diff --git a/src/MANYBODY/pair_edip.cpp b/src/MANYBODY/pair_edip.cpp
index 837ec2a3c7..0efa509cb3 100644
--- a/src/MANYBODY/pair_edip.cpp
+++ b/src/MANYBODY/pair_edip.cpp
@@ -527,7 +527,7 @@ void PairEDIP::allocateGrids(void)
    pre-calculated structures
 ------------------------------------------------------------------------- */
 
-void PairEDIP::allocatePreLoops(void)
+void PairEDIP::allocatePreLoops()
 {
   int nthreads = comm->nthreads;
 
@@ -545,7 +545,7 @@ void PairEDIP::allocatePreLoops(void)
    deallocate grids
 ------------------------------------------------------------------------- */
 
-void PairEDIP::deallocateGrids(void)
+void PairEDIP::deallocateGrids()
 {
   memory->destroy(cutoffFunction);
   memory->destroy(cutoffFunctionDerived);
@@ -562,7 +562,7 @@ void PairEDIP::deallocateGrids(void)
    deallocate preLoops
 ------------------------------------------------------------------------- */
 
-void PairEDIP::deallocatePreLoops(void)
+void PairEDIP::deallocatePreLoops()
 {
   memory->destroy(preInvR_ij);
   memory->destroy(preExp3B_ij);
@@ -597,7 +597,7 @@ void PairEDIP::settings(int narg, char ** /*arg*/)
 
 /* ---------------------------------------------------------------------- */
 
-void PairEDIP::initGrids(void)
+void PairEDIP::initGrids()
 {
   int l;
   int numGridPointsOneCutoffFunction;
diff --git a/src/MANYBODY/pair_tersoff_table.cpp b/src/MANYBODY/pair_tersoff_table.cpp
index 94c4d88b09..752a47f128 100644
--- a/src/MANYBODY/pair_tersoff_table.cpp
+++ b/src/MANYBODY/pair_tersoff_table.cpp
@@ -506,7 +506,7 @@ void PairTersoffTable::compute(int eflag, int vflag)
 
 /* ---------------------------------------------------------------------- */
 
-void PairTersoffTable::deallocatePreLoops(void)
+void PairTersoffTable::deallocatePreLoops()
 {
     memory->destroy(preGtetaFunction);
     memory->destroy(preGtetaFunctionDerived);
@@ -514,7 +514,7 @@ void PairTersoffTable::deallocatePreLoops(void)
     memory->destroy(preCutoffFunctionDerived);
 }
 
-void PairTersoffTable::allocatePreLoops(void)
+void PairTersoffTable::allocatePreLoops()
 {
   deallocatePreLoops();
   memory->create(preGtetaFunction,leadingDimensionInteractionList,
@@ -538,7 +538,7 @@ void PairTersoffTable::deallocateGrids()
   memory->destroy(betaZetaPowerDerived);
 }
 
-void PairTersoffTable::allocateGrids(void)
+void PairTersoffTable::allocateGrids()
 {
   int   i, j, k, l;
 
diff --git a/src/MC/fix_gcmc.cpp b/src/MC/fix_gcmc.cpp
index 3420c23e82..38007b8e2e 100644
--- a/src/MC/fix_gcmc.cpp
+++ b/src/MC/fix_gcmc.cpp
@@ -68,9 +68,9 @@ enum{MOVEATOM,MOVEMOL}; // movemode
 
 FixGCMC::FixGCMC(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  idregion(nullptr), full_flag(0), ngroups(0), groupstrings(nullptr), ngrouptypes(0), grouptypestrings(nullptr),
-  grouptypebits(nullptr), grouptypes(nullptr), local_gas_list(nullptr), molcoords(nullptr), molq(nullptr), molimage(nullptr),
-  random_equal(nullptr), random_unequal(nullptr),
+  idregion(nullptr), full_flag(false), ngroups(0), groupstrings(nullptr), ngrouptypes(0),
+  grouptypestrings(nullptr), grouptypebits(nullptr), grouptypes(nullptr), local_gas_list(nullptr),
+  molcoords(nullptr), molq(nullptr), molimage(nullptr), random_equal(nullptr), random_unequal(nullptr),
   fixrigid(nullptr), fixshake(nullptr), idrigid(nullptr), idshake(nullptr)
 {
   if (narg < 11) error->all(FLERR,"Illegal fix gcmc command");
diff --git a/src/MC/fix_widom.cpp b/src/MC/fix_widom.cpp
index f3204ca800..cc0ea7981e 100644
--- a/src/MC/fix_widom.cpp
+++ b/src/MC/fix_widom.cpp
@@ -59,9 +59,8 @@ enum{EXCHATOM,EXCHMOL}; // exchmode
 
 FixWidom::FixWidom(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  idregion(nullptr), full_flag(0),
-  local_gas_list(nullptr), molcoords(nullptr), molq(nullptr), molimage(nullptr),
-  random_equal(nullptr)
+  idregion(nullptr), full_flag(false), local_gas_list(nullptr), molcoords(nullptr),
+  molq(nullptr), molimage(nullptr), random_equal(nullptr)
 {
   if (narg < 8) error->all(FLERR,"Illegal fix widom command");
 
diff --git a/src/MISC/fix_imd.cpp b/src/MISC/fix_imd.cpp
index 199cabc644..87c6886864 100644
--- a/src/MISC/fix_imd.cpp
+++ b/src/MISC/fix_imd.cpp
@@ -75,7 +75,7 @@ negotiate an appropriate license for such distribution."
 #include <sys/file.h>
 #endif
 
-#include <errno.h>
+#include <cerrno>
 
 using namespace LAMMPS_NS;
 using namespace FixConst;
diff --git a/src/MISC/fix_pair_tracker.cpp b/src/MISC/fix_pair_tracker.cpp
index 755025baed..626b148be2 100644
--- a/src/MISC/fix_pair_tracker.cpp
+++ b/src/MISC/fix_pair_tracker.cpp
@@ -12,6 +12,7 @@
 ------------------------------------------------------------------------- */
 
 #include "fix_pair_tracker.h"
+
 #include "atom.h"
 #include "atom_vec.h"
 #include "error.h"
@@ -21,7 +22,7 @@
 #include "tokenizer.h"
 #include "update.h"
 
-#include <string.h>
+#include <cstring>
 
 using namespace LAMMPS_NS;
 using namespace FixConst;
@@ -31,7 +32,7 @@ using namespace FixConst;
 /* ---------------------------------------------------------------------- */
 
 FixPairTracker::FixPairTracker(LAMMPS *lmp, int narg, char **arg) :
-    Fix(lmp, narg, arg), nvalues(0), vector(NULL), array(NULL), pack_choice(NULL)
+    Fix(lmp, narg, arg), nvalues(0), vector(nullptr), array(nullptr), pack_choice(nullptr)
 {
   if (narg < 3) error->all(FLERR, "Illegal fix pair/tracker command");
   local_flag = 1;
@@ -140,8 +141,8 @@ FixPairTracker::FixPairTracker(LAMMPS *lmp, int narg, char **arg) :
 
   nmax = 0;
   ncount = 0;
-  vector = NULL;
-  array = NULL;
+  vector = nullptr;
+  array = nullptr;
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/input.cpp b/src/input.cpp
index 3a17b3f533..6cb0a56585 100644
--- a/src/input.cpp
+++ b/src/input.cpp
@@ -46,7 +46,7 @@
 #include "variable.h"
 
 #include <cstring>
-#include <errno.h>
+#include <cerrno>
 #include <cctype>
 #include <sys/stat.h>
 #include <unistd.h>