diff --git a/.github/scripts/codespell-ignore.words b/.github/scripts/codespell-ignore.words
index b64de09d2b52..1a5e106400a6 100644
--- a/.github/scripts/codespell-ignore.words
+++ b/.github/scripts/codespell-ignore.words
@@ -4,7 +4,6 @@
 anonymou
 aNULL
 bu
-Buss
 clen
 CNA
 hel
diff --git a/.github/scripts/codespell.sh b/.github/scripts/codespell.sh
index a14d36a53be8..ac5f27dc479f 100755
--- a/.github/scripts/codespell.sh
+++ b/.github/scripts/codespell.sh
@@ -14,6 +14,7 @@ codespell \
   --skip 'docs/THANKS' \
   --skip 'projects/OS400/*' \
   --skip 'projects/vms/*' \
+  --skip 'RELEASE-NOTES' \
   --skip 'scripts/wcurl' \
   --ignore-regex '.*spellchecker:disable-line' \
   --ignore-words '.github/scripts/codespell-ignore.words' \
diff --git a/CMake/win32-cache.cmake b/CMake/win32-cache.cmake
index 3ddbfce7d385..c4ee6fd1b9b4 100644
--- a/CMake/win32-cache.cmake
+++ b/CMake/win32-cache.cmake
@@ -154,7 +154,6 @@ set(HAVE_SIGSETJMP 0)
 set(HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID 1)
 set(HAVE_SOCKET 1)
 set(HAVE_SOCKETPAIR 0)
-set(HAVE_STRDUP 1)
 set(HAVE_STRERROR_R 0)
 set(HAVE_STROPTS_H 0)
 set(HAVE_STRUCT_SOCKADDR_STORAGE 1)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8103f98aec41..20cd74542b4c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -193,9 +193,6 @@ if(WIN32)
 
   # Apply to all feature checks
   list(APPEND CMAKE_REQUIRED_DEFINITIONS "-DWIN32_LEAN_AND_MEAN")
-  if(MSVC)
-    list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_CRT_NONSTDC_NO_DEPRECATE")  # for strdup() detection
-  endif()
 
   set(CURL_TARGET_WINDOWS_VERSION "" CACHE STRING "Minimum target Windows version as hex string")
   if(CURL_TARGET_WINDOWS_VERSION)
@@ -1537,7 +1534,6 @@ check_symbol_exists("send"            "${CURL_INCLUDES}" HAVE_SEND)  # proto/bsd
 check_function_exists("sendmsg"       HAVE_SENDMSG)
 check_function_exists("sendmmsg"      HAVE_SENDMMSG)
 check_symbol_exists("select"          "${CURL_INCLUDES}" HAVE_SELECT)  # proto/bsdsocket.h sys/select.h sys/socket.h
-check_symbol_exists("strdup"          "string.h" HAVE_STRDUP)
 check_symbol_exists("memrchr"         "string.h" HAVE_MEMRCHR)
 check_symbol_exists("alarm"           "unistd.h" HAVE_ALARM)
 check_symbol_exists("fcntl"           "fcntl.h" HAVE_FCNTL)
@@ -1575,11 +1571,14 @@ check_function_exists("getrlimit"     HAVE_GETRLIMIT)
 check_function_exists("setlocale"     HAVE_SETLOCALE)
 check_function_exists("setrlimit"     HAVE_SETRLIMIT)
 
