diff --git a/setserial-c99.patch b/setserial-c99.patch
new file mode 100644
index 0000000..4dbb16b
--- /dev/null
+++ b/setserial-c99.patch
@@ -0,0 +1,28 @@
+Avoid implicit int and implicit function declarations.
+
+A mostly equivalent patch has been posted upstream:
+
+<https://sourceforge.net/p/setserial/discussion/7060/thread/95d874c12c/?limit=25#1643>
+
+diff --git a/setserial.c b/setserial.c
+index f184e98db07ceb59..d181327fc568ede4 100644
+--- a/setserial.c
++++ b/setserial.c
+@@ -15,6 +15,9 @@
+ #include <termios.h>
+ #include <string.h>
+ #include <errno.h>
++#include <sys/ioctl.h>
++#include <unistd.h>
++#include <stdlib.h>
+ 
+ #ifdef HAVE_ASM_IOCTLS_H
+ #include <asm/ioctls.h>
+@@ -714,6 +717,7 @@ fprintf(stderr, "\t* port\t\tset the I/O port\n");
+ 	exit(1);
+ }
+ 
++int
+ main(int argc, char **argv)
+ {
+ 	int	get_flag = 0, wild_intr_flag = 0;
diff --git a/setserial-configure-c99.patch b/setserial-configure-c99.patch
new file mode 100644
index 0000000..f9ed88d
--- /dev/null
+++ b/setserial-configure-c99.patch
@@ -0,0 +1,17 @@
+This fixes a generic C99 compatibility issue in autoconf-generated
+code.  No need to upstream this because the issue goes away if
+autoconf is re-run.
+
+diff --git a/configure b/configure
+index 9d6084aa38e5d6e8..4d948816e1112d13 100755
+--- a/configure
++++ b/configure
+@@ -689,7 +689,7 @@ cat > conftest.$ac_ext << EOF
+ #line 690 "configure"
+ #include "confdefs.h"
+ 
+-main(){return(0);}
++int main(){return(0);}
+ EOF
+ if { (eval echo configure:695: \"$ac_link\") 1>&5; (eval $ac_link) 2>&5; } && test -s conftest${ac_exeext}; then
+   ac_cv_prog_cc_works=yes
diff --git a/setserial.spec b/setserial.spec
index 2bc8b66..5a12344 100644
--- a/setserial.spec
+++ b/setserial.spec
@@ -3,7 +3,7 @@
 Summary: A utility for configuring serial ports
 Name: setserial
 Version: 2.17
-Release: 56%{?dist}
+Release: 57%{?dist}
 Source: https://sourceforge.net/projects/setserial/files/setserial/%{version}/%{name}-%{version}.tar.gz
 Patch0: setserial-2.17-fhs.patch
 Patch1: setserial-2.17-rc.patch
@@ -11,6 +11,8 @@ Patch2: setserial-2.17-readme.patch
 Patch3: setserial-2.17-spelling.patch
 Patch4: setserial-hayesesp.patch
 Patch5: setserial-aarch64.patch
+Patch6: setserial-configure-c99.patch
+Patch7: setserial-c99.patch
 License: GPL+
 URL: http://setserial.sourceforge.net/
 ExcludeArch: s390 s390x
@@ -44,6 +46,8 @@ rm -f config.cache
 
 # Support aarch64 (bug #926522).
 %patch5 -p1 -b .aarch64
+%patch6 -p1
+%patch7 -p1
 
 %build
 %set_build_flags
@@ -64,6 +68,9 @@ make install DESTDIR=${RPM_BUILD_ROOT}
 %{_mandir}/man*/*
 
 %changelog
+* Thu Apr 13 2023 Florian Weimer <fweimer@redhat.com> - 2.17-57
+- Port to C99
+
 * Sat Jan 21 2023 Fedora Release Engineering <releng@fedoraproject.org> - 2.17-56
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_38_Mass_Rebuild