diff --git a/apps/gdaltindex_lib.cpp b/apps/gdaltindex_lib.cpp
index 68c15081642c..8bec93493ed2 100644
--- a/apps/gdaltindex_lib.cpp
+++ b/apps/gdaltindex_lib.cpp
@@ -147,7 +147,7 @@ static bool PatternMatch(const char *input, const char *pattern)
         }
         else
         {
-            if (tolower(*pattern) != tolower(*input))
+            if (CPLTolower(*pattern) != CPLTolower(*input))
             {
                 return false;
             }
diff --git a/frmts/netcdf/netcdfdataset.cpp b/frmts/netcdf/netcdfdataset.cpp
index b170b0e65823..68cf0c28cad4 100644
--- a/frmts/netcdf/netcdfdataset.cpp
+++ b/frmts/netcdf/netcdfdataset.cpp
@@ -3425,14 +3425,14 @@ void netCDFDataset::SetProjectionFromVar(
         for (unsigned int i = 0;
              i < strlen(poDS->papszDimName[poDS->nXDimID]) && i < 3; i++)
         {
-            szDimNameX[i] = (char)tolower(static_cast<unsigned char>(
+            szDimNameX[i] = (char)CPLTolower(static_cast<unsigned char>(
                 (poDS->papszDimName[poDS->nXDimID])[i]));
         }
         szDimNameX[3] = '\0';
         // for( unsigned int i = 0;
         //      (i < strlen(poDS->papszDimName[poDS->nYDimID])
         //                        && i < 3 ); i++ ) {
-        //    szDimNameY[i]=(char)tolower(static_cast<unsigned char>((poDS->papszDimName[poDS->nYDimID])[i]));
+        //    szDimNameY[i]=(char)CPLTolower(static_cast<unsigned char>((poDS->papszDimName[poDS->nYDimID])[i]));
         // }
         // szDimNameY[3] = '\0';
     }
diff --git a/frmts/pcidsk/sdk/core/pcidsk_utils.cpp b/frmts/pcidsk/sdk/core/pcidsk_utils.cpp
index 169c3891ff48..45c1dbd7f256 100644
--- a/frmts/pcidsk/sdk/core/pcidsk_utils.cpp
+++ b/frmts/pcidsk/sdk/core/pcidsk_utils.cpp
@@ -40,6 +40,12 @@
 #include <cstdarg>
 #include <iostream>
 
+extern "C"
+{
+int CPL_DLL CPLToupper(int c);
+int CPL_DLL CPLTolower(int c);
+}
+
 #if !defined(va_copy) && defined(__va_copy)
 #define va_copy __va_copy
 #endif
@@ -104,8 +110,7 @@ std::string &PCIDSK::UCaseStr( std::string &target )
 {
     for( unsigned int i = 0; i < target.size(); i++ )
     {
-        if( islower(static_cast<unsigned char>(target[i])) )
-            target[i] = (char) toupper(static_cast<unsigned char>(target[i]));
+        target[i] = (char) CPLToupper(static_cast<unsigned char>(target[i]));
     }
 
     return target;
@@ -409,10 +414,8 @@ int PCIDSK::pci_strcasecmp( const char *string1, const char *string2 )
         char c1 = string1[i];
         char c2 = string2[i];
 
-        if( islower(static_cast<unsigned char>(c1)) )
-            c1 = (char) toupper(static_cast<unsigned char>(c1));
-        if( islower(static_cast<unsigned char>(c2)) )
-            c2 = (char) toupper(static_cast<unsigned char>(c2));
+        c1 = (char) CPLToupper(static_cast<unsigned char>(c1));
+        c2 = (char) CPLToupper(static_cast<unsigned char>(c2));
 
         if( c1 < c2 )
             return -1;
@@ -447,10 +450,8 @@ int PCIDSK::pci_strncasecmp( const char *string1, const char *string2, size_t le
         char c1 = string1[i];
         char c2 = string2[i];
 
