get_config: fix locale-dependent decimal separator in %.17e output

snprintf respects LC_NUMERIC; in de_DE and similar locales it replaces
'.' with ',' in float output, producing strings like "8,00e+00" that
are not valid TOML.

Add h5z_zfp_fmt_double() helper that calls snprintf("%.17e") then
replaces the first comma with a dot. Since %.17e places exactly one
decimal separator in the mantissa, a single-pass scan suffices. Use
it at all four float-formatting sites (rate and accuracy in both the
memory-format and header-format paths of get_config).

Author: brtnfld

src/H5Zzfp.c

@@ -1053,6 +1053,21 @@ H5Z_zfp_set_config(const char *params, unsigned *flags,
  * value as a float rather than an integer (%.17g drops the decimal point for
  * whole numbers like 8.0 → "8", which TOML would read as an integer).
  * --------------------------------------------------------------------------- */
+/* Format a double as %.17e into buf[n] with the decimal separator always '.'
+ * regardless of LC_NUMERIC locale.  Returns the snprintf-style character count
+ * (negative on encoding error, >= n if the buffer was too small). */
+static int
+h5z_zfp_fmt_double(char *buf, size_t n, double val)
+{
+    int   r = snprintf(buf, n, "%.17e", val);
+    char *p;
+    /* Some locales (e.g. de_DE) replace '.' with ',' in printf float output.
+     * TOML requires '.'; scan and fix the one decimal separator if present. */
+    for (p = buf; *p; p++)
+        if (*p == ',') { *p = '.'; break; }
+    return r;
+}
+
 static herr_t
 H5Z_zfp_get_config(unsigned flags, size_t cd_nelmts, const unsigned cd_values[],
     char *buf, size_t *buf_size)
@@ -1089,7 +1104,8 @@ H5Z_zfp_get_config(unsigned flags, size_t cd_nelmts, const unsigned cd_values[],
                     return -1;
                 }
                 memcpy(&rate, &cd_values[2], sizeof(double));
-                nchars = snprintf(tmp, sizeof(tmp), "mode = \"rate\", rate = %.17e", rate);
+                { char d[32]; h5z_zfp_fmt_double(d, sizeof(d), rate);
+                  nchars = snprintf(tmp, sizeof(tmp), "mode = \"rate\", rate = %s", d); }
                 break;
             }
 
@@ -1111,7 +1127,8 @@ H5Z_zfp_get_config(unsigned flags, size_t cd_nelmts, const unsigned cd_values[],
                     return -1;
                 }
                 memcpy(&acc, &cd_values[2], sizeof(double));
-                nchars = snprintf(tmp, sizeof(tmp), "mode = \"accuracy\", acc = %.17e", acc);
+                { char d[32]; h5z_zfp_fmt_double(d, sizeof(d), acc);
+                  nchars = snprintf(tmp, sizeof(tmp), "mode = \"accuracy\", acc = %s", d); }
                 break;
             }
 
@@ -1187,7 +1204,8 @@ H5Z_zfp_get_config(unsigned flags, size_t cd_nelmts, const unsigned cd_values[],
                     zfp_type ztype = Z zfp_field_type(zfld);
                     double rate = Z zfp_stream_rate(zstr, ztype, zdims);
 #endif
-                    nchars = snprintf(tmp, sizeof(tmp), "mode = \"rate\", rate = %.17e", rate);
+                    { char d[32]; h5z_zfp_fmt_double(d, sizeof(d), rate);
+                      nchars = snprintf(tmp, sizeof(tmp), "mode = \"rate\", rate = %s", d); }
                     rc = 0;
                     break;
                 }
@@ -1199,7 +1217,8 @@ H5Z_zfp_get_config(unsigned flags, size_t cd_nelmts, const unsigned cd_values[],
                 }
                 case zfp_mode_fixed_accuracy: {
                     double acc = Z zfp_stream_accuracy(zstr);
-                    nchars = snprintf(tmp, sizeof(tmp), "mode = \"accuracy\", acc = %.17e", acc);
+                    { char d[32]; h5z_zfp_fmt_double(d, sizeof(d), acc);
+                      nchars = snprintf(tmp, sizeof(tmp), "mode = \"accuracy\", acc = %s", d); }
                     rc = 0;
                     break;
                 }