diff --git a/mariadb-c99.patch b/mariadb-c99.patch
new file mode 100644
index 0000000..ce01dd7
--- /dev/null
+++ b/mariadb-c99.patch
@@ -0,0 +1,21 @@
+rocksdb: Define _GNU_SOURCE during fallocate CMake probe
+
+The glibc headers declare fallocate only if _GNU_SOURCE is defined.
+Upstream rocksdb does not need this because the probe is run with the
+C++ compiler, and current g++ versions define _GNU_SOURCE
+automatically.
+
+Submitted upstream: <https://github.com/MariaDB/server/pull/2593>
+
+
+diff -ur mariadb-10.5.18-downstream_modified.orig/storage/rocksdb/build_rocksdb.cmake mariadb-10.5.18-downstream_modified/storage/rocksdb/build_rocksdb.cmake
+--- mariadb-10.5.18-downstream_modified.orig/storage/rocksdb/build_rocksdb.cmake	2022-11-02 13:37:22.000000000 +0100
++++ mariadb-10.5.18-downstream_modified/storage/rocksdb/build_rocksdb.cmake	2023-04-11 09:42:17.295837813 +0200
+@@ -138,6 +138,7 @@
+ if(WITH_FALLOCATE AND UNIX)
+   include(CheckCSourceCompiles)
+   CHECK_C_SOURCE_COMPILES("
++#define _GNU_SOURCE
+ #include <fcntl.h>
+ #include <linux/falloc.h>
+ int main() {
diff --git a/mariadb.spec b/mariadb.spec
index 65cfe6d..1524270 100644
--- a/mariadb.spec
+++ b/mariadb.spec
@@ -150,7 +150,7 @@
 
 Name:             mariadb
 Version:          10.5.18
-Release:          2%{?with_debug:.debug}%{?dist}
+Release:          3%{?with_debug:.debug}%{?dist}
 Epoch:            3
 
 Summary:          A very fast and robust SQL database server
@@ -212,6 +212,7 @@ Patch10:          %{pkgnamepatch}-ssl-cipher-tests.patch
 #   https://gcc.gnu.org/gcc-13/porting_to.html
 Patch11:          mariadb-10.5-gcc13.patch
 Patch12:          rocksdb-6.8-gcc13.patch
+Patch13:          mariadb-c99.patch
 
 BuildRequires:    make
 BuildRequires:    cmake gcc-c++
@@ -746,6 +747,7 @@ rm -r storage/rocksdb/
 %patch11 -p1
 %if %{with rocksdb}
 %patch12 -p1 -d storage/rocksdb/rocksdb/
+%patch13 -p1
 %endif
 
 # generate a list of tests that fail, but are not disabled by upstream
@@ -1646,6 +1648,9 @@ fi
 %endif
 
 %changelog
+* Tue Apr 11 2023 Florian Weimer <fweimer@redhat.com> - 3:10.5.18-3
+- Port to C99
+
 * Thu Jan 19 2023 Fedora Release Engineering <releng@fedoraproject.org> - 3:10.5.18-2
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_38_Mass_Rebuild