diff --git a/src/Anon.cc b/src/Anon.cc
index 024cda89ea..7780a4dfd5 100644
--- a/src/Anon.cc
+++ b/src/Anon.cc
@@ -1,9 +1,9 @@
 #include "zeek/Anon.h"
 
-#include <assert.h>
-#include <stdlib.h>
 #include <sys/time.h>
 #include <unistd.h>
+#include <cassert>
+#include <cstdlib>
 
 #include "zeek/Event.h"
 #include "zeek/ID.h"
diff --git a/src/Base64.cc b/src/Base64.cc
index 7e76f42520..1cf3f1b924 100644
--- a/src/Base64.cc
+++ b/src/Base64.cc
@@ -2,7 +2,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <math.h>
+#include <cmath>
 
 #include "zeek/Conn.h"
 #include "zeek/Reporter.h"
diff --git a/src/Conn.cc b/src/Conn.cc
index 59052c2cb9..5beebf1bc0 100644
--- a/src/Conn.cc
+++ b/src/Conn.cc
@@ -5,7 +5,7 @@
 #include "zeek/zeek-config.h"
 
 #include <binpac.h>
-#include <ctype.h>
+#include <cctype>
 
 #include "zeek/Desc.h"
 #include "zeek/Event.h"
diff --git a/src/DFA.h b/src/DFA.h
index 2f1517a8ae..5ebd3c98c9 100644
--- a/src/DFA.h
+++ b/src/DFA.h
@@ -2,8 +2,8 @@
 
 #pragma once
 
-#include <assert.h>
 #include <sys/types.h> // for u_char
+#include <cassert>
 #include <map>
 #include <string>
 
diff --git a/src/DNS_Mgr.cc b/src/DNS_Mgr.cc
index 33bb667afa..f1fff3f864 100644
--- a/src/DNS_Mgr.cc
+++ b/src/DNS_Mgr.cc
@@ -4,25 +4,25 @@
 
 #include "zeek/zeek-config.h"
 
-#include <errno.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#include <stdlib.h>
 #include <sys/select.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <algorithm>
+#include <cerrno>
+#include <cstdlib>
 #include <vector>
 
 #ifdef TIME_WITH_SYS_TIME
 #include <sys/time.h>
-#include <time.h>
+#include <ctime>
 #elif defined(HAVE_SYS_TIME_H)
 #include <sys/time.h>
 #else
-#include <time.h>
+#include <ctime>
 #endif
 
 #include <ztd/out_ptr.hpp>
diff --git a/src/DbgBreakpoint.cc b/src/DbgBreakpoint.cc
index 01dcbb059a..32c87655e1 100644
--- a/src/DbgBreakpoint.cc
+++ b/src/DbgBreakpoint.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <assert.h>
+#include <cassert>
 
 #include "zeek/Debug.h"
 #include "zeek/Desc.h"
diff --git a/src/Debug.cc b/src/Debug.cc
index d63cef512c..7053369835 100644
--- a/src/Debug.cc
+++ b/src/Debug.cc
@@ -4,10 +4,10 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
-#include <signal.h>
-#include <stdarg.h>
-#include <stdio.h>
+#include <cctype>
+#include <csignal>
+#include <cstdarg>
+#include <cstdio>
 #include <string>
 
 #ifdef HAVE_READLINE
diff --git a/src/DebugCmds.cc b/src/DebugCmds.cc
index 9b23c67622..1f5041a59d 100644
--- a/src/DebugCmds.cc
+++ b/src/DebugCmds.cc
@@ -5,10 +5,10 @@
 
 #include "zeek/zeek-config.h"
 
-#include <assert.h>
 #include <regex.h>
-#include <string.h>
 #include <sys/types.h>
+#include <cassert>
+#include <cstring>
 
 #include "zeek/DbgBreakpoint.h"
 #include "zeek/Debug.h"
diff --git a/src/DebugCmds.h b/src/DebugCmds.h
index 35a0a773cd..3eb0ec6b2c 100644
--- a/src/DebugCmds.h
+++ b/src/DebugCmds.h
@@ -3,7 +3,7 @@
 
 #pragma once
 
-#include <stdlib.h>
+#include <cstdlib>
 #include <deque>
 #include <string>
 #include <vector>
diff --git a/src/DebugLogger.cc b/src/DebugLogger.cc
index bb6a5b5f20..f95db79139 100644
--- a/src/DebugLogger.cc
+++ b/src/DebugLogger.cc
@@ -2,8 +2,8 @@
 
 #include "zeek/DebugLogger.h"
 
-#include <stdlib.h>
 #include <unistd.h>
+#include <cstdlib>
 
 #include "zeek/RunState.h"
 #include "zeek/plugin/Plugin.h"
diff --git a/src/Desc.cc b/src/Desc.cc
index 2c97c05a6b..315c39abe5 100644
--- a/src/Desc.cc
+++ b/src/Desc.cc
@@ -4,10 +4,10 @@
 
 #include "zeek/zeek-config.h"
 
-#include <errno.h>
-#include <math.h>
-#include <stdlib.h>
-#include <string.h>
+#include <cerrno>
+#include <cmath>
+#include <cstdlib>
+#include <cstring>
 
 #include "zeek/3rdparty/ConvertUTF.h"
 #include "zeek/File.h"
@@ -179,7 +179,7 @@ void ODesc::Add(double d, bool no_exp)
 			return v < 0 ? -v < tolerance : v < tolerance;
 		};
 
-		if ( approx_equal(d, nearbyint(d), 1e-9) && isfinite(d) && ! strchr(tmp, 'e') )
+		if ( approx_equal(d, nearbyint(d), 1e-9) && std::isfinite(d) && ! strchr(tmp, 'e') )
 			// disambiguate from integer
 			Add(".0");
 		}
diff --git a/src/Dict.cc b/src/Dict.cc
index 6e5e584898..0ccbf81b39 100644
--- a/src/Dict.cc
+++ b/src/Dict.cc
@@ -7,9 +7,9 @@
 #ifdef HAVE_MEMORY_H
 #include <memory.h>
 #endif
-#include <signal.h>
 #include <algorithm>
 #include <climits>
+#include <csignal>
 #include <fstream>
 
