plotting/grid.c, bug fix, avoid string buffer overflow

Prevent a crash of example/FFT_test.cir.
Buffer 'buf' in function drawlingrid() has been too small.
Increase 'buf' size, and add some protection.

The example file examples/plot/test-small-plot.cir
shows the limits of plotting:
  difference in two numbers at digit 14 is plotted well.
                            at digit 15 is plotted with bugs.
                            at digit 16 is reduced to integer.

consider compiling with
  -O1 -D_FORTIFY_SOURCE=2
for gcc, and
  /GS
for visual studio.

examples/plot/test-small-plot.cir

@@ -0,0 +1,116 @@
+resolution test for plotting
+.control
+let xvar = vector(4)
+let yvar = vector(4)
+
+* --------------------
+* integer
+let yvar[0] = 1.0
+let yvar[1] = 1.0
+let yvar[2] = 1.0
+let yvar[3] = 1.0
+
+plot yvar vs xvar title integer 1
+
+* --------------------
+* 14 digits
+let yvar[0] = 1.0
+let yvar[1] = 1.00000000000001
+let yvar[2] = 0.99999999999999
+let yvar[3] = 1.0
+
+plot yvar vs xvar title '14 digits'
+
+* --------------------
+* 15 digits
+let yvar[0] = 1.0
+let yvar[1] = 1.000000000000001
+let yvar[2] = 0.999999999999999
+let yvar[3] = 1.0
+
+plot yvar vs xvar title '15 digits'
+
+* --------------------
+* 16 digits
+let yvar[0] = 1.0
+let yvar[1] = 1.0000000000000001
+let yvar[2] = 0.9999999999999999
+let yvar[3] = 1.0
+
+plot yvar vs xvar title '16 digits'
+
+* --------------------
+* 14 digits plus exponent
+let yvar[0] = 1.0e-14
+let yvar[1] = 1.00000000000001e-14
+let yvar[2] = 0.99999999999999e-14
+let yvar[3] = 1.0e-14
+
+plot yvar vs xvar title '14 digits plus exponent'
+
+* --------------------
+* 15 digits plus exponent
+let yvar[0] = 1.0e-14
+let yvar[1] = 1.000000000000001e-14
+let yvar[2] = 0.999999999999999e-14
+let yvar[3] = 1.0e-14
+
+plot yvar vs xvar title '15 digits plus exponent'
+
+* --------------------
+* 16 digits plus exponent
+let yvar[0] = 1.0e-14
+let yvar[1] = 1.0000000000000001e-14
+let yvar[2] = 0.9999999999999999e-14
+let yvar[3] = 1.0e-14
+
+plot yvar vs xvar title '16 digits plus exponent'
+
+* --------------------
+* negative 14 digits plus exponent
+let yvar[0] = -1.0e-14
+let yvar[1] = -1.00000000000001e-14
+let yvar[2] = -0.99999999999999e-14
+let yvar[3] = -1.0e-14
+
+plot yvar vs xvar title 'neg 14 digits plus exponent'
+
+* --------------------
+* negative 15 digits plus exponent
+let yvar[0] = -1.0e-14
+let yvar[1] = -1.000000000000001e-14
+let yvar[2] = -0.999999999999999e-14
+let yvar[3] = -1.0e-14
+
+plot yvar vs xvar title 'neg 15 digits plus exponent'
+
+* --------------------
+* negative 16 digits plus exponent
+let yvar[0] = -1.0e-14
+let yvar[1] = -1.0000000000000001e-14
+let yvar[2] = -0.9999999999999999e-14
+let yvar[3] = -1.0e-14
+
+plot yvar vs xvar title 'neg 16 digits plus exponent'
+
+* --------------------
+* 15 digits plus big exponent
+let yvar[0] = 1.0e-200
+let yvar[1] = 1.000000000000001e-200
+let yvar[2] = 0.999999999999999e-200
+let yvar[3] = 1.0e-200
+
+plot yvar vs xvar title '15 digits plus big exponent'
+
+* --------------------
+* negative 15 digits plus big exponent
+let yvar[0] = -1.0e-200
+let yvar[1] = -1.000000000000001e-200
+let yvar[2] = -0.999999999999999e-200
+let yvar[3] = -1.0e-200
+
+plot yvar vs xvar title 'neg 15 digits plus big exponent'
+
+.endc
+
+.end

src/frontend/plotting/grid.c

@@ -365,10 +365,10 @@ lingrid(GRAPH *graph, double lo, double hi, double delta, int type, Axis axis)
         buf[0] = '\0';
     }
 
-    if ((s = ft_typabbrev(type)) != NULL)
-        (void) strncat(buf, s, sizeof(buf) - strlen(buf) - 1);
-    else
-        (void) strncat(buf, "Units", sizeof(buf) - strlen(buf) - 1);
+    s = ft_typabbrev(type);
+    if (!s)
+        s = "Units";
+    strncat(buf, s, sizeof(buf) - strlen(buf) - 1);
 
     if (delta == 0.0) {
         int     i;
@@ -527,7 +527,7 @@ drawlingrid(GRAPH *graph, char *units, int spacing, int nsp, double dst, double
         if (j == 0)
             SetLinestyle(1);
 
-        (void) sprintf(buf, "%.*f", digits + 1, m * mag / 100.0);
+        snprintf(buf, sizeof(buf), "%.*f", digits + 1, m * mag / 100.0);
 
         if (axis == x_axis)
             DevDrawText(buf, graph->viewportxoff + i -
@@ -629,10 +629,12 @@ loggrid(GRAPH *graph, double lo, double hi, int type, Axis axis)
     dd[0] = pow(10.0, (double) lmt);
     dd[1] = pow(10.0, (double) hmt);
 
-    if ((s = ft_typabbrev(type)) != NULL)
-        (void) strcpy(buf, s);
-    else
-        (void) strcpy(buf, "Units");
+    buf[0] = '\0';
+
+    s = ft_typabbrev(type);
+    if (!s)
+        s = "Units";
+    strncat(buf, s, sizeof(buf) - strlen(buf) - 1);
 
     if (axis == x_axis) {
         (void) strcpy(graph->grid.xaxis.log.units, buf);

src/include/ngspice/graph.h

@@ -17,7 +17,7 @@ Author: 1988 Jeffrey M. Hsu
 
 struct _keyed;
 
-#define GRAPH_UNITS_LENGTH  16
+#define GRAPH_UNITS_LENGTH  20
 
 /* Device-independent data structure for plots. */