diff -r b7e2e1d7ed22 -r 85859399a0cc tests/test_printf.c
--- a/tests/test_printf.c	Tue Jan 16 23:12:43 2024 +0100
+++ b/tests/test_printf.c	Tue Jan 16 23:13:01 2024 +0100
@@ -294,6 +294,107 @@
     free(expected);
 }
 
+CX_TEST(test_sprintf_no_realloc) {
+    char *buf = malloc(16);
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+    CxAllocator *alloc = &talloc.base;
+    CX_TEST_DO {
+        char *oldbuf = buf;
+        size_t len = cx_sprintf_a(alloc, &buf, 16, "Test %d %s", 47, "string");
+        CX_TEST_ASSERT(oldbuf == buf);
+        CX_TEST_ASSERT(len == 14);
+        CX_TEST_ASSERT(0 == memcmp(buf, "Test 47 string", 15));
+        CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+    }
+    cx_testing_allocator_destroy(&talloc);
+    free(buf);
+}
+
+CX_TEST(test_sprintf_realloc) {
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+    CxAllocator *alloc = &talloc.base;
+    char *buf = cxMalloc(alloc, 8);
+    CX_TEST_DO {
+        size_t len = cx_sprintf_a(alloc, &buf, 8, "Test %d %s", 47, "foobar");
+        CX_TEST_ASSERT(len == 14);
+        CX_TEST_ASSERT(0 == memcmp(buf, "Test 47 foobar", 15));
+        cxFree(alloc, buf);
+        CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+    }
+    cx_testing_allocator_destroy(&talloc);
+}
+
+CX_TEST(test_sprintf_realloc_to_fit_terminator) {
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+    CxAllocator *alloc = &talloc.base;
+    // make it so that only the zero-terminator does not fit
+    char *buf = cxMalloc(alloc, 14);
+    CX_TEST_DO {
+        size_t len = cx_sprintf_a(alloc, &buf, 14, "Test %d %s", 13, "string");
+        CX_TEST_ASSERT(len == 14);
+        CX_TEST_ASSERT(0 == memcmp(buf, "Test 13 string", 15));
+        cxFree(alloc, buf);
+        CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+    }
+    cx_testing_allocator_destroy(&talloc);
+}
+
+CX_TEST(test_sprintf_s_no_alloc) {
+    char buf[16];
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+    CxAllocator *alloc = &talloc.base;
+    CX_TEST_DO {
+        char *str;
+        size_t len = cx_sprintf_sa(alloc, buf, 16, &str, "Test %d %s", 47, "string");
+        CX_TEST_ASSERT(str == buf);
+        CX_TEST_ASSERT(len == 14);
+        CX_TEST_ASSERT(0 == memcmp(buf, "Test 47 string", 15));
+        CX_TEST_ASSERT(0 == memcmp(str, "Test 47 string", 15));
+        CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+    }
+    cx_testing_allocator_destroy(&talloc);
+}
+
+CX_TEST(test_sprintf_s_alloc) {
+    char buf[16];
+    memcpy(buf, "0123456789abcdef", 16);
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+    CxAllocator *alloc = &talloc.base;
+    CX_TEST_DO {
+        char *str;
+        size_t len = cx_sprintf_sa(alloc, buf, 16, &str, "Hello %d %s", 4711, "larger string");
+        CX_TEST_ASSERT(str != buf);
+        CX_TEST_ASSERT(len == 24);
+        CX_TEST_ASSERT(0 == memcmp(str, "Hello 4711 larger string", 25));
+        cxFree(alloc, str);
+        CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+    }
+    cx_testing_allocator_destroy(&talloc);
+}
+
+CX_TEST(test_sprintf_s_alloc_to_fit_terminator) {
+    char buf[16];
+    memcpy(buf, "0123456789abcdef", 16);
+    CxTestingAllocator talloc;
+    cx_testing_allocator_init(&talloc);
+    CxAllocator *alloc = &talloc.base;
+    CX_TEST_DO {
+        char *str;
+        size_t len = cx_sprintf_sa(alloc, buf, 16, &str, "Hello %d %s", 112, "string");
+        CX_TEST_ASSERT(str != buf);
+        CX_TEST_ASSERT(len == 16);
+        CX_TEST_ASSERT(0 == memcmp(str, "Hello 112 string", 17)); // include terminator
+        cxFree(alloc, str);
+        CX_TEST_ASSERT(cx_testing_allocator_verify(&talloc));
+    }
+    cx_testing_allocator_destroy(&talloc);
+}
+
 CxTestSuite *cx_test_suite_printf(void) {
     CxTestSuite *suite = cx_test_suite_new("printf");
 
@@ -303,6 +404,12 @@
     cx_test_register(suite, test_fprintf);
     cx_test_register(suite, test_asprintf);
     cx_test_register(suite, test_asprintf_large_string);
+    cx_test_register(suite, test_sprintf_no_realloc);
+    cx_test_register(suite, test_sprintf_realloc);
+    cx_test_register(suite, test_sprintf_realloc_to_fit_terminator);
+    cx_test_register(suite, test_sprintf_s_no_alloc);
+    cx_test_register(suite, test_sprintf_s_alloc);
+    cx_test_register(suite, test_sprintf_s_alloc_to_fit_terminator);
 
     return suite;
 }