 #include "zeek/3rdparty/doctest.h"
diff --git a/src/File.cc b/src/File.cc
index 00fa141b3a..b2a167740f 100644
--- a/src/File.cc
+++ b/src/File.cc
@@ -7,19 +7,19 @@
 #include <sys/types.h>
 #ifdef TIME_WITH_SYS_TIME
 #include <sys/time.h>
-#include <time.h>
+#include <ctime>
 #else
 #ifdef HAVE_SYS_TIME_H
 #include <sys/time.h>
 #else
-#include <time.h>
+#include <ctime>
 #endif
 #endif
-#include <errno.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <unistd.h>
 #include <algorithm>
+#include <cerrno>
 
 #include "zeek/Attr.h"
 #include "zeek/Desc.h"
diff --git a/src/Flare.cc b/src/Flare.cc
index c1121caabc..e2f42fa34e 100644
--- a/src/Flare.cc
+++ b/src/Flare.cc
@@ -2,9 +2,9 @@
 
 #include "zeek/Flare.h"
 
-#include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <cerrno>
 
 #include "zeek/Reporter.h"
 
diff --git a/src/Func.cc b/src/Func.cc
index 5b39e453aa..54cbfa1d4d 100644
--- a/src/Func.cc
+++ b/src/Func.cc
@@ -6,15 +6,15 @@
 #include "zeek/zeek-config.h"
 
 #include <broker/error.hh>
-#include <ctype.h>
-#include <errno.h>
 #include <netdb.h>
 #include <netinet/in.h>
-#include <signal.h>
-#include <stdlib.h>
 #include <sys/param.h>
 #include <unistd.h>
 #include <algorithm>
+#include <cctype>
+#include <cerrno>
+#include <csignal>
+#include <cstdlib>
 
 #include "zeek/Base64.h"
 #include "zeek/Debug.h"
diff --git a/src/Hash.h b/src/Hash.h
index 0b58971cff..a1ffaa1698 100644
--- a/src/Hash.h
+++ b/src/Hash.h
@@ -19,7 +19,7 @@
 
 #pragma once
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "zeek/util.h" // for bro_int_t
 
diff --git a/src/IntSet.cc b/src/IntSet.cc
index be9557aaf2..554e673f37 100644
--- a/src/IntSet.cc
+++ b/src/IntSet.cc
@@ -2,10 +2,7 @@
 
 #include "zeek/zeek-config.h"
 
-#ifdef HAVE_MEMORY_H
-#include <memory.h>
-#endif
-#include <stdlib.h>
+#include <cstring>
 
 namespace zeek::detail
 	{
diff --git a/src/IntSet.h b/src/IntSet.h
index 6e2895f5ed..65b0e22cf9 100644
--- a/src/IntSet.h
+++ b/src/IntSet.h
@@ -6,7 +6,7 @@
 
 #pragma once
 
-#include <string.h>
+#include <cstring>
 
 namespace zeek::detail
 	{
diff --git a/src/Obj.cc b/src/Obj.cc
index 3a36e39e49..8b4fffaa7e 100644
--- a/src/Obj.cc
+++ b/src/Obj.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "zeek/Desc.h"
 #include "zeek/File.h"
diff --git a/src/Obj.h b/src/Obj.h
index a62ca4886a..bc2eba6aa4 100644
--- a/src/Obj.h
+++ b/src/Obj.h
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <limits.h>
+#include <climits>
 
 namespace zeek
 	{
diff --git a/src/Pipe.cc b/src/Pipe.cc
index 5bc6796bed..bf6cb46054 100644
--- a/src/Pipe.cc
+++ b/src/Pipe.cc
@@ -2,9 +2,9 @@
 
 #include "zeek/Pipe.h"
 
-#include <errno.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <cerrno>
 #include <cstdio>
 
 #include "zeek/Reporter.h"
diff --git a/src/PolicyFile.cc b/src/PolicyFile.cc
index c1bd4eac42..31e44dcdd9 100644
--- a/src/PolicyFile.cc
+++ b/src/PolicyFile.cc
@@ -2,11 +2,11 @@
 
 #include "zeek/zeek-config.h"
 
-#include <assert.h>
-#include <errno.h>
-#include <stdio.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <cassert>
+#include <cerrno>
+#include <cstdio>
 #include <map>
 #include <string>
 #include <vector>
diff --git a/src/PriorityQueue.cc b/src/PriorityQueue.cc
index 4a0397b004..b5d38ca67c 100644
--- a/src/PriorityQueue.cc
+++ b/src/PriorityQueue.cc
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdio.h>
-#include <stdlib.h>
+#include <cstdio>
+#include <cstdlib>
 
 #include "zeek/Reporter.h"
 #include "zeek/util.h"
diff --git a/src/PriorityQueue.h b/src/PriorityQueue.h
index d0e27219fa..cac07f6bbb 100644
--- a/src/PriorityQueue.h
+++ b/src/PriorityQueue.h
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <math.h>
-#include <stdint.h>
+#include <cmath>
+#include <cstdint>
 
 namespace zeek::detail
 	{
diff --git a/src/Queue.h b/src/Queue.h
index 13f5522dd8..7600aacce4 100644
--- a/src/Queue.h
+++ b/src/Queue.h
@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include <string.h>
+#include <cstring>
 #include <iterator>
 
 // Queue.h --
diff --git a/src/RE.cc b/src/RE.cc
index dbff44b0f7..f5fb352079 100644
--- a/src/RE.cc
+++ b/src/RE.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 #include <utility>
 
 #include "zeek/CCL.h"
diff --git a/src/RE.h b/src/RE.h
index 06fdd5f391..e30fbb91a4 100644
--- a/src/RE.h
+++ b/src/RE.h
@@ -2,8 +2,8 @@
 
