diff --git a/netpbm-c99.patch b/netpbm-c99.patch
new file mode 100644
index 0000000..ed0a887
--- /dev/null
+++ b/netpbm-c99.patch
@@ -0,0 +1,46 @@
+Include "pm.h" for the declaration of the overflow_add function.  This
+avoids an implicit function declaration.  Future C compilers will
+require that all functions are declared explicitly.  Implicit function
+declarations are a language feature that was removed in 1999.
+
+Likewise, include "pm.h" for the newly added declarations of realloc2.
+This fixes a pointer truncation bug in pjtoppm.
+
+Both issues are specific to downstream patches.
+
+diff --git a/converter/pbm/pbmtoppa/pbm.c b/converter/pbm/pbmtoppa/pbm.c
+index 1c8d236219b297e1..d63cab63b13f2051 100644
+--- a/converter/pbm/pbmtoppa/pbm.c
++++ b/converter/pbm/pbmtoppa/pbm.c
+@@ -12,6 +12,7 @@
+ #include <stdlib.h>
+ #include <string.h>
+ 
++#include "pm.h"
+ #include "ppapbm.h"
+ 
+ int make_pbm_stat(pbm_stat* pbm,FILE* fptr)
+diff --git a/converter/ppm/pjtoppm.c b/converter/ppm/pjtoppm.c
+index c458c06457c2b9bd..5e93a6acb8d6f198 100644
+--- a/converter/ppm/pjtoppm.c
++++ b/converter/ppm/pjtoppm.c
+@@ -10,6 +10,7 @@
+ ** implied warranty.
+ */
+ 
++#include "pm.h"
+ #include "ppm.h"
+ #include "mallocvar.h"
+ 
+diff --git a/lib/pm.h b/lib/pm.h
+index 5037c2519c565fbc..435445d5e7f52fbf 100644
+--- a/lib/pm.h
++++ b/lib/pm.h
+@@ -457,6 +457,7 @@ pm_parse_height(const char * const arg);
+ 
+ void *malloc2(int, int);
+ void *malloc3(int, int, int);
++void *realloc2(void * a, int b, int c);
+ #define overflow2(a,b) __overflow2(a,b)
+ void __overflow2(int, int);
+ void overflow3(int, int, int);
diff --git a/netpbm.spec b/netpbm.spec
index a99239b..23cb95d 100644
--- a/netpbm.spec
+++ b/netpbm.spec
@@ -1,7 +1,7 @@
 Summary:         A library for handling different graphics file formats
 Name:            netpbm
 Version:         11.00.00
-Release:         1%{?dist}
+Release:         2%{?dist}
 # See copyright_summary for details
 License:         BSD and GPLv2 and IJG and MIT and Public Domain
 URL: http://netpbm.sourceforge.net/
@@ -29,6 +29,7 @@ Patch14:         netpbm-pamtojpeg2k.patch
 Patch15:         netpbm-manfix.patch
 Patch16:         netpbm-jasper.patch
 Patch17:         netpbm-libdir-so.patch
+Patch18:         netpbm-c99.patch
 
 BuildRequires: make
 BuildRequires:   libjpeg-devel, libpng-devel, libtiff-devel, flex, gcc, jbigkit-devel
@@ -229,6 +230,9 @@ popd
 %doc userguide/*
 
 %changelog
+* Fri Dec  2 2022 Florian Weimer <fweimer@redhat.com> - 11.00.00-2
+- Port downstream-specific patches to C99
+
 * Tue Oct 04 2022 Josef Ridky <jridky@redhat.com> - 11.00.00-1
 - New upstream release 11.00.00 (#2130384)