-if(NOT WIN32)
+if(WIN32)
+  set(HAVE_STRDUP 1)  # to not define local implementation. curl uses _strdup() on Windows.
+else()
   check_function_exists("if_nametoindex"  HAVE_IF_NAMETOINDEX)  # net/if.h
   check_function_exists("realpath"        HAVE_REALPATH)
   check_function_exists("sched_yield"     HAVE_SCHED_YIELD)
   check_symbol_exists("strcasecmp"        "string.h" HAVE_STRCASECMP)
+  check_symbol_exists("strdup"            "string.h" HAVE_STRDUP)
   check_symbol_exists("stricmp"           "string.h" HAVE_STRICMP)
   check_symbol_exists("strcmpi"           "string.h" HAVE_STRCMPI)
 endif()
diff --git a/docs/examples/.checksrc b/docs/examples/.checksrc
index 8c7b0c901e80..a9c6b4ed503f 100644
--- a/docs/examples/.checksrc
+++ b/docs/examples/.checksrc
@@ -5,6 +5,7 @@
 allowfunc atoi
 allowfunc atol
 allowfunc calloc
+allowfunc close
 allowfunc fclose
 allowfunc fdopen
 allowfunc fopen
diff --git a/docs/internals/CODE_STYLE.md b/docs/internals/CODE_STYLE.md
index 8be6592df5b8..6167eaad001f 100644
--- a/docs/internals/CODE_STYLE.md
+++ b/docs/internals/CODE_STYLE.md
@@ -353,6 +353,7 @@ This is the full list of functions generally banned.
     atoi
     atol
     calloc
+    close
     CreateFile
     CreateFileA
     CreateFileW
diff --git a/lib/cf-socket.c b/lib/cf-socket.c
index ebad57f371d0..8aa211367bfa 100644
--- a/lib/cf-socket.c
+++ b/lib/cf-socket.c
@@ -349,7 +349,7 @@ static CURLcode socket_open(struct Curl_easy *data,
   if(fcntl(*sockfd, F_SETFD, FD_CLOEXEC) < 0) {
     failf(data, "fcntl set CLOEXEC: %s",
           curlx_strerror(SOCKERRNO, errbuf, sizeof(errbuf)));
-    close(*sockfd);
+    sclose(*sockfd);
     *sockfd = CURL_SOCKET_BAD;
     return CURLE_COULDNT_CONNECT;
   }
diff --git a/lib/curl_fopen.c b/lib/curl_fopen.c
index 9559c0d0c4f5..3d5ee5002dbc 100644
--- a/lib/curl_fopen.c
+++ b/lib/curl_fopen.c
@@ -147,7 +147,7 @@ CURLcode Curl_fopen(struct Curl_easy *data, const char *filename,
 
 fail:
   if(fd != -1) {
-    close(fd);
+    curlx_close(fd);
     unlink(tempstore);
   }
 
diff --git a/lib/curl_setup.h b/lib/curl_setup.h
index 750ba90d2d4c..178694c66edf 100644
--- a/lib/curl_setup.h
+++ b/lib/curl_setup.h
@@ -89,10 +89,6 @@
 #ifdef _MSC_VER
 /* Disable Visual Studio warnings: 4127 "conditional expression is constant" */
 #pragma warning(disable:4127)
-/* Avoid VS2005 and upper complaining about portable C functions. */
-#ifndef _CRT_NONSTDC_NO_DEPRECATE  /* mingw-w64 v2+. MS SDK ~10+/~VS2017+. */
-#define _CRT_NONSTDC_NO_DEPRECATE  /* for close(), fileno(), unlink(), etc. */
-#endif
 #ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS  /* for getenv(), tests: sscanf() */
 #endif
@@ -808,6 +804,13 @@
 #  define read(fd, buf, count)  (ssize_t)_read(fd, buf, curlx_uztoui(count))
 #  undef  write
 #  define write(fd, buf, count) (ssize_t)_write(fd, buf, curlx_uztoui(count))
+/* Avoid VS2005+ _CRT_NONSTDC_NO_DEPRECATE warnings about non-portable funcs */
+#  undef fileno
+#  define fileno(fh) _fileno(fh)
+#  undef unlink
+#  define unlink(fn) _unlink(fn)
+#  undef isatty
+#  define isatty(fd) _isatty(fd)
 #endif
 
