Fixed bacnet_snprintf() to be able to return the full length of a string when the size is zero. (#1164)

CHANGELOG.md

@@ -24,7 +24,7 @@ The git repositories are hosted at the following sites:
   Integrated the new types into the application data framework with
   application tag assignments. (#1163)
 * Added library specific string manipulation utilities including strcmp,
-  strncmp, and snprintf with offset functions. (#1163)
+  strncmp, and snprintf with offset functions. (#1163)(#1164)
 * Added default option to bactext name functions so that NULL can be
   returned when a name does not exist. (#1160)
 * Added library specific ltrim, rtrim, and trim string functions. (#1159)

src/bacnet/bacstr.c

@@ -2019,7 +2019,7 @@ char *bacnet_stptok(const char *s, char *tok, size_t toklen, const char *brk)
 /**
  * @brief General purpose print formatter snprintf() function with offset
  * @param buffer - destination string
- * @param size - length of the destination string
+ * @param size - length of the destination string, or zero for length
  * @param offset - offset into the buffer to start the string
  * @param format - format string
  * @return total number of characters written from the beginning of buffer
@@ -2027,19 +2027,35 @@ char *bacnet_stptok(const char *s, char *tok, size_t toklen, const char *brk)
 int bacnet_snprintf(
     char *buffer, size_t size, int offset, const char *format, ...)
 {
-    int length = 0;
+    int length = 0, write_length = 0;
     va_list args;
+    char *write_buffer = NULL;
+    size_t write_size = 0;
 
+    if (offset < 0) {
+        return offset;
+    }
     if (offset < size) {
-        size -= offset;
+        write_size = size - offset;
         if (buffer) {
-            buffer += offset;
+            write_buffer = buffer + offset;
+        }
+    } else if (size == 0) {
+        write_size = 0;
+        /* when size is zero, nothing is written to the buffer */
+    } else {
+        return size;
+    }
+    va_start(args, format);
+    length = vsnprintf(write_buffer, write_size, format, args);
+    va_end(args);
+    write_length = offset + length;
+    if (size != 0) {
+        /* limit the return value to the size of the buffer */
+        if (write_length > size) {
+            write_length = size;
         }
-        va_start(args, format);
-        length = vsnprintf(buffer, size, format, args);
-        va_end(args);
-        return offset + length;
     }
 
-    return size;
+    return write_length;
 }

test/bacnet/bacdest/src/main.c

@@ -121,7 +121,7 @@ static void test_BACnetDestination_ASCII(void)
     zassert_true(len > 0, NULL);
     status = bacnet_destination_same(&destination, &test_destination);
     zassert_true(status, NULL);
-    /* get the length */
+    /* get the length without NULL termination */
     null_len = bacnet_destination_to_ascii(&test_destination, NULL, 0);
     if (null_len > 0) {
         test_ascii = calloc(null_len, sizeof(char));
@@ -132,7 +132,15 @@ static void test_BACnetDestination_ASCII(void)
             while (--test_len) {
                 len = bacnet_destination_to_ascii(
                     &test_destination, test_ascii, test_len);
-                zassert_equal(len, null_len, NULL);
+                if (test_len > 0) {
+                    zassert_equal(
+                        len, test_len, "%s len=%d test_len=%d", test_ascii, len,
+                        test_len);
+                } else {
+                    zassert_equal(
+                        len, null_len, "%s len=%d null_len=%d", test_ascii, len,
+                        null_len);
+                }
             }
             free(test_ascii);
         }

test/bacnet/bacstr/src/main.c

@@ -790,10 +790,27 @@ ZTEST(bacstr_tests, test_bacnet_snprintf)
 static void test_bacnet_snprintf(void)
 #endif
 {
-    int buf_len = 0, str_len;
+    int buf_len = 0, str_len, null_len = 0, test_null_len = 0;
     int i;
     char str[30] = "";
+    const char *null_string = "REALLY BIG NULL STRING";
+    const char *one_char_string = "1";
+    const char *two_char_string = "12";
 
+    /* one char */
+    buf_len = 0;
+    str_len = 1;
+    buf_len = bacnet_snprintf(str, str_len, buf_len, "%s", one_char_string);
+    zassert_equal(buf_len, str_len, "buf_len=%d str_len=%d", buf_len, str_len);
+    /* two char */
+    buf_len = 0;
+    str_len = 2;
+    buf_len = bacnet_snprintf(str, str_len, buf_len, "%s", two_char_string);
+    zassert_equal(buf_len, str_len, "buf_len=%d str_len=%d", buf_len, str_len);
+    buf_len = bacnet_snprintf(str, str_len, buf_len, "%s", two_char_string);
+    zassert_equal(buf_len, str_len, "buf_len=%d str_len=%d", buf_len, str_len);
+    /* large chars */
+    buf_len = 0;
     str_len = sizeof(str);
     for (i = 0; i < 5; i++) {
         /* appending formatted strings */
@@ -801,10 +818,14 @@ static void test_bacnet_snprintf(void)
         buf_len =
             bacnet_snprintf(str, str_len, buf_len, "REALLY BIG STRING BASS");
         buf_len = bacnet_snprintf(str, str_len, buf_len, "}");
+        /* appending to a NULL string for length */
+        null_len = bacnet_snprintf(NULL, 0, null_len, null_string);
+        test_null_len += strlen(null_string);
     }
     zassert_equal(buf_len, str_len, "buf_len=%d str_len=%d", buf_len, str_len);
     zassert_equal(
         str[buf_len - 1], 0, "str[%d]=%c", buf_len - 1, str[buf_len - 1]);
+    zassert_equal(null_len, test_null_len, "null_len=%d", null_len);
 }
 
 /**