Ensure correct behaviour of printf formatting

* Add configuration check to ensure that compiler and runtime support
  C99 length modifier 'z'
* Add emv_debug_test to ensure that debugging mechanism correctly
  applies C99 length modifier 'z'
* Let emv_debug_internal() and emv_str_list_add() specify format
  checking according to the compiler being used

The above is intended to address the paradoxes created by MinGW's use of
MSVCRT. MSYS2's MinGW provides its own printf() family of functions to
compensate for MSVCRT's lack of C99 compatibility, but unfortunately
still issues a warning for length modifiers that would otherwise work
with its own printf() family of functions.

Author: leonlynch

src/CMakeLists.txt

@@ -7,6 +7,38 @@
 
 cmake_minimum_required(VERSION 3.16)
 
+# Check whether C99 length modifier 'z' is supported
+include(CheckCSourceRuns)
+check_c_source_runs("
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+#if defined(__clang__)
+	__attribute__((format(printf, 1, 4)))
+#elif defined(__GNUC__)
+	__attribute__((format(gnu_printf, 1, 4)))
+#endif
+static int test_formatting(const char* fmt, char* s, size_t s_len, ...)
+{
+	int r;
+	va_list ap;
+	va_start(ap, s_len);
+	r = vsnprintf(s, s_len, fmt, ap);
+	va_end(ap);
+	return r;
+}
+int main(void)
+{
+	char s[16] = { 0 };
+	size_t x = 1337;
+	return test_formatting(\"s=%zu\", s, sizeof(s), x) != 6 || strcmp(s, \"s=1337\") != 0;
+}"
+C99_length_modifier_z_is_supported
+)
+if(NOT C99_length_modifier_z_is_supported)
+	message(FATAL_ERROR "C99 length modifier 'z' not supported")
+endif()
+
 # Check for time.h and clock_gettime()
 include(CheckIncludeFile)
 check_include_file(time.h HAVE_TIME_H)

src/emv_debug.h

@@ -108,7 +108,17 @@ void emv_debug_internal(
 	const void* buf,
 	size_t buf_len,
 	...
-) __attribute__((format(printf, 4, 7)));
+)
+#if defined(__clang__)
+	// Check for Clang first because it also defines __GNUC__
+	__attribute__((format(printf, 4, 7)));
+#elif defined(__GNUC__)
+	// Use gnu_printf for GCC and MSYS2's MinGW
+	__attribute__((format(gnu_printf, 4, 7)));
+#else
+	// Otherwise no format string checks
+	;
+#endif
 
 #ifdef EMV_DEBUG_ENABLED
 

src/emv_strings.c

@@ -31,6 +31,17 @@
 #include <stdio.h> // for vsnprintf() and snprintf()
 #include <ctype.h>
 
+#if defined(__clang__)
+	// Check for Clang first because it also defines __GNUC__
+	#define ATTRIBUTE_FORMAT_PRINTF(str_idx, va_idx) __attribute__((format(printf, str_idx, va_idx)))
+#elif defined(__GNUC__)
+	// Use gnu_printf for GCC and MSYS2's MinGW
+	#define ATTRIBUTE_FORMAT_PRINTF(str_idx, va_idx) __attribute__((format(gnu_printf, str_idx, va_idx)))
+#else
+	// Otherwise no format string checks
+	#define ATTRIBUTE_FORMAT_PRINTF(str_idx, va_idx)
+#endif
+
 struct str_itr_t {
 	char* ptr;
 	size_t len;
@@ -40,7 +51,7 @@ struct str_itr_t {
 static int emv_tlv_value_get_string(const struct emv_tlv_t* tlv, enum emv_format_t format, size_t max_format_len, char* value_str, size_t value_str_len);
 static int emv_uint_to_str(uint32_t value, char* str, size_t str_len);
 static void emv_str_list_init(struct str_itr_t* itr, char* buf, size_t len);
-static void emv_str_list_add(struct str_itr_t* itr, const char* fmt, ...) __attribute__((format(printf, 2, 3)));
+static void emv_str_list_add(struct str_itr_t* itr, const char* fmt, ...) ATTRIBUTE_FORMAT_PRINTF(2, 3);
 static int emv_country_alpha2_code_get_string(const uint8_t* buf, size_t buf_len, char* str, size_t str_len);
 static int emv_country_alpha3_code_get_string(const uint8_t* buf, size_t buf_len, char* str, size_t str_len);
 static int emv_country_numeric_code_get_string(const uint8_t* buf, size_t buf_len, char* str, size_t str_len);
@@ -1559,7 +1570,7 @@ static void emv_str_list_init(struct str_itr_t* itr, char* buf, size_t len)
 	itr->len = len;
 }
 
-__attribute__((format(printf, 2, 3)))
+ATTRIBUTE_FORMAT_PRINTF(2, 3)
 static void emv_str_list_add(struct str_itr_t* itr, const char* fmt, ...)
 {
 	int r;

tests/CMakeLists.txt

@@ -8,6 +8,17 @@
 cmake_minimum_required(VERSION 3.16)
 
 if (BUILD_TESTING)
+	add_executable(emv_debug_test emv_debug_test.c)
+	target_link_libraries(emv_debug_test PRIVATE print_helpers emv)
+	add_test(emv_debug_test emv_debug_test)
+	string(CONCAT emv_debug_test_regex
+		"\\[APP\\] asdf=1337[\r\n]"
+	)
+	set_tests_properties(emv_debug_test
+		PROPERTIES
+			PASS_REGULAR_EXPRESSION ${emv_debug_test_regex}
+	)
+
 	add_executable(emv_atr_parse_test emv_atr_parse_test.c)
 	target_link_libraries(emv_atr_parse_test PRIVATE print_helpers emv)
 	add_test(emv_atr_parse_test emv_atr_parse_test)

tests/emv_debug_test.c

@@ -0,0 +1,43 @@
+/**
+ * @file emv_debug_test.c
+ * @brief Unit tests for EMV debug implementation
+ *
+ * Copyright (c) 2023 Leon Lynch
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this program. If not, see
+ * <https://www.gnu.org/licenses/>.
+ */
+
+#define EMV_DEBUG_SOURCE EMV_DEBUG_SOURCE_APP
+#include "emv_debug.h"
+#include "print_helpers.h"
+
+#include <stdint.h>
+#include <stdio.h>
+
+int main(void)
+{
+	int r;
+	size_t asdf = 1337;
+
+	// Enable debug output
+	r = emv_debug_init(EMV_DEBUG_SOURCE_ALL, EMV_DEBUG_ALL, &print_emv_debug);
+	if (r) {
+		fprintf(stderr, "emv_debug_init() failed; r=%d\n", r);
+		return 1;
+	}
+
+	// Test whether compiler supports length modifier z both at compile-time and runtime
+	emv_debug_error("asdf=%zu", asdf);
+}