diff --git a/argtable-c99.patch b/argtable-c99.patch
new file mode 100644
index 0000000..6d21d7a
--- /dev/null
+++ b/argtable-c99.patch
@@ -0,0 +1,17 @@
+Include <ctype.h> for toupper, isspace.  This avoids implicit function
+declarations and should improve compatibility with future compilers
+which default to stricter C99 mode.
+
+diff --git a/src/arg_int.c b/src/arg_int.c
+index 29c20e55cccb53ee..7145bb31c5dd8f90 100644
+--- a/src/arg_int.c
++++ b/src/arg_int.c
+@@ -31,6 +31,8 @@ USA.
+ #include "argtable2.h"
+ #include <limits.h>
+ 
++#include <ctype.h>
++
+ /* local error codes */
+ enum {EMINCOUNT=1,EMAXCOUNT,EBADINT,EOVERFLOW};
+ 
diff --git a/argtable.spec b/argtable.spec
index 5d788e3..4b8c069 100644
--- a/argtable.spec
+++ b/argtable.spec
@@ -3,10 +3,11 @@
 Summary: Cross platform C library for parsing GNU style command line arguments
 Name: argtable
 Version: 2.13
-Release: 23%{?dist}
+Release: 24%{?dist}
 License: LGPLv2+
 Url: http://argtable.sourceforge.net/
 Source: http://prdownloads.sourceforge.net/argtable/%{name}2-13.tar.gz
+Patch0: argtable-c99.patch
 BuildRequires:  gcc
 BuildRequires: pkgconfig
 BuildRequires: make
@@ -29,7 +30,7 @@ Requires: %{name}%{?_isa} = %{version}-%{release}
 Header and development files for using argtable
 
 %prep
-%setup -q -n %{name}2-13
+%autosetup -p1 -n %{name}2-13
 
 %build
 %configure --disable-static --docdir=%{_pkgdocdir}
@@ -63,6 +64,9 @@ install -pm 644 AUTHORS ChangeLog COPYING README ${RPM_BUILD_ROOT}%{_pkgdocdir}
 %ldconfig_scriptlets
 
 %changelog
+* Tue Dec  6 2022 Florian Weimer <fweimer@redhat.com> - 2.13-24
+- Port to C99 (#2151244)
+
 * Wed Jul 20 2022 Fedora Release Engineering <releng@fedoraproject.org> - 2.13-23
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_37_Mass_Rebuild