diff --git a/libsmi-c99.patch b/libsmi-c99.patch
new file mode 100644
index 0000000..6ffead5
--- /dev/null
+++ b/libsmi-c99.patch
@@ -0,0 +1,36 @@
+Do not define _ISOC11_SOURCE in parser-sming.y
+
+It overrides _DEFAULT_SOURCE, thereby hiding the declaration of timegm
+in <time.h>.  This will result in build failures with future compilers
+that do not support implicit function declarations.
+
+Submitted upstream:
+
+<https://lists.ibr.cs.tu-bs.de/hyperkitty/list/libsmi@ibr.cs.tu-bs.de/thread/MI6XPAR7JE2AY6UZIMXA3Q7JGD635JRR/>
+
+diff --git a/lib/parser-sming.c b/lib/parser-sming.c
+index 823b5a73773275f7..00a1391820f21a16 100644
+--- a/lib/parser-sming.c
++++ b/lib/parser-sming.c
+@@ -227,7 +227,7 @@
+     
+ #ifdef BACKEND_SMING
+ 
+-#define _ISOC99_SOURCE
++
+ #include <stdio.h>
+ #include <errno.h>
+ #include <stdlib.h>
+diff --git a/lib/parser-sming.y b/lib/parser-sming.y
+index 555e570613ad689f..32bddeac93eac7e8 100644
+--- a/lib/parser-sming.y
++++ b/lib/parser-sming.y
+@@ -17,7 +17,7 @@
+     
+ #ifdef BACKEND_SMING
+ 
+-#define _ISOC99_SOURCE
++
+ #include <stdio.h>
+ #include <errno.h>
+ #include <stdlib.h>
diff --git a/libsmi-configure-c99.patch b/libsmi-configure-c99.patch
new file mode 100644
index 0000000..10f7e00
--- /dev/null
+++ b/libsmi-configure-c99.patch
@@ -0,0 +1,60 @@
+Avoid implicit ints and implicit function declarations, to prevent
+build failures with future compilers.
+
+It's not clear if old compilers really use <stdio.h> for the exit
+function.  Most of these checks would be better served by using
+#error, so that they can use AC_TRY_LINK and be compatible with
+cross-compilation.
+
+Submitted upstream:
+
+<https://lists.ibr.cs.tu-bs.de/hyperkitty/list/libsmi@ibr.cs.tu-bs.de/thread/MI6XPAR7JE2AY6UZIMXA3Q7JGD635JRR/>
+
+diff --git a/configure.in b/configure.in
+index 9b678e2d915a789d..c032538163bd05ab 100644
+--- a/configure.in
++++ b/configure.in
+@@ -42,8 +42,8 @@ AC_ARG_WITH(cc,
+ CC="$withval")
+ 
+ AC_MSG_CHECKING([for additional required compiler flags])
+-AC_TRY_RUN([#include <stdio.h>
+-  main() {
++AC_TRY_RUN([#include <stdlib.h>
++  int main(void) {
+   #ifdef __SUNPRO_C
+         exit(0);
+   #else
+@@ -100,8 +100,8 @@ dirseparator="$withval", dirseparator="")
+ 
+ AC_MSG_CHECKING([for path separator character])
+ if test "$pathseparator" = "" ; then
+-  AC_TRY_RUN([#include <stdio.h>
+-    main() {
++  AC_TRY_RUN([#include <stdlib.h>
++    int main(void) {
+     #ifdef _WIN32
+ 	  exit(0);
+     #else
+@@ -114,8 +114,8 @@ echo $pathseparator
+ 
+ AC_MSG_CHECKING([for dir separator character])
+ if test "$dirseparator" = "" ; then
+-  AC_TRY_RUN([#include <stdio.h>
+-    main() {
++  AC_TRY_RUN([#include <stdlib.h>
++    int main(void) {
+     #ifdef _WIN32
+ 	  exit(0);
+     #else
+@@ -140,8 +140,8 @@ AC_MSG_CHECKING([for 64 bit types])
+ # "warning: decimal constant is so large that it is unsigned"
+ # if used with the (correct) value -9223372036854775808LL.
+ #
+-AC_TRY_RUN([#include <stdio.h>
+-      main() {
++AC_TRY_RUN([#include <stdlib.h>
++      int main(void) {
+ 	  long long ll;
+ 	  unsigned long long ull;
+ 	  exit(0);
diff --git a/libsmi.spec b/libsmi.spec
index b97b181..dab7240 100644
--- a/libsmi.spec
+++ b/libsmi.spec
@@ -1,6 +1,6 @@
 Name:           libsmi
 Version:        0.4.8
-Release:        32%{?dist}
+Release:        33%{?dist}
 Summary:        A library to access SMI MIB information
 
 License:        GPLv2+ and BSD
@@ -12,6 +12,8 @@ Patch0:		libsmi-0.4.8-wget111.patch
 Patch1:		libsmi-0.4.8-CVE-2010-2891.patch
 Patch2:		libsmi-0.4.8-symbols-clash.patch
 Patch3:		libsmi-0.4.8-format-security-fix.patch
+Patch4: libsmi-configure-c99.patch
+Patch5: libsmi-c99.patch
 
 BuildRequires:  libtool
 BuildRequires:  flex, bison
@@ -47,9 +49,15 @@ libsmi-based applications.
 %patch1 -p1 -b .CVE-2010-2891
 %patch2 -p1 -b .clash
 %patch3 -p1 -b .format-security
+%patch4 -p1
+%patch5 -p1
+# Avoid re-running bison.  The regeneration appears to break the generated
+# C code.
+touch -r lib/parser-sming.y lib/parser-sming.c
 cp %{SOURCE2} .
 
 %build
+autoreconf -iv
 %configure \
     --enable-smi \
     --enable-sming \
@@ -98,6 +106,9 @@ make check ||:
 
 
 %changelog
+* Fri Apr 14 2023 Florian Weimer <fweimer@redhat.com> - 0.4.8-33
+- Port to C99
+
 * Thu Jan 19 2023 Fedora Release Engineering <releng@fedoraproject.org> - 0.4.8-32
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_38_Mass_Rebuild