Index: flang/unittests/Decimal/quick-sanity-test.cpp
===================================================================
--- flang/unittests/Decimal/quick-sanity-test.cpp
+++ flang/unittests/Decimal/quick-sanity-test.cpp
@@ -53,7 +53,8 @@
       ++expo;
     }
     if (q >= buffer && q < buffer + sizeof buffer) {
-      std::snprintf(q + result.length, sizeof buffer, "e%d", expo);
+      std::snprintf(q + result.length,
+          buffer + sizeof buffer - (q + result.length), "e%d", expo);
     }
     const char *p{q};
     auto rflags{ConvertDecimalToFloat(&p, &y, RoundNearest)};
Index: flang/unittests/Decimal/thorough-test.cpp
===================================================================
--- flang/unittests/Decimal/thorough-test.cpp
+++ flang/unittests/Decimal/thorough-test.cpp
@@ -51,7 +51,8 @@
       if (*q == '-' || *q == '+') {
         ++expo;
       }
-      std::snprintf(q + result.length, sizeof buffer, "e%d", expo);
+      std::snprintf(q + result.length,
+          buffer + sizeof buffer - (q + result.length), "e%d", expo);
     }
     const char *p{q};
     auto rflags{ConvertDecimalToFloat(&p, &y, RoundNearest)};