 #pragma once
 
-#include <ctype.h>
 #include <sys/types.h> // for u_char
+#include <cctype>
 #include <map>
 #include <set>
 #include <string>
diff --git a/src/RandTest.cc b/src/RandTest.cc
index 5062c6968d..554e9db2c5 100644
--- a/src/RandTest.cc
+++ b/src/RandTest.cc
@@ -14,7 +14,7 @@
 
 #include "zeek/RandTest.h"
 
-#include <math.h>
+#include <cmath>
 
 constexpr double log2of10 = 3.32192809488736234787;
 
diff --git a/src/RandTest.h b/src/RandTest.h
index fa44827c89..91812f1b81 100644
--- a/src/RandTest.h
+++ b/src/RandTest.h
@@ -2,7 +2,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdint.h>
+#include <cstdint>
 
 #define RT_MONTEN                                                                                  \
 	6 /* Bytes used as Monte Carlo                                                                 \
diff --git a/src/Reassem.h b/src/Reassem.h
index 6ffb2571e7..8deff8d5d3 100644
--- a/src/Reassem.h
+++ b/src/Reassem.h
@@ -2,10 +2,10 @@
 
 #pragma once
 
-#include <assert.h>
-#include <string.h>
 #include <sys/types.h> // for u_char
+#include <cassert>
 #include <cstdint>
+#include <cstring>
 #include <map>
 
 #include "zeek/Obj.h"
diff --git a/src/Rule.h b/src/Rule.h
index 480e92bb20..72e41e1ece 100644
--- a/src/Rule.h
+++ b/src/Rule.h
@@ -1,7 +1,7 @@
 #pragma once
 
-#include <limits.h>
-#include <stdint.h>
+#include <climits>
+#include <cstdint>
 #include <map>
 #include <string>
 
diff --git a/src/RuleCondition.h b/src/RuleCondition.h
index e32a4553cc..8e688a2ca6 100644
--- a/src/RuleCondition.h
+++ b/src/RuleCondition.h
@@ -1,7 +1,7 @@
 #pragma once
 
-#include <stdint.h> // for u_char
 #include <sys/types.h> // for u_char
+#include <cstdint> // for u_char
 
 #include "zeek/util.h"
 
diff --git a/src/RuleMatcher.h b/src/RuleMatcher.h
index 6713a623c0..f4418dc1a3 100644
--- a/src/RuleMatcher.h
+++ b/src/RuleMatcher.h
@@ -1,7 +1,7 @@
 #pragma once
 
-#include <limits.h>
 #include <sys/types.h> // for u_char
+#include <climits>
 #include <functional>
 #include <map>
 #include <set>
diff --git a/src/RunState.cc b/src/RunState.cc
index 5e44eb8129..052a9ae39c 100644
--- a/src/RunState.cc
+++ b/src/RunState.cc
@@ -7,18 +7,18 @@
 #include <sys/types.h>
 #ifdef TIME_WITH_SYS_TIME
 #include <sys/time.h>
-#include <time.h>
+#include <ctime>
 #else
 #ifdef HAVE_SYS_TIME_H
 #include <sys/time.h>
 #else
-#include <time.h>
+#include <ctime>
 #endif
 #endif
 
-#include <signal.h>
-#include <stdlib.h>
 #include <unistd.h>
+#include <csignal>
+#include <cstdlib>
 
 extern "C"
 	{
diff --git a/src/ScannedFile.cc b/src/ScannedFile.cc
index 0e25b47a27..13eb87206f 100644
--- a/src/ScannedFile.cc
+++ b/src/ScannedFile.cc
@@ -1,7 +1,7 @@
 #include "zeek/ScannedFile.h"
 
-#include <errno.h>
-#include <limits.h> // for PATH_MAX
+#include <cerrno>
+#include <climits> // for PATH_MAX
 
 #include "zeek/DebugLogger.h"
 #include "zeek/Reporter.h"
diff --git a/src/SerializationFormat.cc b/src/SerializationFormat.cc
index dc619adf95..42cff63cdd 100644
--- a/src/SerializationFormat.cc
+++ b/src/SerializationFormat.cc
@@ -1,6 +1,6 @@
 #include "zeek/SerializationFormat.h"
 
-#include <ctype.h>
+#include <cctype>
 
 #include "zeek/DebugLogger.h"
 #include "zeek/IPAddr.h"
diff --git a/src/SerializationFormat.h b/src/SerializationFormat.h
index e0eb139285..44215792bb 100644
--- a/src/SerializationFormat.h
+++ b/src/SerializationFormat.h
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdint.h>
+#include <cstdint>
 #include <string>
 
 struct in_addr;
diff --git a/src/SmithWaterman.cc b/src/SmithWaterman.cc
index c02fe2bc37..239fccca5a 100644
--- a/src/SmithWaterman.cc
+++ b/src/SmithWaterman.cc
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
 #include <algorithm>
+#include <cctype>
 
 #include "zeek/Reporter.h"
 #include "zeek/Val.h"
diff --git a/src/Stats.h b/src/Stats.h
index dd1d3e7418..403edd069b 100644
--- a/src/Stats.h
+++ b/src/Stats.h
@@ -4,10 +4,10 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdint.h>
 #include <sys/resource.h>
 #include <sys/time.h>
 #include <sys/types.h>
+#include <cstdint>
 #include <memory>
 
 namespace zeek
diff --git a/src/Trigger.cc b/src/Trigger.cc
index 36e2a7acd4..5aa320f2bc 100644
--- a/src/Trigger.cc
+++ b/src/Trigger.cc
@@ -1,7 +1,7 @@
 #include "zeek/Trigger.h"
 
-#include <assert.h>
 #include <algorithm>
+#include <cassert>
 
 #include "zeek/DebugLogger.h"
 #include "zeek/Desc.h"
diff --git a/src/UID.h b/src/UID.h
index 1738200000..25fdb1ba73 100644
--- a/src/UID.h
+++ b/src/UID.h
@@ -2,7 +2,7 @@
 
