minimal-printf: Fix handling of the two character sequence %%

The two character sequence %% is used in standard implementations of
printf to print a single %. This is because % is essentially printf's
escape character for format specifiers and as \% cannot work printf
uses %%.
Therefore to be compatible with string buffers containing
%%, minimal-printf also needs to only print a single %.

TESTS/mbed_platform/minimal-printf/compliance/main.cpp

@@ -404,6 +404,21 @@ static control_t test_printf_x(const size_t call_count)
     return CaseNext;
 }
 
+static control_t test_printf_percent(const size_t call_count)
+{
+    int result_baseline;
+    int result_minimal;
+    int result_file;
+
+    result_minimal = mbed_printf("%% \r\n");
+    result_file = mbed_fprintf(stderr, "%% \r\n");
+    result_baseline = printf("%% \r\n");
+    TEST_ASSERT_EQUAL_INT(result_baseline, result_minimal);
+    TEST_ASSERT_EQUAL_INT(result_baseline, result_file);
+
+    return CaseNext;
+}
+
 /******************************************************************************/
 /*                                                                            */
 /* SNPRINTF                                                                   */
@@ -721,6 +736,34 @@ static control_t test_snprintf_x(const size_t call_count)
     return CaseNext;
 }
 
+static control_t test_snprintf_percent(const size_t call_count)
+{
+    char buffer_baseline[100];
+    char buffer_minimal[100];
+    int result_baseline;
+    int result_minimal;
+
+    result_minimal = mbed_snprintf(buffer_minimal, sizeof(buffer_minimal), "%% \r\n");
+    result_baseline = snprintf(buffer_baseline, sizeof(buffer_baseline), "%% \r\n");
+    TEST_ASSERT_EQUAL_STRING(buffer_baseline, buffer_minimal);
+    TEST_ASSERT_EQUAL_INT(result_baseline, result_minimal);
+
+    return CaseNext;
+}
+
+static control_t test_snprintf_unsupported_specifier(const size_t call_count)
+{
+    char buffer_minimal[100];
+
+    TEST_ASSERT_NOT_EQUAL(
+        0,
+        mbed_snprintf(buffer_minimal, sizeof(buffer_minimal), "%a \r\n", 5)
+    );
+    TEST_ASSERT_EQUAL_STRING("%a \r\n", buffer_minimal);
+
+    return CaseNext;
+}
+
 #if MBED_CONF_PLATFORM_MINIMAL_PRINTF_ENABLE_FLOATING_POINT
 static control_t test_printf_f(const size_t call_count)
 {
@@ -902,6 +945,9 @@ Case cases[] = {
     Case("snprintf %u", test_snprintf_u),
     Case("printf %x", test_printf_x),
     Case("snprintf %x", test_snprintf_x),
+    Case("printf %%", test_printf_percent),
+    Case("snprintf %%", test_snprintf_percent),
+    Case("snprintf unsupported specifier", test_snprintf_unsupported_specifier),
 #if MBED_CONF_PLATFORM_MINIMAL_PRINTF_ENABLE_FLOATING_POINT
     Case("printf %f", test_printf_f),
     Case("snprintf %f", test_snprintf_f),

platform/source/minimal-printf/mbed_printf_implementation.c

@@ -626,19 +626,14 @@ int mbed_minimal_formatted_string(char *buffer, size_t length, const char *forma
 
                     mbed_minimal_formatted_string_void_pointer(buffer, length, &result, value, stream);
                 } else {
-                    /* write all characters between format beginning and unrecognied modifier */
+                    // Unrecognised, or `%%`. Print the `%` that led us in.
-                    while (index < next_index) {
+                    mbed_minimal_formatted_string_character(buffer, length, &result, '%', stream);
-                        mbed_minimal_formatted_string_character(buffer, length, &result, format[index], stream);
+                    if (next == '%') {
-                        index++;
+                        // Continue printing loop after `%%`
-                    }
+                        index = next_index;
-
-                    /* if this is not the end of the string, write unrecognized modifier */
-                    if (next != '\0') {
-                        mbed_minimal_formatted_string_character(buffer, length, &result, format[index], stream);
-                    } else {
-                        /* break out of for loop */
-                        break;
                     }
+                    // Otherwise we continue the printing loop after the leading `%`, so an
+                    // unrecognised thing like "Blah = %a" will just come out as "Blah = %a"
                 }
             } else
                 /* not a format specifier */