-        if( islower(static_cast<unsigned char>(c1)) )
-            c1 = (char) toupper(static_cast<unsigned char>(c1));
-        if( islower(static_cast<unsigned char>(c2)) )
-            c2 = (char) toupper(static_cast<unsigned char>(c2));
+        c1 = (char) CPLToupper(static_cast<unsigned char>(c1));
+        c2 = (char) CPLToupper(static_cast<unsigned char>(c2));
 
         if( c1 < c2 )
             return -1;
diff --git a/frmts/pds/pds4dataset.cpp b/frmts/pds/pds4dataset.cpp
index dd532d3676f4..9862a3ea01bb 100644
--- a/frmts/pds/pds4dataset.cpp
+++ b/frmts/pds/pds4dataset.cpp
@@ -1421,7 +1421,7 @@ static CPLString FixupTableFilename(const CPLString &osFilename)
     if (!osExt.empty())
     {
         CPLString osTry(osFilename);
-        if (islower(static_cast<unsigned char>(osExt[0])))
+        if (osExt[0] >= 'a' && osExt[0] <= 'z')
         {
             osTry = CPLResetExtension(osFilename, osExt.toupper());
         }
diff --git a/gcore/gdal_mdreader.cpp b/gcore/gdal_mdreader.cpp
index 2d78e4de842c..18684e492c9e 100644
--- a/gcore/gdal_mdreader.cpp
+++ b/gcore/gdal_mdreader.cpp
@@ -1031,7 +1031,7 @@ static bool GDAL_IMD_AA2R(char ***ppapszIMD)
             CPLString osLine;
             osTarget.Printf(
                 "IMAGE_1.%c%s",
-                tolower(static_cast<unsigned char>(keylist[iKey][0])),
+                CPLTolower(static_cast<unsigned char>(keylist[iKey][0])),
                 keylist[iKey] + 1);
 
             osLine = osTarget + "=" + osValue;
diff --git a/gcore/gdal_misc.cpp b/gcore/gdal_misc.cpp
index 0e9feaad5da8..a00be33fda88 100644
--- a/gcore/gdal_misc.cpp
+++ b/gcore/gdal_misc.cpp
@@ -2316,9 +2316,9 @@ int GDALReadWorldFile2(const char *pszBaseFilename, const char *pszExtension,
     for (int i = 0; szExtUpper[i] != '\0'; i++)
     {
         szExtUpper[i] = static_cast<char>(
-            toupper(static_cast<unsigned char>(szExtUpper[i])));
+            CPLToupper(static_cast<unsigned char>(szExtUpper[i])));
         szExtLower[i] = static_cast<char>(
-            tolower(static_cast<unsigned char>(szExtLower[i])));
+            CPLTolower(static_cast<unsigned char>(szExtLower[i])));
     }
 
     const char *pszTFW = CPLResetExtension(pszBaseFilename, szExtLower);
diff --git a/ogr/ogr_srs_pci.cpp b/ogr/ogr_srs_pci.cpp
index 089720c2dd00..67f650a61a81 100644
--- a/ogr/ogr_srs_pci.cpp
+++ b/ogr/ogr_srs_pci.cpp
@@ -236,7 +236,7 @@ OGRErr OGRSpatialReference::importFromPCI(const char *pszProj,
 
             if (nCode >= -99 && nCode <= 999)
                 snprintf(szEarthModel, sizeof(szEarthModel), "%c%03d",
-                         toupper(static_cast<unsigned char>(*pszEM)), nCode);
+                         CPLToupper(static_cast<unsigned char>(*pszEM)), nCode);
 
             break;
         }
diff --git a/ogr/ogrsf_frmts/avc/avc_binwr.cpp b/ogr/ogrsf_frmts/avc/avc_binwr.cpp
index 2e7828179d32..5febe68bc65a 100644
--- a/ogr/ogrsf_frmts/avc/avc_binwr.cpp
+++ b/ogr/ogrsf_frmts/avc/avc_binwr.cpp
@@ -1793,7 +1793,8 @@ AVCBinFile *AVCBinWriteCreateTable(const char *pszInfoPath,
         for (i = 0; *pszPtr != '\0' && *pszPtr != '.' && *pszPtr != ' ';
              i++, pszPtr++)
         {
-            szCoverName[i] = (char)tolower(static_cast<unsigned char>(*pszPtr));
+            szCoverName[i] =
+                (char)CPLTolower(static_cast<unsigned char>(*pszPtr));
         }
         szCoverName[i] = '\0';
 