 #pragma once
 
-#include <string.h>
+#include <cstring>
 #include <string>
 
 #include "zeek/util.h" // for bro_int_t
diff --git a/src/Val.cc b/src/Val.cc
index 203e7398e0..f8c09aa677 100644
--- a/src/Val.cc
+++ b/src/Val.cc
@@ -6,12 +6,12 @@
 
 #include <netdb.h>
 #include <netinet/in.h>
-#include <stdio.h>
-#include <stdlib.h>
 #include <sys/param.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <cmath>
+#include <cstdio>
+#include <cstdlib>
 #include <set>
 
 #include "zeek/Attr.h"
diff --git a/src/ZeekString.cc b/src/ZeekString.cc
index 3161cd2ef8..2021e4696b 100644
--- a/src/ZeekString.cc
+++ b/src/ZeekString.cc
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
 #include <algorithm>
+#include <cctype>
 #include <iostream>
 #include <sstream> // Needed for unit testing
 
diff --git a/src/analyzer/protocol/dce-rpc/DCE_RPC.cc b/src/analyzer/protocol/dce-rpc/DCE_RPC.cc
index 109542371e..fced8fbefa 100644
--- a/src/analyzer/protocol/dce-rpc/DCE_RPC.cc
+++ b/src/analyzer/protocol/dce-rpc/DCE_RPC.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 #include <map>
 #include <string>
 
diff --git a/src/analyzer/protocol/dns/DNS.cc b/src/analyzer/protocol/dns/DNS.cc
index 788de2a181..c8e4a684d9 100644
--- a/src/analyzer/protocol/dns/DNS.cc
+++ b/src/analyzer/protocol/dns/DNS.cc
@@ -5,10 +5,10 @@
 #include "zeek/zeek-config.h"
 
 #include <arpa/inet.h>
-#include <ctype.h>
 #include <netinet/in.h>
 #include <sys/socket.h>
 #include <sys/types.h>
+#include <cctype>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/finger/Finger.cc b/src/analyzer/protocol/finger/Finger.cc
index 155a1bc14b..f0306fd174 100644
--- a/src/analyzer/protocol/finger/Finger.cc
+++ b/src/analyzer/protocol/finger/Finger.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
+#include <cctype>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/ftp/FTP.cc b/src/analyzer/protocol/ftp/FTP.cc
index 594da3606a..1142e092ff 100644
--- a/src/analyzer/protocol/ftp/FTP.cc
+++ b/src/analyzer/protocol/ftp/FTP.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "zeek/Base64.h"
 #include "zeek/Event.h"
diff --git a/src/analyzer/protocol/gnutella/Gnutella.cc b/src/analyzer/protocol/gnutella/Gnutella.cc
index 4fc37de9f6..064cc13948 100644
--- a/src/analyzer/protocol/gnutella/Gnutella.cc
+++ b/src/analyzer/protocol/gnutella/Gnutella.cc
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
 #include <algorithm>
+#include <cctype>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/http/HTTP.cc b/src/analyzer/protocol/http/HTTP.cc
index 3d3b185853..96f275176c 100644
--- a/src/analyzer/protocol/http/HTTP.cc
+++ b/src/analyzer/protocol/http/HTTP.cc
@@ -4,10 +4,10 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
-#include <math.h>
-#include <stdlib.h>
 #include <algorithm>
+#include <cctype>
+#include <cmath>
+#include <cstdlib>
 #include <string>
 
 #include "zeek/Event.h"
diff --git a/src/analyzer/protocol/ident/Ident.cc b/src/analyzer/protocol/ident/Ident.cc
index 47946d5f8a..618bc2fa2f 100644
--- a/src/analyzer/protocol/ident/Ident.cc
+++ b/src/analyzer/protocol/ident/Ident.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
+#include <cctype>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/login/Login.cc b/src/analyzer/protocol/login/Login.cc
index db90dc359c..7b3de46000 100644
--- a/src/analyzer/protocol/login/Login.cc
+++ b/src/analyzer/protocol/login/Login.cc
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
-#include <stdlib.h>
+#include <cctype>
+#include <cstdlib>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/login/NVT.cc b/src/analyzer/protocol/login/NVT.cc
index 894aced4da..80f35eabe0 100644
--- a/src/analyzer/protocol/login/NVT.cc
+++ b/src/analyzer/protocol/login/NVT.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/mime/MIME.h b/src/analyzer/protocol/mime/MIME.h
index cdde47c5c7..80dbca02b7 100644
--- a/src/analyzer/protocol/mime/MIME.h
+++ b/src/analyzer/protocol/mime/MIME.h
@@ -1,8 +1,8 @@
 #pragma once
 
-#include <assert.h>
 #include <openssl/evp.h>
-#include <stdio.h>
+#include <cassert>
+#include <cstdio>
 #include <queue>
 #include <vector>
 
diff --git a/src/analyzer/protocol/ncp/NCP.cc b/src/analyzer/protocol/ncp/NCP.cc
index d6ecf6d05b..e915d5023f 100644
--- a/src/analyzer/protocol/ncp/NCP.cc
+++ b/src/analyzer/protocol/ncp/NCP.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 #include <map>
 #include <string>
 
diff --git a/src/analyzer/protocol/netbios/NetbiosSSN.cc b/src/analyzer/protocol/netbios/NetbiosSSN.cc
index 18efafa742..a90f4456b6 100644
--- a/src/analyzer/protocol/netbios/NetbiosSSN.cc
+++ b/src/analyzer/protocol/netbios/NetbiosSSN.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
+#include <cctype>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/ntp/ntp-analyzer.pac b/src/analyzer/protocol/ntp/ntp-analyzer.pac
index f2d4b6fe2f..c202e40daf 100644
--- a/src/analyzer/protocol/ntp/ntp-analyzer.pac
+++ b/src/analyzer/protocol/ntp/ntp-analyzer.pac
@@ -1,6 +1,6 @@
 
