diff --git a/README.md b/README.md
index 2566ac8..807f647 100644
--- a/README.md
+++ b/README.md
@@ -59,8 +59,8 @@ contracts are always checked at compile-time. If the contract's predicate isn't
 program won't compile. This is a huge advantage over using `<cassert>` or the GSL's `Expects` and
 `Ensures` macros.
 
-When optimisations are diabled and `NDEBUG` is not defined as a macro, the contract will check your
-predicate at run-time. If the predicate fails, then a diagnostic will be emit, and the program will
+When optimisations are disabled and `NDEBUG` is not defined as a macro, the contract will check your
+predicate at run-time. If the predicate fails, then a diagnostic will be emitted, and the program will
 crash.
 
 When optimisations are enabled, and `NDEBUG` remains undefined, the program will emit a diagnostic
@@ -195,9 +195,18 @@ generation than when the contract isn't used. [See for yourself][__builtin_unrea
 	</tbody>
 </table>
 
-*Rudimentary testing has identified that neither GCC nor Clang perform optimisations <em>before</em>
+\*Rudimentary testing has identified that neither GCC nor Clang perform optimisations <em>before</em>
 the contract.
 
+### Configuring diagnostics
+
+By default, diagnostic messages are printed to `stderr` by `std::fwrite`. If `CJDB_USE_IOSTREAM` is
+defined as a macro, messages are printed with `std::cerr.write` instead. If `CJDB_SKIP_STDIO` is
+defined as a macro, there is no dependency on either `cstdio` or `iostream` and printing diagnostic
+messages is a no-op. If you would like to customize how diagnostics are printed, you may set the
+function pointer `cjdb::print_error` to any function or lambda with the signature
+`void(std::string_view)`.
+
 ### Assertions
 
 You should use assertions to indicate when a programming error has occurred in the middle of your
diff --git a/include/cjdb/contracts.hpp b/include/cjdb/contracts.hpp
index b13c846..3b167ba 100644
--- a/include/cjdb/contracts.hpp
+++ b/include/cjdb/contracts.hpp
@@ -4,10 +4,17 @@
 #ifndef CJDB_CONTRACTS_HPP
 #define CJDB_CONTRACTS_HPP
 
-#include <cstdio>
 #include <string_view>
 #include <type_traits>
 
+#ifdef CJDB_USE_IOSTREAM
+	#include <iostream>
+#elif !defined(CJDB_SKIP_STDIO)
+	#include <cerrno>
+	#include <cstdio>
+	#include <system_error>
+#endif // CJDB_USE_IOSTREAM
+
 // clang-tidy doesn't yet support this
 //
 // #ifndef __cpp_lib_is_constant_evaluated
@@ -24,7 +31,22 @@
 	#define CJDB_PRETTY_FUNCTION __PRETTY_FUNCTION__
 #endif // _MSC_VER
 
-namespace cjdb::contracts_detail {
+namespace cjdb {
+	using print_error_fn = void(std::string_view);
+	inline print_error_fn* print_error = [](std::string_view message)
+	{
+	#ifdef CJDB_USE_IOSTREAM
+		std::cerr.write(message.data(), static_cast<std::streamsize>(message.size()));
+	#elif !defined(CJDB_SKIP_STDIO)
+		if (auto const len = message.size();
+			std::fwrite(message.data(), sizeof(char), len, stderr) < len) [[unlikely]]
+		{
+			throw std::system_error{errno, std::system_category()};
+		}
+	#endif // CJDB_USE_IOSTREAM
+	};
+
+namespace contracts_detail {
 	#ifdef NDEBUG
 		inline constexpr auto is_debug = false;
 	#else
@@ -34,12 +56,19 @@ namespace cjdb::contracts_detail {
 	struct contract_impl_fn {
 		constexpr void operator()(bool const result,
 		                          std::string_view const message,
-		                          std::string_view const function) const noexcept
+		                          std::string_view const function) const noexcept(!is_debug)
 		{
 			if (not result) {
 				if (not std::is_constant_evaluated()) {
 					if constexpr (is_debug) {
-						std::fprintf(stderr, "%s in `%s`\n", message.data(), function.data());
+					#ifdef _WIN32
+						constexpr auto& suffix = "`\r\n";
+					#else
+						constexpr auto& suffix = "`\n";
+					#endif // _WIN32
+						::cjdb::print_error(message);
+						::cjdb::print_error(function);
+						::cjdb::print_error(suffix);
 					}
 				}
 			#ifdef _MSC_VER
@@ -65,11 +94,12 @@ namespace cjdb::contracts_detail {
 		}
 	};
 	inline constexpr auto matches_bool = matches_bool_fn{};
-} // namespace cjdb::contracts_detail
+} // namespace contracts_detail
+} // namespace cjdb
 
 #define CJDB_CONTRACT_IMPL(CJDB_KIND, ...) \
    ::cjdb::contracts_detail::contract_impl(::cjdb::contracts_detail::matches_bool(__VA_ARGS__), \
-      __FILE__ ":" CJDB_TO_STRING(__LINE__) ": " CJDB_KIND " `" #__VA_ARGS__ "` failed",        \
+      __FILE__ ":" CJDB_TO_STRING(__LINE__) ": " CJDB_KIND " `" #__VA_ARGS__ "` failed in `", \
       CJDB_PRETTY_FUNCTION)
 
 
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 9ae06ef..60fceda 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -3,15 +3,22 @@
 #
 function(build_contract filename)
 	set(target "${filename}")
+	set(target_ios "${filename}_ios")
 	add_executable("${target}" "${filename}.cpp")
+	add_executable("${target_ios}" "${filename}.cpp")
 	if(MSVC)
 		target_compile_options("${target}" PRIVATE "/permissive-")
+		target_compile_options("${target_ios}" PRIVATE "/permissive-" "/DCJDB_USE_IOSTREAM")
+	else()
+		target_compile_options("${target_ios}" PRIVATE "-DCJDB_USE_IOSTREAM")
 	endif()
 	target_include_directories("${target}" PRIVATE "${CMAKE_SOURCE_DIR}/include")
+	target_include_directories("${target_ios}" PRIVATE "${CMAKE_SOURCE_DIR}/include")
 endfunction()
 
 build_contract(pass)
 add_test(test.pass pass)
+add_test(test.pass_ios pass_ios)
 
 function(test_contract target expected_output)
 	set(args "${CMAKE_SOURCE_DIR}/test/check-failure.py"
@@ -23,6 +30,10 @@ function(test_contract target expected_output)
 	add_test(NAME "test.${target}"
 	         COMMAND python3 ${args}
 	         WORKING_DIRECTORY "${CMAKE_BINARY_DIR}")
+	list(TRANSFORM args APPEND "_ios" AT 1)
+	add_test(NAME "test.${target}_ios"
+	         COMMAND python3 ${args}
+	         WORKING_DIRECTORY "${CMAKE_BINARY_DIR}")
 endfunction()
 
 function(test_quiet_contract target expected_output)