@@ -1802,13 +1803,14 @@ AVCBinFile *AVCBinWriteCreateTable(const char *pszInfoPath,
 
         for (i = 0; i < 3 && *pszPtr != '\0' && *pszPtr != ' '; i++, pszPtr++)
         {
-            szExt[i] = (char)tolower(static_cast<unsigned char>(*pszPtr));
+            szExt[i] = (char)CPLTolower(static_cast<unsigned char>(*pszPtr));
         }
         szExt[i] = '\0';
 
         for (i = 0; *pszPtr != '\0' && *pszPtr != ' '; i++, pszPtr++)
         {
-            szSubclass[i] = (char)tolower(static_cast<unsigned char>(*pszPtr));
+            szSubclass[i] =
+                (char)CPLTolower(static_cast<unsigned char>(*pszPtr));
         }
         szSubclass[i] = '\0';
 
@@ -1980,7 +1982,7 @@ AVCBinFile *_AVCBinWriteCreateDBFTable(const char *pszPath,
          i++, pszDBFBasename++)
     {
         psFile->pszFilename[i] =
-            (char)tolower(static_cast<unsigned char>(*pszDBFBasename));
+            (char)CPLTolower(static_cast<unsigned char>(*pszDBFBasename));
     }
 
     strcat(psFile->pszFilename, ".dbf");
diff --git a/ogr/ogrsf_frmts/avc/avc_e00gen.cpp b/ogr/ogrsf_frmts/avc/avc_e00gen.cpp
index d0b848c1dd48..2d1b49d92064 100644
--- a/ogr/ogrsf_frmts/avc/avc_e00gen.cpp
+++ b/ogr/ogrsf_frmts/avc/avc_e00gen.cpp
@@ -169,7 +169,7 @@ const char *AVCE00GenStartSection(AVCE00GenInfo *psInfo, AVCFileType eType,
         for (i = 0; pszClassName[i] != '\0'; i++)
         {
             psInfo->pszBuf[i] =
-                (char)toupper(static_cast<unsigned char>(pszClassName[i]));
+                (char)CPLToupper(static_cast<unsigned char>(pszClassName[i]));
         }
         psInfo->pszBuf[i] = '\0';
     }
diff --git a/ogr/ogrsf_frmts/avc/avc_e00read.cpp b/ogr/ogrsf_frmts/avc/avc_e00read.cpp
index d397573412c8..255ac4ba05c6 100644
--- a/ogr/ogrsf_frmts/avc/avc_e00read.cpp
+++ b/ogr/ogrsf_frmts/avc/avc_e00read.cpp
@@ -1008,7 +1008,7 @@ static int _AVCE00ReadBuildSqueleton(AVCE00ReadPtr psInfo, char **papszCoverDir)
         CPLSPrintf("EXP  0 %s%s.E00", szCWD, osCoverPathTruncated.c_str()));
     pcTmp = pszEXPPath;
     for (; *pcTmp != '\0'; pcTmp++)
-        *pcTmp = (char)toupper(static_cast<unsigned char>(*pcTmp));
+        *pcTmp = (char)CPLToupper(static_cast<unsigned char>(*pcTmp));
 
     /*-----------------------------------------------------------------
      * EXP Header
@@ -1309,7 +1309,8 @@ static int _AVCE00ReadBuildSqueleton(AVCE00ReadPtr psInfo, char **papszCoverDir)
                                      papszCoverDir[iFile]);
                 pcTmp = (char *)szFname;
                 for (; *pcTmp != '\0'; pcTmp++)
-                    *pcTmp = (char)toupper(static_cast<unsigned char>(*pcTmp));
+                    *pcTmp =
+                        (char)CPLToupper(static_cast<unsigned char>(*pcTmp));
                 papszCoverDir[iFile][nLen - 4] = '.';
 
                 papszTables = CSLAddString(papszTables, szFname);
diff --git a/ogr/ogrsf_frmts/avc/avc_e00write.cpp b/ogr/ogrsf_frmts/avc/avc_e00write.cpp
index 789feddfa9fb..67f5578cc3a4 100644
--- a/ogr/ogrsf_frmts/avc/avc_e00write.cpp
+++ b/ogr/ogrsf_frmts/avc/avc_e00write.cpp
@@ -480,7 +480,8 @@ static void _AVCE00WriteRenameTable(AVCTableDef *psTableDef,
 
     snprintf(szNewName, sizeof(szNewName), "%s", pszNewCoverName);
     for (i = 0; szNewName[i] != '\0'; i++)
-        szNewName[i] = (char)toupper(static_cast<unsigned char>(szNewName[i]));
+        szNewName[i] =
+            (char)CPLToupper(static_cast<unsigned char>(szNewName[i]));
 