 %extern{
-	#include <math.h>
+	#include <cmath>
 	#define FRAC_16 pow(2,-16)
 	#define FRAC_32 pow(2,-32)
 	// NTP defines the epoch from 1900, not 1970
diff --git a/src/analyzer/protocol/pop3/POP3.cc b/src/analyzer/protocol/pop3/POP3.cc
index bc88a279cd..602cb89214 100644
--- a/src/analyzer/protocol/pop3/POP3.cc
+++ b/src/analyzer/protocol/pop3/POP3.cc
@@ -5,7 +5,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <ctype.h>
+#include <cctype>
 #include <string>
 #include <vector>
 
diff --git a/src/analyzer/protocol/rpc/RPC.cc b/src/analyzer/protocol/rpc/RPC.cc
index 4628749309..4064e67881 100644
--- a/src/analyzer/protocol/rpc/RPC.cc
+++ b/src/analyzer/protocol/rpc/RPC.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 #include <string>
 
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/rpc/XDR.cc b/src/analyzer/protocol/rpc/XDR.cc
index dddae9b154..40101aca78 100644
--- a/src/analyzer/protocol/rpc/XDR.cc
+++ b/src/analyzer/protocol/rpc/XDR.cc
@@ -4,8 +4,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <string.h>
 #include <algorithm>
+#include <cstring>
 
 #include "zeek/analyzer/protocol/rpc/events.bif.h"
 
diff --git a/src/analyzer/protocol/smtp/SMTP.cc b/src/analyzer/protocol/smtp/SMTP.cc
index d7d5b73870..c6f1c2d92f 100644
--- a/src/analyzer/protocol/smtp/SMTP.cc
+++ b/src/analyzer/protocol/smtp/SMTP.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "zeek/Event.h"
 #include "zeek/NetVar.h"
diff --git a/src/analyzer/protocol/tcp/TCP_Endpoint.cc b/src/analyzer/protocol/tcp/TCP_Endpoint.cc
index 67c40e9d2c..904518138b 100644
--- a/src/analyzer/protocol/tcp/TCP_Endpoint.cc
+++ b/src/analyzer/protocol/tcp/TCP_Endpoint.cc
@@ -2,7 +2,7 @@
 
 #include "zeek/analyzer/protocol/tcp/TCP_Endpoint.h"
 
-#include <errno.h>
+#include <cerrno>
 
 #include "zeek/Event.h"
 #include "zeek/File.h"
diff --git a/src/binpac-lib.pac b/src/binpac-lib.pac
index 317218c5b2..93669a98e5 100644
--- a/src/binpac-lib.pac
+++ b/src/binpac-lib.pac
@@ -1,5 +1,5 @@
 %extern{
-#include <string.h>
+#include <cstring>
 #include <binpac_bytestring.h>
 %}
 
diff --git a/src/fuzzers/FuzzBuffer.cc b/src/fuzzers/FuzzBuffer.cc
index cea03f6111..5529e98531 100644
--- a/src/fuzzers/FuzzBuffer.cc
+++ b/src/fuzzers/FuzzBuffer.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/fuzzers/FuzzBuffer.h"
 
-#include <string.h>
+#include <cstring>
 
 namespace zeek::detail
 	{
diff --git a/src/input/readers/ascii/Ascii.cc b/src/input/readers/ascii/Ascii.cc
index 4dbaf4f9e3..dd23039127 100644
--- a/src/input/readers/ascii/Ascii.cc
+++ b/src/input/readers/ascii/Ascii.cc
@@ -2,10 +2,10 @@
 
 #include "zeek/input/readers/ascii/Ascii.h"
 
-#include <errno.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <cerrno>
 #include <sstream>
 
 #include "zeek/input/readers/ascii/ascii.bif.h"
diff --git a/src/input/readers/benchmark/Benchmark.cc b/src/input/readers/benchmark/Benchmark.cc
index 89beb3a9eb..89b56bf846 100644
--- a/src/input/readers/benchmark/Benchmark.cc
+++ b/src/input/readers/benchmark/Benchmark.cc
@@ -2,10 +2,10 @@
 
 #include "zeek/input/readers/benchmark/Benchmark.h"
 
-#include <errno.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <cerrno>
 
 #include "zeek/input/readers/benchmark/benchmark.bif.h"
 #include "zeek/threading/Manager.h"
diff --git a/src/input/readers/config/Config.cc b/src/input/readers/config/Config.cc
index f2a2a83590..e019008f81 100644
--- a/src/input/readers/config/Config.cc
+++ b/src/input/readers/config/Config.cc
@@ -2,11 +2,11 @@
 
 #include "zeek/input/readers/config/Config.h"
 
-#include <errno.h>
 #include <regex.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <cerrno>
 #include <sstream>
 #include <unordered_set>
 
diff --git a/src/input/readers/raw/Raw.cc b/src/input/readers/raw/Raw.cc
index a8ba5df1c8..8c7fdd0f31 100644
--- a/src/input/readers/raw/Raw.cc
+++ b/src/input/readers/raw/Raw.cc
@@ -2,15 +2,15 @@
 
 #include "zeek/input/readers/raw/Raw.h"
 
-#include <errno.h>
 #include <fcntl.h>
-#include <signal.h>
-#include <stdio.h>
-#include <stdlib.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
+#include <cerrno>
+#include <csignal>
+#include <cstdio>
+#include <cstdlib>
 
 #include "zeek/input/readers/raw/Plugin.h"
 #include "zeek/input/readers/raw/raw.bif.h"
diff --git a/src/iosource/BPF_Program.cc b/src/iosource/BPF_Program.cc
index c0d07cf6ee..39aabc257b 100644
--- a/src/iosource/BPF_Program.cc
+++ b/src/iosource/BPF_Program.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <string.h>
+#include <cstring>
 
 #include "zeek/util.h"
 
diff --git a/src/iosource/Manager.cc b/src/iosource/Manager.cc
index 90f2a6e3ad..5eb9859429 100644
--- a/src/iosource/Manager.cc
+++ b/src/iosource/Manager.cc
@@ -2,7 +2,7 @@
 
