diff --git a/hist/hbook/src/THbookFile.cxx b/hist/hbook/src/THbookFile.cxx
index fa86b23cc900d..d7fa9cc0a3e3c 100644
--- a/hist/hbook/src/THbookFile.cxx
+++ b/hist/hbook/src/THbookFile.cxx
@@ -137,6 +137,11 @@ static Int_t gLastEntry = -1;
 # define type_of_call
 # define DEFCHAR  const char*
 # define PASSCHAR(string) string
+
+// As recommended in
+// https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html
+using fortran_charlen_t = size_t;
+
 #else
 # define hlimit  HLIMIT
 # define hldir   HLDIR
@@ -177,8 +182,8 @@ static Int_t gLastEntry = -1;
 extern "C" void  type_of_call hlimit(const int&);
 #ifndef WIN32
 extern "C" void  type_of_call hropen(const int&,DEFCHAR,DEFCHAR,DEFCHAR,
-                        const int&,const int&,const int,const int,const int);
-extern "C" void  type_of_call hrend(DEFCHAR,const int);
+                        const int&,const int&,fortran_charlen_t,fortran_charlen_t,fortran_charlen_t);
+extern "C" void  type_of_call hrend(DEFCHAR,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hropen(const int&,DEFCHAR,DEFCHAR,DEFCHAR,
                         const int&,const int&);
@@ -189,7 +194,7 @@ extern "C" void  type_of_call hrin(const int&,const int&,const int&);
 extern "C" void  type_of_call hnoent(const int&,const int&);
 #ifndef WIN32
 extern "C" void  type_of_call hgive(const int&,DEFCHAR,const int&,const float&,const float&,
-   const int&,const float&,const float&,const int&,const int&,const int);
+   const int&,const float&,const float&,const int&,const int&,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hgive(const int&,DEFCHAR,const int&,const float&,const float&,
    const int&,const float&,const float&,const int&,const int&);
@@ -198,8 +203,8 @@ extern "C" void  type_of_call hgive(const int&,DEFCHAR,const int&,const float&,c
   //SUBROUTINE HGNT1(IDD,BLKNA1,VAR,IOFFST,NVAR,IDNEVT,IERROR)
 #ifndef WIN32
 extern "C" void  type_of_call hgiven(const int&,DEFCHAR,const int&,DEFCHAR,
-   const float&,const float&,const int,const int);
-extern "C" void  type_of_call hgnt1(const int&,DEFCHAR,DEFCHAR,const int&,const int&,const int&,const int&,const int,const int);
+   const float&,const float&,fortran_charlen_t,fortran_charlen_t);
+extern "C" void  type_of_call hgnt1(const int&,DEFCHAR,DEFCHAR,const int&,const int&,const int&,const int&,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hgiven(const int&,DEFCHAR,const int&,DEFCHAR,
    const float&,const float&);
@@ -207,15 +212,15 @@ extern "C" void  type_of_call hgnt1(const int&,DEFCHAR,DEFCHAR,const int&,const
 #endif
 
 #ifndef WIN32
-extern "C" void  type_of_call hntvar2(const int&,const int&,DEFCHAR,DEFCHAR,DEFCHAR,int&,int&,int&,int&,int&,const int,const int, const int);
-extern "C" void  type_of_call hntvar3(const int&,const int&,DEFCHAR, const int);
+extern "C" void  type_of_call hntvar2(const int&,const int&,DEFCHAR,DEFCHAR,DEFCHAR,int&,int&,int&,int&,int&,fortran_charlen_t,fortran_charlen_t, fortran_charlen_t);
+extern "C" void  type_of_call hntvar3(const int&,const int&,DEFCHAR, fortran_charlen_t);
 #else
 extern "C" void  type_of_call hntvar2(const int&,const int&,DEFCHAR,DEFCHAR,DEFCHAR,int&,int&,int&,int&,int&);
 extern "C" void  type_of_call hntvar3(const int&,const int&,DEFCHAR);
 #endif
 
 #ifndef WIN32
-extern "C" void  type_of_call hbnam(const int&,DEFCHAR,const int&,DEFCHAR,const int&,const int, const int);
+extern "C" void  type_of_call hbnam(const int&,DEFCHAR,const int&,DEFCHAR,const int&,fortran_charlen_t, fortran_charlen_t);
 #else
 extern "C" void  type_of_call hbnam(const int&,DEFCHAR,const int&,DEFCHAR,const int&);
 #endif
@@ -237,8 +242,8 @@ extern "C" void  type_of_call hix(const int&,const int&,const float&);
 extern "C" void  type_of_call hijxy(const int&,const int&,const int&,const float&,const float&);
 extern "C" float type_of_call hije(const int&,const int&,const int&);
 #ifndef WIN32
-extern "C" void  type_of_call hcdir(DEFCHAR,DEFCHAR ,const int,const int);
-extern "C" void  type_of_call hldir(DEFCHAR,DEFCHAR ,const int,const int);
+extern "C" void  type_of_call hcdir(DEFCHAR,DEFCHAR ,fortran_charlen_t,fortran_charlen_t);
+extern "C" void  type_of_call hldir(DEFCHAR,DEFCHAR ,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hcdir(DEFCHAR,DEFCHAR);
 extern "C" void  type_of_call hldir(DEFCHAR,DEFCHAR);
diff --git a/main/src/h2root.cxx b/main/src/h2root.cxx
index b374c16c2ffaf..c0ca8d5f0db44 100644
--- a/main/src/h2root.cxx
+++ b/main/src/h2root.cxx
@@ -131,6 +131,11 @@ void MAIN__() {}
 # define type_of_call
 # define DEFCHAR  const char*
 # define PASSCHAR(string) string
+
+// As recommended in
+// https://gcc.gnu.org/onlinedocs/gfortran/Argument-passing-conventions.html
+using fortran_charlen_t = size_t;
+
 #else
 # define hlimit  HLIMIT
 # define hropen  HROPEN
@@ -169,7 +174,7 @@ void MAIN__() {}
 extern "C" void  type_of_call hlimit(const int&);
 #ifndef WIN32
 extern "C" void  type_of_call hropen(const int&,DEFCHAR,DEFCHAR,DEFCHAR,
-                        const int&,const int&,const int,const int,const int);
+                        const int&,const int&,fortran_charlen_t,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hropen(const int&,DEFCHAR,DEFCHAR,DEFCHAR,
                         const int&,const int&);
@@ -179,7 +184,7 @@ extern "C" void  type_of_call hrin(const int&,const int&,const int&);
 extern "C" void  type_of_call hnoent(const int&,const int&);
 #ifndef WIN32
 extern "C" void  type_of_call hgive(const int&,DEFCHAR,const int&,const float&,const float&,
-   const int&,const float&,const float&,const int&,const int&,const int);
+   const int&,const float&,const float&,const int&,const int&,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hgive(const int&,DEFCHAR,const int&,const float&,const float&,
    const int&,const float&,const float&,const int&,const int&);
@@ -187,29 +192,37 @@ extern "C" void  type_of_call hgive(const int&,DEFCHAR,const int&,const float&,c
 