     /*-----------------------------------------------------------------
      * Extract components from the current table name.
@@ -662,7 +663,7 @@ static int _AVCE00WriteCreateCoverFile(AVCE00WritePtr psInfo, AVCFileType eType,
      * Make sure filename is all lowercase and attempt to create the file
      *----------------------------------------------------------------*/
     for (i = 0; szFname[i] != '\0'; i++)
-        szFname[i] = (char)tolower(static_cast<unsigned char>(szFname[i]));
+        szFname[i] = (char)CPLTolower(static_cast<unsigned char>(szFname[i]));
 
     if (nStatus == 0)
     {
@@ -961,8 +962,8 @@ int AVCE00DeleteCoverage(const char *pszCoverToDelete)
         {
             /* Convert table filename to lowercases */
             for (j = 0; papszFiles[i] && papszFiles[i][j] != '\0'; j++)
-                papszFiles[i][j] =
-                    (char)tolower(static_cast<unsigned char>(papszFiles[i][j]));
+                papszFiles[i][j] = (char)CPLTolower(
+                    static_cast<unsigned char>(papszFiles[i][j]));
 
             /* Delete the .DAT file */
             pszFname = CPLSPrintf("%s%s.dat", pszInfoPath, papszFiles[i]);
diff --git a/ogr/ogrsf_frmts/dxf/ogr_autocad_services.cpp b/ogr/ogrsf_frmts/dxf/ogr_autocad_services.cpp
index cf19f856544e..7e37fbf2b9ec 100644
--- a/ogr/ogrsf_frmts/dxf/ogr_autocad_services.cpp
+++ b/ogr/ogrsf_frmts/dxf/ogr_autocad_services.cpp
@@ -67,7 +67,7 @@ CPLString ACTextUnescape(const char *pszRawInput, const char *pszEncoding,
             if (pszInput[1] == ' ')
                 osResult += '^';
             else
-                osResult += static_cast<char>(toupper(
+                osResult += static_cast<char>(CPLToupper(
                                 static_cast<unsigned char>(pszInput[1]))) ^
                             0x40;
             pszInput++;
diff --git a/ogr/ogrsf_frmts/edigeo/ogredigeodatasource.cpp b/ogr/ogrsf_frmts/edigeo/ogredigeodatasource.cpp
index aeda2eb0fd9a..d966bd15f764 100644
--- a/ogr/ogrsf_frmts/edigeo/ogredigeodatasource.cpp
+++ b/ogr/ogrsf_frmts/edigeo/ogredigeodatasource.cpp
@@ -188,10 +188,8 @@ VSILFILE *OGREDIGEODataSource::OpenFile(const char *pszType,
     VSILFILE *fp = VSIFOpenL(osFilename, "rb");
     if (fp == nullptr)
     {
-        CPLString osExtLower = osExt;
-        for (int i = 0; i < (int)osExt.size(); i++)
-            osExtLower[i] = (char)tolower(static_cast<unsigned char>(osExt[i]));
-        CPLString osFilename2 = CPLFormCIFilename(
+        const CPLString osExtLower = CPLString(osExt).tolower();
+        const CPLString osFilename2 = CPLFormCIFilename(
             CPLGetPath(pszName), osTmp.c_str(), osExtLower.c_str());
         fp = VSIFOpenL(osFilename2, "rb");
         if (fp == nullptr)
diff --git a/ogr/ogrsf_frmts/elastic/ogrelasticlayer.cpp b/ogr/ogrsf_frmts/elastic/ogrelasticlayer.cpp
index bb60c85390dc..ebfbdeea7799 100644
--- a/ogr/ogrsf_frmts/elastic/ogrelasticlayer.cpp
+++ b/ogr/ogrsf_frmts/elastic/ogrelasticlayer.cpp
@@ -1415,7 +1415,8 @@ static void decode_geohash_bbox(const char *geohash, double lat[2],
     hashlen = static_cast<int>(strlen(geohash));
     for (i = 0; i < hashlen; i++)
     {
-        c = static_cast<char>(tolower(static_cast<unsigned char>(geohash[i])));
+        c = static_cast<char>(
+            CPLTolower(static_cast<unsigned char>(geohash[i])));
         cd = static_cast<char>(strchr(BASE32, c) - BASE32);
         for (j = 0; j < 5; j++)
         {
diff --git a/ogr/ogrsf_frmts/gmlas/ogrgmlasutils.cpp b/ogr/ogrsf_frmts/gmlas/ogrgmlasutils.cpp
index 82f0de613598..910b98501417 100644
--- a/ogr/ogrsf_frmts/gmlas/ogrgmlasutils.cpp
+++ b/ogr/ogrsf_frmts/gmlas/ogrgmlasutils.cpp
@@ -52,8 +52,7 @@ CPLString OGRGMLASTruncateIdentifier(const CPLString &osName,
         const char *pszToken = papszTokens[j];
         bool bIsCamelCase = false;
         // Split parts like camelCase or CamelCase into several tokens
-        if (pszToken[0] != '\0' &&
-            islower(static_cast<unsigned char>(pszToken[1])))
+        if (pszToken[0] != '\0' && pszToken[1] >= 'a' && pszToken[1] <= 'z')
         {
             bIsCamelCase = true;
             bool bLastIsLower = true;
@@ -63,7 +62,7 @@ CPLString OGRGMLASTruncateIdentifier(const CPLString &osName,
             osCurrentPart += pszToken[1];
             for (int k = 2; pszToken[k]; ++k)
             {
-                if (isupper(static_cast<unsigned char>(pszToken[k])))
+                if (pszToken[k] >= 'A' && pszToken[k] <= 'Z')
                 {
                     if (!bLastIsLower)
                     {
diff --git a/ogr/ogrsf_frmts/hana/ogrhanadatasource.cpp b/ogr/ogrsf_frmts/hana/ogrhanadatasource.cpp
index 8527bbebfc34..bb1c698eaefe 100644
--- a/ogr/ogrsf_frmts/hana/ogrhanadatasource.cpp
+++ b/ogr/ogrsf_frmts/hana/ogrhanadatasource.cpp
@@ -367,8 +367,8 @@ GetGeometryColumnDescription(odbc::Connection &conn, const CPLString &query,
             columnName.end(),
             [](char ch1, char ch2)
             {
-                return std::toupper(static_cast<unsigned char>(ch1)) ==
-                       std::toupper(static_cast<unsigned char>(ch2));
+                return CPLToupper(static_cast<unsigned char>(ch1)) ==
+                       CPLToupper(static_cast<unsigned char>(ch2));
             });
 