 #include "zeek/iosource/Manager.h"
 
-#include <assert.h>
+#include <cassert>
 // These two files have to remain in the same order or FreeBSD builds
 // stop working.
 // clang-format off
diff --git a/src/iosource/Packet.h b/src/iosource/Packet.h
index c93a39a82e..dd8a7430c7 100644
--- a/src/iosource/Packet.h
+++ b/src/iosource/Packet.h
@@ -2,8 +2,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdint.h>
 #include <sys/types.h> // for u_char
+#include <cstdint>
 #include <string>
 
 #if defined(__OpenBSD__)
diff --git a/src/iosource/pcap/Dumper.cc b/src/iosource/pcap/Dumper.cc
index 3cf7a0dcf7..3e7b5e32bb 100644
--- a/src/iosource/pcap/Dumper.cc
+++ b/src/iosource/pcap/Dumper.cc
@@ -2,8 +2,8 @@
 
 #include "zeek/iosource/pcap/Dumper.h"
 
-#include <errno.h>
 #include <sys/stat.h>
+#include <cerrno>
 
 #include "zeek/RunState.h"
 #include "zeek/iosource/PktSrc.h"
diff --git a/src/logging/writers/ascii/Ascii.cc b/src/logging/writers/ascii/Ascii.cc
index 269eef5277..7df607a115 100644
--- a/src/logging/writers/ascii/Ascii.cc
+++ b/src/logging/writers/ascii/Ascii.cc
@@ -3,11 +3,11 @@
 #include "zeek/logging/writers/ascii/Ascii.h"
 
 #include <dirent.h>
-#include <errno.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <cerrno>
 #include <cstdio>
 #include <ctime>
 #include <memory>
diff --git a/src/logging/writers/sqlite/SQLite.cc b/src/logging/writers/sqlite/SQLite.cc
index 440c62d01c..4fd2e54e90 100644
--- a/src/logging/writers/sqlite/SQLite.cc
+++ b/src/logging/writers/sqlite/SQLite.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <errno.h>
+#include <cerrno>
 #include <string>
 #include <vector>
 
diff --git a/src/module_util.cc b/src/module_util.cc
index 121907546b..58c72dbf3b 100644
--- a/src/module_util.cc
+++ b/src/module_util.cc
@@ -3,7 +3,7 @@
 
 #include "zeek/module_util.h"
 
-#include <string.h>
+#include <cstring>
 #include <iostream>
 #include <string>
 
diff --git a/src/net_util.h b/src/net_util.h
index 90a22f0d50..f9016c930d 100644
--- a/src/net_util.h
+++ b/src/net_util.h
@@ -27,7 +27,7 @@ enum IPFamily
 // clang-format off
 #include <sys/types.h>
 #include <arpa/inet.h>
-#include <assert.h>
+#include <cassert>
 #include <netinet/in.h>
 #include <netinet/in_systm.h>
 #include <netinet/ip.h>
diff --git a/src/parse.y b/src/parse.y
index d2bb078322..9d529698b1 100644
--- a/src/parse.y
+++ b/src/parse.y
@@ -78,9 +78,9 @@
 %type <when_clause> when_head when_start when_clause
 
 %{
-#include <stdlib.h>
-#include <string.h>
-#include <assert.h>
+#include <cstdlib>
+#include <cstring>
+#include <cassert>
 
 #include <set>
 #include <string>
diff --git a/src/plugin/Manager.cc b/src/plugin/Manager.cc
index 64cfcbf151..ff044ddbe2 100644
--- a/src/plugin/Manager.cc
+++ b/src/plugin/Manager.cc
@@ -4,10 +4,10 @@
 
 #include <dirent.h>
 #include <dlfcn.h>
-#include <errno.h>
 #include <glob.h>
-#include <limits.h> // for PATH_MAX
 #include <sys/stat.h>
+#include <cerrno>
+#include <climits> // for PATH_MAX
 #include <cstdlib>
 #include <fstream>
 #include <optional>
diff --git a/src/probabilistic/CardinalityCounter.cc b/src/probabilistic/CardinalityCounter.cc
index b203fa6642..8a58c4b25e 100644
--- a/src/probabilistic/CardinalityCounter.cc
+++ b/src/probabilistic/CardinalityCounter.cc
@@ -3,8 +3,8 @@
 #include "zeek/probabilistic/CardinalityCounter.h"
 
 #include <broker/data.hh>
-#include <math.h>
-#include <stdint.h>
+#include <cmath>
+#include <cstdint>
 #include <utility>
 
 #include "zeek/Reporter.h"
diff --git a/src/probabilistic/CardinalityCounter.h b/src/probabilistic/CardinalityCounter.h
index 205988cc72..8088f43f76 100644
--- a/src/probabilistic/CardinalityCounter.h
+++ b/src/probabilistic/CardinalityCounter.h
@@ -3,7 +3,7 @@
 #pragma once
 
 #include <broker/expected.hh>
-#include <stdint.h>
+#include <cstdint>
 #include <memory>
 #include <vector>
 
diff --git a/src/re-parse.y b/src/re-parse.y
index dc4695d924..75d1ba5350 100644
--- a/src/re-parse.y
+++ b/src/re-parse.y
@@ -1,7 +1,7 @@
 // parse.y - parser for flex input
 
 %{
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "zeek/RE.h"
 #include "zeek/CCL.h"
diff --git a/src/rule-scan.l b/src/rule-scan.l
index 128292043d..a20e21751b 100644
--- a/src/rule-scan.l
+++ b/src/rule-scan.l
@@ -1,5 +1,5 @@
 %{
-#include <string.h>
+#include <cstring>
 #include <string>
 #include <sys/types.h>
 #include <sys/socket.h>
@@ -247,4 +247,3 @@ void rules_parse_input()
 	rules_parse();
 	yy_delete_buffer(rules_buffer);
 	}
-
diff --git a/src/scan.l b/src/scan.l
index cca6ddd0f5..6b4b10fde6 100644
--- a/src/scan.l
+++ b/src/scan.l
@@ -1,8 +1,7 @@
 %{
 // See the file "COPYING" in the main distribution directory for copyright.
 
