))
-static void __wasilibc_populate_preopens(void) {
+void __wasilibc_populate_preopens(void) {
+    // Fast path: If the preopens are already initialized, do nothing.
+    if (preopens_populated) {
+        return;
+    }
+
+    LOCK(lock);
+
+    // Check whether another thread initialized the preopens already.
+    if (preopens_populated) {
+        UNLOCK(lock);
+        return;
+    }
+
     // Skip stdin, stdout, and stderr, and count up until we reach an invalid
     // file descriptor.
     for (__wasi_fd_t fd = 3; fd != 0; ++fd) {
@@ -249,7 +268,7 @@ static void __wasilibc_populate_preopens(void) {
                 goto oserr;
             prefix[prestat.u.dir.pr_name_len] = '\0';
 
-            if (internal_register_preopened_fd(fd, prefix) != 0)
+            if (internal_register_preopened_fd_unlocked(fd, prefix) != 0)
                 goto software;
             free(prefix);
 
@@ -260,6 +279,11 @@ static void __wasilibc_populate_preopens(void) {
         }
     }
 
+    // Preopens are now initialized.
+    preopens_populated = true;
+
+    UNLOCK(lock);
+
     return;
 oserr:
     _Exit(EX_OSERR);