         if (it != preparedQuery.end())
@@ -1285,8 +1285,8 @@ std::pair<OGRErr, CPLString> OGRHanaDataSource::LaunderName(const char *name)
             if (c == '-' || c == '#')
                 newName[i] = '_';
             else
-                newName[i] =
-                    static_cast<char>(toupper(static_cast<unsigned char>(c)));
+                newName[i] = static_cast<char>(
+                    CPLToupper(static_cast<unsigned char>(c)));
         }
         else
         {
diff --git a/ogr/ogrsf_frmts/ili/ili2reader.cpp b/ogr/ogrsf_frmts/ili/ili2reader.cpp
index 0b343b9ebb8d..577c06217572 100644
--- a/ogr/ogrsf_frmts/ili/ili2reader.cpp
+++ b/ogr/ogrsf_frmts/ili/ili2reader.cpp
@@ -72,10 +72,10 @@ int cmpStr(const string &s1, const string &s2)
 
     while (p1 != s1.end() && p2 != s2.end())
     {
-        if (toupper(static_cast<unsigned char>(*p1)) !=
-            toupper(static_cast<unsigned char>(*p2)))
-            return (toupper(static_cast<unsigned char>(*p1)) <
-                    toupper(static_cast<unsigned char>(*p2)))
+        if (CPLToupper(static_cast<unsigned char>(*p1)) !=
+            CPLToupper(static_cast<unsigned char>(*p2)))
+            return (CPLToupper(static_cast<unsigned char>(*p1)) <
+                    CPLToupper(static_cast<unsigned char>(*p2)))
                        ? -1
                        : 1;
         ++p1;
diff --git a/ogr/ogrsf_frmts/mitab/mitab_feature.cpp b/ogr/ogrsf_frmts/mitab/mitab_feature.cpp
index b79679cff706..4145fb265a39 100644
--- a/ogr/ogrsf_frmts/mitab/mitab_feature.cpp
+++ b/ogr/ogrsf_frmts/mitab/mitab_feature.cpp
@@ -1820,7 +1820,7 @@ const char *TABCustomPoint::GetSymbolStyleString(double dfAngle) const
     for (i = 0; i < 7 && *pszPtr != '\0' && *pszPtr != ' '; i++, pszPtr++)
     {
         szLowerExt[i] =
-            static_cast<char>(tolower(static_cast<unsigned char>(*pszPtr)));
+            static_cast<char>(CPLTolower(static_cast<unsigned char>(*pszPtr)));
     }
     szLowerExt[i] = '\0';
 