 /*
diff --git a/lib/curlx/fopen.h b/lib/curlx/fopen.h
index a395590d8e0f..e78a474b975c 100644
--- a/lib/curlx/fopen.h
+++ b/lib/curlx/fopen.h
@@ -50,14 +50,18 @@ int curlx_win32_open(const char *filename, int oflag, ...);
 int curlx_win32_rename(const char *oldpath, const char *newpath);
 #define CURLX_FOPEN_LOW(fname, mode)       curlx_win32_fopen(fname, mode)
 #define CURLX_FREOPEN_LOW(fname, mode, fh) curlx_win32_freopen(fname, mode, fh)
+#define CURLX_FDOPEN_LOW                   _fdopen
 #define curlx_stat(fname, stp)             curlx_win32_stat(fname, stp)
 #define curlx_open                         curlx_win32_open
+#define curlx_close                        _close
 #define curlx_rename                       curlx_win32_rename
 #else
 #define CURLX_FOPEN_LOW                    fopen
 #define CURLX_FREOPEN_LOW                  freopen
+#define CURLX_FDOPEN_LOW                   fdopen
 #define curlx_stat(fname, stp)             stat(fname, stp)
 #define curlx_open                         open
+#define curlx_close                        close
 #define curlx_rename                       rename
 #endif
 
@@ -71,7 +75,7 @@ int curlx_win32_rename(const char *oldpath, const char *newpath);
 #else
 #define curlx_fopen             CURLX_FOPEN_LOW
 #define curlx_freopen           CURLX_FREOPEN_LOW
-#define curlx_fdopen            fdopen
+#define curlx_fdopen            CURLX_FDOPEN_LOW
 #define curlx_fclose            fclose
 #endif
 
diff --git a/lib/easy.c b/lib/easy.c
index 51936660cee2..cc505b5b3f82 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -99,12 +99,10 @@ static curl_simple_lock s_lock = CURL_SIMPLE_LOCK_INIT;
  * ways, but at this point it must be defined as the system-supplied strdup
  * so the callback pointer is initialized correctly.
  */
-#ifdef HAVE_STRDUP
 #ifdef _WIN32
 #define system_strdup _strdup
-#else
+#elif defined(HAVE_STRDUP)
 #define system_strdup strdup
-#endif
 #else
 #define system_strdup Curl_strdup
 #endif
diff --git a/lib/file.c b/lib/file.c
index 753aed2dbe30..d4cfedff1743 100644
--- a/lib/file.c
+++ b/lib/file.c
@@ -87,7 +87,7 @@ static void file_cleanup(struct FILEPROTO *file)
   Curl_safefree(file->freepath);
   file->path = NULL;
   if(file->fd != -1) {
-    close(file->fd);
+    curlx_close(file->fd);
     file->fd = -1;
   }
 }
@@ -312,7 +312,7 @@ static CURLcode file_upload(struct Curl_easy *data,
   /* treat the negative resume offset value as the case of "-" */
   if(data->state.resume_from < 0) {
     if(fstat(fd, &file_stat)) {
-      close(fd);
+      curlx_close(fd);
       failf(data, "cannot get the size of %s", file->path);
       return CURLE_WRITE_ERROR;
     }
@@ -367,7 +367,7 @@ static CURLcode file_upload(struct Curl_easy *data,
     result = Curl_pgrsUpdate(data);
 
 out:
-  close(fd);
+  curlx_close(fd);
   Curl_multi_xfer_ulbuf_release(data, xfer_ulbuf);
 