-#include <errno.h>
-
+#include <cerrno>
 #include <climits>
 #include <cstdlib>
 
diff --git a/src/script_opt/CPP/Driver.cc b/src/script_opt/CPP/Driver.cc
index 8f27fbbf59..9f70c84d66 100644
--- a/src/script_opt/CPP/Driver.cc
+++ b/src/script_opt/CPP/Driver.cc
@@ -1,8 +1,8 @@
 // See the file "COPYING" in the main distribution directory for copyright.
 
-#include <errno.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <cerrno>
 
 #include "zeek/script_opt/CPP/Compile.h"
 
diff --git a/src/script_opt/CPP/Util.cc b/src/script_opt/CPP/Util.cc
index 510cfd7a00..dad23c8704 100644
--- a/src/script_opt/CPP/Util.cc
+++ b/src/script_opt/CPP/Util.cc
@@ -2,9 +2,9 @@
 
 #include "zeek/script_opt/CPP/Util.h"
 
-#include <errno.h>
 #include <sys/file.h>
 #include <unistd.h>
+#include <cerrno>
 
 #include "zeek/script_opt/StmtOptInfo.h"
 
diff --git a/src/session/Manager.cc b/src/session/Manager.cc
index 3ed9d9741d..062ff83d2f 100644
--- a/src/session/Manager.cc
+++ b/src/session/Manager.cc
@@ -7,8 +7,8 @@
 #include <arpa/inet.h>
 #include <netinet/in.h>
 #include <pcap.h>
-#include <stdlib.h>
 #include <unistd.h>
+#include <cstdlib>
 
 #include "zeek/Desc.h"
 #include "zeek/Event.h"
diff --git a/src/threading/BasicThread.cc b/src/threading/BasicThread.cc
index 8385c5264f..acb1b0087d 100644
--- a/src/threading/BasicThread.cc
+++ b/src/threading/BasicThread.cc
@@ -3,7 +3,7 @@
 #include "zeek/zeek-config.h"
 
 #include <pthread.h>
-#include <signal.h>
+#include <csignal>
 
 #include "zeek/threading/Manager.h"
 #include "zeek/util.h"
diff --git a/src/threading/BasicThread.h b/src/threading/BasicThread.h
index f665462cbe..9d78e8dd05 100644
--- a/src/threading/BasicThread.h
+++ b/src/threading/BasicThread.h
@@ -3,8 +3,8 @@
 
 #include "zeek/zeek-config.h"
 
-#include <stdint.h>
 #include <atomic>
+#include <cstdint>
 #include <iosfwd>
 #include <thread>
 
diff --git a/src/threading/Formatter.cc b/src/threading/Formatter.cc
index 00975567f2..784aa9033f 100644
--- a/src/threading/Formatter.cc
+++ b/src/threading/Formatter.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <errno.h>
+#include <cerrno>
 
 #include "zeek/3rdparty/bro_inet_ntop.h"
 #include "zeek/threading/MsgThread.h"
diff --git a/src/threading/MsgThread.cc b/src/threading/MsgThread.cc
index f8fceae59c..f4c0f94acb 100644
--- a/src/threading/MsgThread.cc
+++ b/src/threading/MsgThread.cc
@@ -1,8 +1,8 @@
 #include "zeek/threading/MsgThread.h"
 
 #include <fcntl.h>
-#include <signal.h>
 #include <unistd.h>
+#include <csignal>
 
 #include "zeek/DebugLogger.h"
 #include "zeek/Desc.h"
diff --git a/src/threading/Queue.h b/src/threading/Queue.h
index b005476f57..69d53f1606 100644
--- a/src/threading/Queue.h
+++ b/src/threading/Queue.h
@@ -1,8 +1,8 @@
 #pragma once
 
-#include <stdint.h>
 #include <sys/time.h>
 #include <condition_variable>
+#include <cstdint>
 #include <deque>
 #include <mutex>
 #include <queue>
diff --git a/src/threading/formatters/Ascii.cc b/src/threading/formatters/Ascii.cc
index d475f11399..cb76083b5a 100644
--- a/src/threading/formatters/Ascii.cc
+++ b/src/threading/formatters/Ascii.cc
@@ -4,7 +4,7 @@
 
 #include "zeek/zeek-config.h"
 
-#include <errno.h>
+#include <cerrno>
 #include <sstream>
 
 #include "zeek/Desc.h"
diff --git a/src/threading/formatters/JSON.cc b/src/threading/formatters/JSON.cc
index 774b231f39..2fe9e646fe 100644
--- a/src/threading/formatters/JSON.cc
+++ b/src/threading/formatters/JSON.cc
@@ -8,10 +8,10 @@
 #define __STDC_LIMIT_MACROS
 #endif
 
-#include <errno.h>
-#include <math.h>
 #include <rapidjson/internal/ieee754.h>
-#include <stdint.h>
+#include <cerrno>
+#include <cmath>
+#include <cstdint>
 #include <sstream>
 
 #include "zeek/Desc.h"
diff --git a/src/util.cc b/src/util.cc
index 830ceaf81b..02c4b0dbbe 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -11,20 +11,20 @@
 #include <mach/task.h>
 #endif
 
-#include <ctype.h>
-#include <errno.h>
 #include <fcntl.h>
 #include <libgen.h>
 #include <openssl/md5.h>
 #include <openssl/sha.h>
-#include <signal.h>
-#include <stdarg.h>
-#include <stdio.h>
-#include <stdlib.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <cctype>
+#include <cerrno>
+#include <csignal>
+#include <cstdarg>
+#include <cstdio>
+#include <cstdlib>
 
 #if defined(HAVE_MALLINFO) || defined(HAVE_MALLINFO2)
 #include <malloc.h>
diff --git a/src/util.h b/src/util.h
index 936f7424d7..af1fa3646c 100644
--- a/src/util.h
+++ b/src/util.h
@@ -15,13 +15,13 @@
 #endif
 