@@ -6314,7 +6314,7 @@ const char *TABText::GetLabelStyleString() const
         for (int i = 0; pszTextString[i]; ++i)
             if (isalpha(static_cast<unsigned char>(pszTextString[i])))
                 pszTextString[i] = static_cast<char>(
-                    toupper(static_cast<unsigned char>(pszTextString[i])));
+                    CPLToupper(static_cast<unsigned char>(pszTextString[i])));
 
     /* Escape the double quote chars and expand the text */
     char *pszTmpTextString = nullptr;
diff --git a/ogr/ogrsf_frmts/mitab/mitab_indfile.cpp b/ogr/ogrsf_frmts/mitab/mitab_indfile.cpp
index 165c08f31185..b270897639b6 100644
--- a/ogr/ogrsf_frmts/mitab/mitab_indfile.cpp
+++ b/ogr/ogrsf_frmts/mitab/mitab_indfile.cpp
@@ -599,8 +599,8 @@ GByte *TABINDFile::BuildKey(int nIndexNumber, const char *pszStr)
     int i = 0;
     for (i = 0; i < nKeyLength && pszStr[i] != '\0'; i++)
     {
-        m_papbyKeyBuffers[nIndexNumber - 1][i] =
-            static_cast<GByte>(toupper(static_cast<unsigned char>(pszStr[i])));
+        m_papbyKeyBuffers[nIndexNumber - 1][i] = static_cast<GByte>(
+            CPLToupper(static_cast<unsigned char>(pszStr[i])));
     }
 
     /* Pad the end of the buffer with '\0' */
diff --git a/ogr/ogrsf_frmts/mitab/mitab_utils.cpp b/ogr/ogrsf_frmts/mitab/mitab_utils.cpp
index 3d9d62f80b53..ffb2d3ba70ba 100644
--- a/ogr/ogrsf_frmts/mitab/mitab_utils.cpp
+++ b/ogr/ogrsf_frmts/mitab/mitab_utils.cpp
@@ -240,8 +240,8 @@ GBool TABAdjustFilenameExtension(char *pszFname)
     for (int i = static_cast<int>(strlen(pszFname)) - 1;
          i >= 0 && pszFname[i] != '.'; i--)
     {
-        pszFname[i] =
-            static_cast<char>(toupper(static_cast<unsigned char>(pszFname[i])));
+        pszFname[i] = static_cast<char>(
+            CPLToupper(static_cast<unsigned char>(pszFname[i])));
     }
 
     if (VSIStatL(pszFname, &sStatBuf) == 0)
@@ -253,8 +253,8 @@ GBool TABAdjustFilenameExtension(char *pszFname)
     for (int i = static_cast<int>(strlen(pszFname)) - 1;
          i >= 0 && pszFname[i] != '.'; i--)
     {
-        pszFname[i] =
-            static_cast<char>(tolower(static_cast<unsigned char>(pszFname[i])));
+        pszFname[i] = static_cast<char>(
+            CPLTolower(static_cast<unsigned char>(pszFname[i])));
     }
 
