diff --git a/src/compare.c b/src/compare.c
index 0e4763b33..a2a964892 100644
--- a/src/compare.c
+++ b/src/compare.c
@@ -971,7 +971,7 @@ pixCompareGray(PIX        *pix1,
                PIX       **ppixdiff)
 {
 char            buf[64];
-static l_int32  index = 0;
+static l_atomic index = 0;
 l_int32         d1, d2, same, first, last;
 GPLOT          *gplot;
 NUMA           *na, *nac;
@@ -1078,7 +1078,7 @@ pixCompareRGB(PIX        *pix1,
               PIX       **ppixdiff)
 {
 char            buf[64];
-static l_int32  index = 0;
+static l_atomic index = 0;
 l_int32         rsame, gsame, bsame, same, first, rlast, glast, blast, last;
 l_float32       rdiff, gdiff, bdiff;
 GPLOT          *gplot;
diff --git a/src/gplot.c b/src/gplot.c
index 194f4194f..26db0ff43 100644
--- a/src/gplot.c
+++ b/src/gplot.c
@@ -761,7 +761,7 @@ gplotSimplePix1(NUMA        *na,
                 const char  *title)
 {
 char            buf[64];
-static l_int32  index;
+static l_atomic index;
 GPLOT          *gplot;
 PIX            *pix;
 
@@ -803,7 +803,7 @@ gplotSimplePix2(NUMA        *na1,
                 const char  *title)
 {
 char            buf[64];
-static l_int32  index;
+static l_atomic index;
 GPLOT          *gplot;
 PIX            *pix;
 
@@ -844,7 +844,7 @@ gplotSimplePixN(NUMAA       *naa,
                 const char  *title)
 {
 char            buf[64];
-static l_int32  index;
+static l_atomic index;
 GPLOT          *gplot;
 PIX            *pix;
 
diff --git a/src/regutils.h b/src/regutils.h
index 2f1d5e4a8..d3983bed1 100644
--- a/src/regutils.h
+++ b/src/regutils.h
@@ -120,7 +120,7 @@ struct L_RegParams
     char    *testname;  /*!< name of test, without '_reg'                     */
     char    *tempfile;  /*!< name of temp file for compare mode output        */
     l_int32  mode;      /*!< generate, compare or display                     */
-    l_int32  index;     /*!< index into saved files for this test; 0-based    */
+    l_atomic index;     /*!< index into saved files for this test; 0-based    */
     l_int32  success;   /*!< overall result of the test                       */
     l_int32  display;   /*!< 1 if in display mode; 0 otherwise                */
     L_TIMER  tstart;    /*!< marks beginning of the reg test                  */
diff --git a/src/writefile.c b/src/writefile.c
index 04990260d..781e7d574 100644
--- a/src/writefile.c
+++ b/src/writefile.c
@@ -924,7 +924,7 @@ pixDisplayWithTitle(PIX         *pixs,
 {
 char           *tempname;
 char            buffer[Bufsize];
-static l_int32  index = 0;  /* caution: not .so or thread safe */
+static l_atomic index = 0;  /* caution: not .so safe */
 l_int32         w, h, d, spp, maxheight, opaque, threeviews;
 l_float32       ratw, rath, ratmin;
 PIX            *pix0, *pix1, *pix2;