   return result;
diff --git a/lib/memdebug.c b/lib/memdebug.c
index 2851bcefa9c6..457034ebdae2 100644
--- a/lib/memdebug.c
+++ b/lib/memdebug.c
@@ -497,8 +497,7 @@ ALLOC_FUNC
 FILE *curl_dbg_fdopen(int filedes, const char *mode,
                       int line, const char *source)
 {
-  /* !checksrc! disable BANNEDFUNC 1 */
-  FILE *res = fdopen(filedes, mode);
+  FILE *res = CURLX_FDOPEN_LOW(filedes, mode);
   if(source)
     curl_dbg_log("FILE %s:%d fdopen(\"%d\",\"%s\") = %p\n",
                  source, line, filedes, mode, (void *)res);
diff --git a/lib/socketpair.c b/lib/socketpair.c
index ec45926237e8..cefbe8523866 100644
--- a/lib/socketpair.c
+++ b/lib/socketpair.c
@@ -64,8 +64,8 @@ static int wakeup_pipe(curl_socket_t socks[2], bool nonblocking)
 #ifdef HAVE_FCNTL
   if(fcntl(socks[0], F_SETFD, FD_CLOEXEC) ||
      fcntl(socks[1], F_SETFD, FD_CLOEXEC)) {
-    close(socks[0]);
-    close(socks[1]);
+    sclose(socks[0]);
+    sclose(socks[1]);
     socks[0] = socks[1] = CURL_SOCKET_BAD;
     return -1;
   }
@@ -73,8 +73,8 @@ static int wakeup_pipe(curl_socket_t socks[2], bool nonblocking)
   if(nonblocking) {
     if(curlx_nonblock(socks[0], TRUE) < 0 ||
        curlx_nonblock(socks[1], TRUE) < 0) {
-      close(socks[0]);
-      close(socks[1]);
+      sclose(socks[0]);
+      sclose(socks[1]);
       socks[0] = socks[1] = CURL_SOCKET_BAD;
       return -1;
     }
@@ -107,8 +107,8 @@ static int wakeup_socketpair(curl_socket_t socks[2], bool nonblocking)
   if(nonblocking) {
     if(curlx_nonblock(socks[0], TRUE) < 0 ||
        curlx_nonblock(socks[1], TRUE) < 0) {
-      close(socks[0]);
-      close(socks[1]);
+      sclose(socks[0]);
+      sclose(socks[1]);
       return -1;
     }
   }
diff --git a/lib/vquic/curl_ngtcp2.c b/lib/vquic/curl_ngtcp2.c
index 57a1543be350..f624c02fa333 100644
--- a/lib/vquic/curl_ngtcp2.c
+++ b/lib/vquic/curl_ngtcp2.c
@@ -55,6 +55,7 @@
 #include "../cf-socket.h"
 #include "../connect.h"
 #include "../progress.h"
+#include "../curlx/fopen.h"
 #include "../curlx/dynbuf.h"
 #include "../http1.h"
 #include "../select.h"
@@ -449,7 +450,7 @@ static void qlog_callback(void *user_data, uint32_t flags,
     ssize_t rc = write(ctx->qlogfd, data, datalen);
     if(rc == -1) {
       /* on write error, stop further write attempts */
-      close(ctx->qlogfd);
+      curlx_close(ctx->qlogfd);
       ctx->qlogfd = -1;
     }
   }
@@ -2140,7 +2141,7 @@ static void cf_ngtcp2_ctx_close(struct cf_ngtcp2_ctx *ctx)
   if(!ctx->initialized)
     return;
   if(ctx->qlogfd != -1) {
-    close(ctx->qlogfd);
+    curlx_close(ctx->qlogfd);
   }
   ctx->qlogfd = -1;
   Curl_vquic_tls_cleanup(&ctx->tls);
diff --git a/scripts/checksrc.pl b/scripts/checksrc.pl
index d065b0bc064b..2fd505b495a9 100755
--- a/scripts/checksrc.pl
+++ b/scripts/checksrc.pl
@@ -70,6 +70,7 @@
     "atoi" => 1,
     "atol" => 1,
     "calloc" => 1,
+    "close" => 1,
     "CreateFile" => 1,
     "CreateFileA" => 1,
     "CreateFileW" => 1,
diff --git a/src/tool_cb_wrt.c b/src/tool_cb_wrt.c
index 17c7d80b0f3d..cc005844af06 100644
--- a/src/tool_cb_wrt.c
+++ b/src/tool_cb_wrt.c
@@ -89,7 +89,7 @@ bool tool_create_output_file(struct OutStruct *outs,
     if(fd != -1) {
       file = curlx_fdopen(fd, "wb");
       if(!file)
-        close(fd);
+        curlx_close(fd);
     }
   }
 
diff --git a/src/tool_dirhie.c b/src/tool_dirhie.c
index 945d8796089d..3f5c987e9547 100644
--- a/src/tool_dirhie.c
+++ b/src/tool_dirhie.c
@@ -23,14 +23,13 @@
  ***************************************************************************/
 #include "tool_setup.h"
 