     if (VSIStatL(pszFname, &sStatBuf) == 0)
diff --git a/ogr/ogrsf_frmts/mssqlspatial/ogrmssqlspatialdatasource.cpp b/ogr/ogrsf_frmts/mssqlspatial/ogrmssqlspatialdatasource.cpp
index 4e64615e77ad..215080cbc260 100644
--- a/ogr/ogrsf_frmts/mssqlspatial/ogrmssqlspatialdatasource.cpp
+++ b/ogr/ogrsf_frmts/mssqlspatial/ogrmssqlspatialdatasource.cpp
@@ -1357,7 +1357,7 @@ char *OGRMSSQLSpatialDataSource::LaunderName(const char *pszSrcName)
     for (i = 0; pszSafeName[i] != '\0'; i++)
     {
         pszSafeName[i] =
-            (char)tolower(static_cast<unsigned char>(pszSafeName[i]));
+            (char)CPLTolower(static_cast<unsigned char>(pszSafeName[i]));
         if (pszSafeName[i] == '-' || pszSafeName[i] == '#')
             pszSafeName[i] = '_';
     }
diff --git a/ogr/ogrsf_frmts/mysql/ogrmysqldatasource.cpp b/ogr/ogrsf_frmts/mysql/ogrmysqldatasource.cpp
index 09636fb357d3..041a777f739a 100644
--- a/ogr/ogrsf_frmts/mysql/ogrmysqldatasource.cpp
+++ b/ogr/ogrsf_frmts/mysql/ogrmysqldatasource.cpp
@@ -1068,7 +1068,7 @@ char *OGRMySQLDataSource::LaunderName(const char *pszSrcName)
     for (int i = 0; pszSafeName[i] != '\0'; i++)
     {
         pszSafeName[i] =
-            (char)tolower(static_cast<unsigned char>(pszSafeName[i]));
+            (char)CPLTolower(static_cast<unsigned char>(pszSafeName[i]));
         if (pszSafeName[i] == '-' || pszSafeName[i] == '#')
             pszSafeName[i] = '_';
     }
diff --git a/ogr/ogrsf_frmts/oci/ogrocisession.cpp b/ogr/ogrsf_frmts/oci/ogrocisession.cpp
index 8de280a93c90..26ec7fb4b755 100644
--- a/ogr/ogrsf_frmts/oci/ogrocisession.cpp
+++ b/ogr/ogrsf_frmts/oci/ogrocisession.cpp
@@ -560,8 +560,8 @@ void OGROCISession::CleanName(char *pszName)
 
     for (i = 0; pszName[i] != '\0'; i++)
     {
-        pszName[i] =
-            static_cast<char>(toupper(static_cast<unsigned char>(pszName[i])));
+        pszName[i] = static_cast<char>(
+            CPLToupper(static_cast<unsigned char>(pszName[i])));
 
         if ((pszName[i] < '0' || pszName[i] > '9') &&
             (pszName[i] < 'A' || pszName[i] > 'Z') && pszName[i] != '_')
diff --git a/ogr/ogrsf_frmts/pgdump/ogrpgdumpdatasource.cpp b/ogr/ogrsf_frmts/pgdump/ogrpgdumpdatasource.cpp
index f4f82e6adca1..b4edaafd5756 100644
--- a/ogr/ogrsf_frmts/pgdump/ogrpgdumpdatasource.cpp
+++ b/ogr/ogrsf_frmts/pgdump/ogrpgdumpdatasource.cpp
@@ -140,7 +140,7 @@ char *OGRPGCommonLaunderName(const char *pszSrcName, const char *pszDebugPrefix,
         if (static_cast<unsigned char>(pszSafeName[i]) <= 127)
         {
             pszSafeName[i] =
-                (char)tolower(static_cast<unsigned char>(pszSafeName[i]));
+                (char)CPLTolower(static_cast<unsigned char>(pszSafeName[i]));
             if (pszSafeName[i] == '\'' || pszSafeName[i] == '-' ||
                 pszSafeName[i] == '#')
             {
diff --git a/ogr/ogrsf_frmts/sqlite/ogrsqlitedatasource.cpp b/ogr/ogrsf_frmts/sqlite/ogrsqlitedatasource.cpp
index ae4584f3bb11..f936fa05ebfb 100644
--- a/ogr/ogrsf_frmts/sqlite/ogrsqlitedatasource.cpp
+++ b/ogr/ogrsf_frmts/sqlite/ogrsqlitedatasource.cpp
@@ -3746,7 +3746,7 @@ char *OGRSQLiteDataSource::LaunderName(const char *pszSrcName)
     for (int i = 0; pszSafeName[i] != '\0'; i++)
     {
         pszSafeName[i] =
-            (char)tolower(static_cast<unsigned char>(pszSafeName[i]));
+            (char)CPLTolower(static_cast<unsigned char>(pszSafeName[i]));
         if (pszSafeName[i] == '\'' || pszSafeName[i] == '-' ||
             pszSafeName[i] == '#')
             pszSafeName[i] = '_';
diff --git a/ogr/swq_op_general.cpp b/ogr/swq_op_general.cpp
index b8e250ed6f54..89e3bb4209fa 100644
--- a/ogr/swq_op_general.cpp
+++ b/ogr/swq_op_general.cpp
@@ -157,8 +157,8 @@ int swq_test_like(const char *input, const char *pattern, char chEscape,
             }
             else if ((!insensitive && *pattern != *input) ||
                      (insensitive &&
-                      tolower(static_cast<unsigned char>(*pattern)) !=
-                          tolower(static_cast<unsigned char>(*input))))
+                      CPLTolower(static_cast<unsigned char>(*pattern)) !=
+                          CPLTolower(static_cast<unsigned char>(*input))))
             {
                 return 0;
             }
diff --git a/port/cpl_conv.cpp b/port/cpl_conv.cpp
index 6779baf4a79c..afcfd3eac95e 100644
--- a/port/cpl_conv.cpp
+++ b/port/cpl_conv.cpp
@@ -327,7 +327,7 @@ char *CPLStrdup(const char *pszString)
  * Convert each characters of the string to lower case.
  *
  * For example, "ABcdE" will be converted to "abcde".
- * This function is locale dependent.
+ * Starting with GDAL 3.9, this function is no longer locale dependent.
  *
  * @param pszString input string to be converted.
  * @return pointer to the same string, pszString.
@@ -344,7 +344,7 @@ char *CPLStrlwr(char *pszString)
     while (*pszTemp)
     {
         *pszTemp =
-            static_cast<char>(tolower(static_cast<unsigned char>(*pszTemp)));
+            static_cast<char>(CPLTolower(static_cast<unsigned char>(*pszTemp)));
         pszTemp++;
     }
 
