fix(printf): fix snprintf buffer termination

Fixes #7 (partly)

printf.c

@@ -355,13 +355,13 @@ static size_t _vsnprintf(char* buffer, size_t buffer_len, const char* format, va
   unsigned int flags, width, precision, n;
   size_t idx = 0U;
 
-  while (idx < buffer_len) {
-    // end reached?
-    if (*format == (char)0) {
-      buffer[idx] = (char)0;
-      break;
-    }
+  // check if buffer is valid
+  if (!buffer) {
+    return 0U;
+  }
 
+  while ((idx < buffer_len) && *format)
+  {
     // format specifier?  %[flags][width][.precision][length]
     if (*format != '%') {
       // no
@@ -582,9 +582,16 @@ static size_t _vsnprintf(char* buffer, size_t buffer_len, const char* format, va
     }
   }
 
+  // termination
+  if (buffer_len > 0U) {
+    buffer[idx == buffer_len ? buffer_len - 1U : idx] = (char)0;
+  }
+
+  // return written chars without terminating \0
   return idx;
 }
 
+
 ///////////////////////////////////////////////////////////////////////////////
 
 int printf(const char* format, ...)

test/test_suite.cpp

@@ -1017,6 +1017,59 @@ TEST_CASE("unknown flag", "[]" ) {
 }
 
 
+TEST_CASE("buffer length", "[]" ) {
+  char buffer[100];
+  int ret;
+
+  // formatted length, this should return '4',
+  // but this feature is not implemented, returning 0
+  ret = test::snprintf(nullptr, 10, "%s", "Test");
+  REQUIRE(ret == 0);
+  ret = test::snprintf(nullptr, 0, "%s", "Test");
+  REQUIRE(ret == 0);
+
+  buffer[0] = (char)0xA5;
+  ret = test::snprintf(buffer, 0, "%s", "Test");
+  REQUIRE(buffer[0] == (char)0xA5);
+  REQUIRE(ret == 0);
+
+  buffer[0] = 0xCC;
+  test::snprintf(buffer, 1, "%s", "Test");
+  REQUIRE(buffer[0] == '\0');
+
+  test::snprintf(buffer, 2, "%s", "Hello");
+  REQUIRE(!strcmp(buffer, "H"));
+}
+
+
+TEST_CASE("ret value", "[]" ) {
+  char buffer[100] ;
+  int ret;
+
+  ret = test::snprintf(buffer, 6, "0%s", "1234");
+  REQUIRE(!strcmp(buffer, "01234"));
+  REQUIRE(ret == 5);
+
+  ret = test::snprintf(buffer, 6, "0%s", "12345");
+  REQUIRE(!strcmp(buffer, "01234"));
+  REQUIRE(ret == 6);  // '5' is truncated
+
+  ret = test::snprintf(buffer, 6, "0%s", "1234567");
+  REQUIRE(!strcmp(buffer, "01234"));
+  REQUIRE(ret == 6);  // '567' are truncated
+
+  ret = test::snprintf(buffer, 10, "hello, world");
+  REQUIRE(ret == 10);
+
+  ret = test::snprintf(buffer, 3, "%d", 10000);
+  REQUIRE(ret == 3);  // '000' are truncated
+  REQUIRE(strlen(buffer) == 2U);
+  REQUIRE(buffer[0] == '1');
+  REQUIRE(buffer[1] == '0');
+  REQUIRE(buffer[2] == '\0');
+}
+
+
 TEST_CASE("misc", "[]" ) {
   char buffer[100];