-#ifdef _WIN32
-#include <direct.h>
-#endif
-
 #include "tool_dirhie.h"
 #include "tool_msgs.h"
 
-#if defined(_WIN32) || (defined(MSDOS) && !defined(__DJGPP__))
+#ifdef _WIN32
+#  include <direct.h>
+#  define mkdir(x, y) _mkdir(x)
+#elif defined(MSDOS) && !defined(__DJGPP__)
 #  define mkdir(x, y) (mkdir)(x)
 #endif
 
diff --git a/src/tool_findfile.c b/src/tool_findfile.c
index 1fcd74f5a194..2745a2eb3ccf 100644
--- a/src/tool_findfile.c
+++ b/src/tool_findfile.c
@@ -74,7 +74,7 @@ static char *checkhome(const char *home, const char *fname, bool dotscore)
       int fd = curlx_open(c, O_RDONLY);
       if(fd >= 0) {
         char *path = curlx_strdup(c);
-        close(fd);
+        curlx_close(fd);
         curl_free(c);
         return path;
       }
diff --git a/src/tool_getpass.c b/src/tool_getpass.c
index a8c7cb243966..99ded8f4c5b5 100644
--- a/src/tool_getpass.c
+++ b/src/tool_getpass.c
@@ -187,7 +187,7 @@ char *getpass_r(const char *prompt, /* prompt to display */
   }
 
   if(STDIN_FILENO != fd)
-    close(fd);
+    curlx_close(fd);
 
   return password; /* return pointer to buffer */
 }
diff --git a/src/tool_operate.c b/src/tool_operate.c
index 13ebe83feea7..1cddf47b00bb 100644
--- a/src/tool_operate.c
+++ b/src/tool_operate.c
@@ -289,7 +289,7 @@ static CURLcode pre_transfer(struct per_transfer *per)
     {
       helpf("cannot open '%s'", per->uploadfile);
       if(per->infd != -1) {
-        close(per->infd);
+        curlx_close(per->infd);
         per->infd = STDIN_FILENO;
       }
       return CURLE_READ_ERROR;
@@ -621,7 +621,7 @@ static CURLcode post_per_transfer(struct per_transfer *per,
   else
 #endif
     if(per->infdopen)
-      close(per->infd);
+      curlx_close(per->infd);
 
   if(per->skip)
     goto skip;
diff --git a/tests/libtest/lib518.c b/tests/libtest/lib518.c
index 8431ed87d734..d405c6dfa058 100644
--- a/tests/libtest/lib518.c
+++ b/tests/libtest/lib518.c
@@ -63,7 +63,7 @@ static void t518_close_file_descriptors(void)
       t518_num_open.rlim_cur < t518_num_open.rlim_max;
       t518_num_open.rlim_cur++)
     if(t518_testfd[t518_num_open.rlim_cur] > 0)
-      close(t518_testfd[t518_num_open.rlim_cur]);
+      curlx_close(t518_testfd[t518_num_open.rlim_cur]);
   curlx_free(t518_testfd);
   t518_testfd = NULL;
 }
@@ -333,7 +333,7 @@ static int t518_test_rlimit(int keep_open)
       for(t518_num_open.rlim_cur = 0;
           t518_testfd[t518_num_open.rlim_cur] >= 0;
           t518_num_open.rlim_cur++)
-        close(t518_testfd[t518_num_open.rlim_cur]);
+        curlx_close(t518_testfd[t518_num_open.rlim_cur]);
       curlx_free(t518_testfd);
       t518_testfd = NULL;
       curlx_free(memchunk);
diff --git a/tests/libtest/lib537.c b/tests/libtest/lib537.c
index 436a9eb0a49b..1bf6fcba0678 100644
--- a/tests/libtest/lib537.c
+++ b/tests/libtest/lib537.c
@@ -60,7 +60,7 @@ static void t537_close_file_descriptors(void)
       t537_num_open.rlim_cur < t537_num_open.rlim_max;
       t537_num_open.rlim_cur++)
     if(t537_testfd[t537_num_open.rlim_cur] > 0)
-      close(t537_testfd[t537_num_open.rlim_cur]);
+      curlx_close(t537_testfd[t537_num_open.rlim_cur]);
   curlx_free(t537_testfd);
   t537_testfd = NULL;
 }
@@ -334,7 +334,7 @@ static int t537_test_rlimit(int keep_open)
       for(t537_num_open.rlim_cur = t537_num_open.rlim_max;
           t537_testfd[t537_num_open.rlim_cur] >= 0;
           t537_num_open.rlim_cur++) {
-        close(t537_testfd[t537_num_open.rlim_cur]);
+        curlx_close(t537_testfd[t537_num_open.rlim_cur]);
         t537_testfd[t537_num_open.rlim_cur] = -1;
       }
 