diff --git a/port/cpl_path.cpp b/port/cpl_path.cpp
index ec5efeaca505..f78c77453bef 100644
--- a/port/cpl_path.cpp
+++ b/port/cpl_path.cpp
@@ -654,8 +654,7 @@ const char *CPLFormCIFilename(const char *pszPath, const char *pszBasename,
     {
         for (size_t i = 0; pszFilename[i] != '\0'; i++)
         {
-            if (islower(static_cast<unsigned char>(pszFilename[i])))
-                pszFilename[i] = static_cast<char>(toupper(pszFilename[i]));
+            pszFilename[i] = static_cast<char>(CPLToupper(pszFilename[i]));
         }
 
         pszFullPath = CPLFormFilename(pszPath, pszFilename, nullptr);
@@ -666,9 +665,8 @@ const char *CPLFormCIFilename(const char *pszPath, const char *pszBasename,
     {
         for (size_t i = 0; pszFilename[i] != '\0'; i++)
         {
-            if (isupper(static_cast<unsigned char>(pszFilename[i])))
-                pszFilename[i] = static_cast<char>(
-                    tolower(static_cast<unsigned char>(pszFilename[i])));
+            pszFilename[i] = static_cast<char>(
+                CPLTolower(static_cast<unsigned char>(pszFilename[i])));
         }
 
         pszFullPath = CPLFormFilename(pszPath, pszFilename, nullptr);
diff --git a/port/cplstring.cpp b/port/cplstring.cpp
index 4928892ce86b..17bfe4174759 100644
--- a/port/cplstring.cpp
+++ b/port/cplstring.cpp
@@ -278,7 +278,7 @@ size_t CPLString::ifind(const char *s, size_t nPos) const
 {
     const char *pszHaystack = c_str();
     const char chFirst =
-        static_cast<char>(::tolower(static_cast<unsigned char>(s[0])));
+        static_cast<char>(CPLTolower(static_cast<unsigned char>(s[0])));
     const size_t nTargetLen = strlen(s);
 
     if (nPos > size())
@@ -288,7 +288,7 @@ size_t CPLString::ifind(const char *s, size_t nPos) const
 
     while (*pszHaystack != '\0')
     {
-        if (chFirst == ::tolower(static_cast<unsigned char>(*pszHaystack)))
+        if (chFirst == CPLTolower(static_cast<unsigned char>(*pszHaystack)))
         {
             if (EQUALN(pszHaystack, s, nTargetLen))
                 return nPos;
@@ -313,7 +313,7 @@ CPLString &CPLString::toupper()
 
 {
     for (size_t i = 0; i < size(); i++)
-        (*this)[i] = static_cast<char>(::toupper((*this)[i]));
+        (*this)[i] = static_cast<char>(CPLToupper((*this)[i]));
 
     return *this;
 }
@@ -330,8 +330,7 @@ CPLString &CPLString::tolower()
 
 {
     for (size_t i = 0; i < size(); i++)
-        (*this)[i] = static_cast<char>(
-            ::tolower(static_cast<unsigned char>((*this)[i])));
+        (*this)[i] = static_cast<char>(CPLTolower((*this)[i]));
 
     return *this;
 }