 #include <libgen.h>
-#include <stdarg.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 #include <array>
 #include <cinttypes>
+#include <cstdarg>
 #include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <memory> // std::unique_ptr
 #include <string>
 #include <string_view>
@@ -29,18 +29,18 @@
 
 #ifdef TIME_WITH_SYS_TIME
 #include <sys/time.h>
-#include <time.h>
+#include <ctime>
 #else
 #ifdef HAVE_SYS_TIME_H
 #include <sys/time.h>
 #else
-#include <time.h>
+#include <ctime>
 #endif
 #endif
 
 #ifdef DEBUG
 
-#include <assert.h>
+#include <cassert>
 
 #define ASSERT(x) assert(x)
 #define DEBUG_MSG(x...) fprintf(stderr, x)
diff --git a/src/zeek-setup.cc b/src/zeek-setup.cc
index 20b9fc5cd3..ca7e5333e3 100644
--- a/src/zeek-setup.cc
+++ b/src/zeek-setup.cc
@@ -7,12 +7,12 @@
 #include <openssl/err.h>
 #include <openssl/opensslv.h>
 #include <openssl/ssl.h>
-#include <signal.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <csignal>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
 #include <list>
 #include <optional>
 
diff --git a/src/zeek.bif b/src/zeek.bif
index 68c4ff2a13..7c6773105b 100644
--- a/src/zeek.bif
+++ b/src/zeek.bif
@@ -8,13 +8,13 @@
 ##! in this file.
 
 %%{ // C segment
-#include <math.h>
+#include <cmath>
 #include <vector>
 #include <algorithm>
 #include <cmath>
 #include <sys/stat.h>
 #include <cstdio>
-#include <time.h>
+#include <ctime>
 
 #include "zeek/digest.h"
 #include "zeek/Reporter.h"
diff --git a/src/zeekygen/Configuration.cc b/src/zeekygen/Configuration.cc
index 97f8aa245a..879aa98203 100644
--- a/src/zeekygen/Configuration.cc
+++ b/src/zeekygen/Configuration.cc
@@ -2,8 +2,8 @@
 
 #include "zeek/zeekygen/Configuration.h"
 
-#include <errno.h>
 #include <algorithm>
+#include <cerrno>
 #include <fstream>
 #include <vector>
 
diff --git a/src/zeekygen/Manager.h b/src/zeekygen/Manager.h
index 772079df3d..a0023dc88e 100644
--- a/src/zeekygen/Manager.h
+++ b/src/zeekygen/Manager.h
@@ -2,8 +2,8 @@
 
 #pragma once
 
-#include <errno.h>
 #include <sys/stat.h>
+#include <cerrno>
 #include <ctime>
 #include <map>
 #include <string>
diff --git a/src/zeekygen/PackageInfo.cc b/src/zeekygen/PackageInfo.cc
index f7ef9b1734..db93b2f517 100644
--- a/src/zeekygen/PackageInfo.cc
+++ b/src/zeekygen/PackageInfo.cc
@@ -2,7 +2,7 @@
 
 #include "zeek/zeekygen/PackageInfo.h"
 
-#include <errno.h>
+#include <cerrno>
 #include <fstream>
 
 #include "zeek/Reporter.h"
diff --git a/src/zeekygen/ReStructuredTextTable.cc b/src/zeekygen/ReStructuredTextTable.cc
index 2f54c37823..ed58960aa4 100644
--- a/src/zeekygen/ReStructuredTextTable.cc
+++ b/src/zeekygen/ReStructuredTextTable.cc
@@ -2,7 +2,7 @@
 
 #include "zeek/zeekygen/ReStructuredTextTable.h"
 
-#include <assert.h>
+#include <cassert>
 
 using namespace std;
 
diff --git a/src/zeekygen/utils.cc b/src/zeekygen/utils.cc
index b79e91fe88..f76cca081a 100644
--- a/src/zeekygen/utils.cc
+++ b/src/zeekygen/utils.cc
@@ -2,8 +2,8 @@
 
 #include "zeek/zeekygen/utils.h"
 
-#include <errno.h>
 #include <sys/stat.h>
+#include <cerrno>
 
 #include "zeek/Func.h"
 #include "zeek/ID.h"
diff --git a/testing/btest/plugins/pktdumper-plugin/src/Foo.cc b/testing/btest/plugins/pktdumper-plugin/src/Foo.cc
index 9f0ad925dd..9c69df4088 100644
--- a/testing/btest/plugins/pktdumper-plugin/src/Foo.cc
+++ b/testing/btest/plugins/pktdumper-plugin/src/Foo.cc
@@ -4,7 +4,7 @@
 #include "RunState.h"
 
 #include <fcntl.h>
-#include <stdio.h>
+#include <cstdio>
 
 using namespace btest::plugin::Demo_Foo;
 
diff --git a/testing/btest/plugins/pktsrc-plugin/src/Foo.cc b/testing/btest/plugins/pktsrc-plugin/src/Foo.cc
index 0b2eb1d2ca..b0d743724a 100644
--- a/testing/btest/plugins/pktsrc-plugin/src/Foo.cc
+++ b/testing/btest/plugins/pktsrc-plugin/src/Foo.cc
@@ -6,7 +6,7 @@ extern "C" {
 }
 
 #include <fcntl.h>
-#include <stdio.h>
+#include <cstdio>
 
 using namespace btest::plugin::Demo_Foo;
 
diff --git a/testing/btest/plugins/reader-plugin/src/Foo.cc b/testing/btest/plugins/reader-plugin/src/Foo.cc
index 8024373833..67e83784b7 100644
--- a/testing/btest/plugins/reader-plugin/src/Foo.cc
+++ b/testing/btest/plugins/reader-plugin/src/Foo.cc
@@ -3,7 +3,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
-#include <errno.h>
+#include <cerrno>
 
 #include "Foo.h"