 #ifndef WIN32
 extern "C" void  type_of_call hgiven(const int&,DEFCHAR,const int&,DEFCHAR,
-   const float&,const float&,const int,const int);
+   const float&,const float&,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hgiven(const int&,DEFCHAR,const int&,DEFCHAR,
    const float&,const float&);
 #endif
 
 #ifndef WIN32
-extern "C" void  type_of_call hntvar2(const int&,const int&,DEFCHAR,DEFCHAR,DEFCHAR,int&,int&,int&,int&,int&,const int,const int, const int);
+extern "C" void  type_of_call hntvar2(const int&,const int&,DEFCHAR,DEFCHAR,DEFCHAR,int&,int&,int&,int&,int&,fortran_charlen_t,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hntvar2(const int&,const int&,DEFCHAR,DEFCHAR,DEFCHAR,int&,int&,int&,int&,int&);
 #endif
 
 #ifndef WIN32
-extern "C" void  type_of_call hbnam(const int&,DEFCHAR,const int&,DEFCHAR,const int&,const int, const int);
+extern "C" void  type_of_call hbnam(const int&,DEFCHAR,const int&,DEFCHAR,const int&,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hbnam(const int&,DEFCHAR,const int&,DEFCHAR,const int&);
 #endif
 
 extern "C" void  type_of_call hprntu(const int&);
-extern "C" void  type_of_call hgnpar(const int&,const char *,const int);
+#ifndef WIN32
+extern "C" void  type_of_call hgnpar(const int&,DEFCHAR,fortran_charlen_t);
+#else
+extern "C" void  type_of_call hgnpar(const int&,DEFCHAR);
+#endif
 extern "C" void  type_of_call hgnf(const int&,const int&,const float&,const int&);
 extern "C" void  type_of_call hgnt(const int&,const int&,const int&);
-extern "C" void  type_of_call rzink(const int&,const int&,const char *,const int);
+#ifndef WIN32
+extern "C" void  type_of_call rzink(const int&,const int&,DEFCHAR,fortran_charlen_t);
+#else
+extern "C" void  type_of_call rzink(const int&,const int&,DEFCHAR);
+#endif
 extern "C" void  type_of_call hdcofl();
 extern "C" void  type_of_call hmaxim(const int&,const float&);
 extern "C" void  type_of_call hminim(const int&,const float&);
@@ -232,14 +245,14 @@ extern "C" double type_of_call hije(const int&,const int&,const int&);
 #endif
 
 #ifndef WIN32
-extern "C" void  type_of_call hcdir(DEFCHAR,DEFCHAR ,const int,const int);
+extern "C" void  type_of_call hcdir(DEFCHAR,DEFCHAR ,fortran_charlen_t,fortran_charlen_t);
 #else
 extern "C" void  type_of_call hcdir(DEFCHAR,DEFCHAR);
 #endif
 
 extern "C" void  type_of_call zitoh(const int&,const int&,const int&);
 #ifndef WIN32
-extern "C" void  type_of_call uhtoc(const int&,const int&,DEFCHAR,int&,const int);
+extern "C" void  type_of_call uhtoc(const int&,const int&,DEFCHAR,int&,fortran_charlen_t);
 #else
 extern "C" void  type_of_call uhtoc(const int&,const int&,DEFCHAR,int&);
 #endif
@@ -312,7 +325,11 @@ int main(int argc, char **argv)
 
    int lun = 10;
 #ifndef WIN32
-   hropen(lun,PASSCHAR("example"),PASSCHAR(file_in),PASSCHAR("px"),record_size,ier,7,strlen(file_in),2);
+   // Reminder: Argument passing convention is that string lengths go as hidden arguments
+   // at the end of the argument list.
+   constexpr auto chdir = "example";
+   constexpr auto opt = "px";
+   hropen(lun, chdir, file_in, opt, record_size, ier, strlen(chdir), strlen(file_in), strlen(opt));
 #else
    hropen(lun,PASSCHAR("example"),PASSCHAR(file_in),PASSCHAR("px"),record_size,ier);
 #endif