diff --git a/tests/libtest/lib568.c b/tests/libtest/lib568.c
index 6106ed8598b9..8c1e5dd7fa29 100644
--- a/tests/libtest/lib568.c
+++ b/tests/libtest/lib568.c
@@ -75,7 +75,7 @@ static CURLcode test_lib568(const char *URL)
     goto test_cleanup;
   }
   fstat(sdp, &file_info);
-  close(sdp);
+  curlx_close(sdp);
 
   sdpf = curlx_fopen(libtest_arg2, "rb");
   if(!sdpf) {
diff --git a/tests/libtest/lib572.c b/tests/libtest/lib572.c
index d1cb80479225..38edd77ecea5 100644
--- a/tests/libtest/lib572.c
+++ b/tests/libtest/lib572.c
@@ -93,7 +93,7 @@ static CURLcode test_lib572(const char *URL)
     goto test_cleanup;
   }
   fstat(params, &file_info);
-  close(params);
+  curlx_close(params);
 
   paramsf = curlx_fopen(libtest_arg2, "rb");
   if(!paramsf) {
diff --git a/tests/server/tftpd.c b/tests/server/tftpd.c
index e1b3a8b936fa..1e0fcc9a48d7 100644
--- a/tests/server/tftpd.c
+++ b/tests/server/tftpd.c
@@ -849,7 +849,7 @@ static void recvtftp(struct testcase *test, const struct formats *pf)
   write_behind(test, pf->f_convert);
   /* close the output file as early as possible after upload completion */
   if(test->ofile > 0) {
-    close(test->ofile);
+    curlx_close(test->ofile);
     test->ofile = 0;
   }
 
@@ -875,7 +875,7 @@ static void recvtftp(struct testcase *test, const struct formats *pf)
 abort:
   /* make sure the output file is closed in case of abort */
   if(test->ofile > 0) {
-    close(test->ofile);
+    curlx_close(test->ofile);
     test->ofile = 0;
   }
   return;
@@ -1305,7 +1305,7 @@ static int test_tftpd(int argc, char **argv)
 tftpd_cleanup:
 
   if(test.ofile > 0)
-    close(test.ofile);
+    curlx_close(test.ofile);
 
   if((peer != sock) && (peer != CURL_SOCKET_BAD))
     sclose(peer);
diff --git a/tests/server/util.c b/tests/server/util.c
index da959f3ed992..1cf62c0e762c 100644
--- a/tests/server/util.c
+++ b/tests/server/util.c
@@ -379,7 +379,7 @@ static void exit_signal_handler(int signum)
 #endif
       static const char msg[] = "exit_signal_handler: called\n";
       (void)!write(fd, msg, sizeof(msg) - 1);
-      close(fd);
+      curlx_close(fd);
     }
     else {
       static const char msg[] = "exit_signal_handler: failed opening ";