diff -r e59b76889f00 -r d31f4d4075dc tests/test_utils.c
--- a/tests/test_utils.c	Wed Dec 20 16:46:14 2023 +0100
+++ b/tests/test_utils.c	Wed Dec 20 17:57:18 2023 +0100
@@ -26,11 +26,230 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include "test_utils.h"
+#include "cx/test.h"
+
+#include "cx/utils.h"
+#include "cx/buffer.h"
+
+CX_TEST(test_stream_bncopy) {
+    CxBuffer source, target;
+    char sbuf[32], tbuf[32];
+    memset(tbuf, 0, 32);
+    cxBufferInit(&source, sbuf, 32, NULL, 0);
+    cxBufferInit(&target, tbuf, 32, NULL, 0);
+    cxBufferPutString(&source, "This is a stream copy test.");
+    cxBufferSeek(&source, 0, SEEK_SET);
+    char tmp[4];
+
+    CX_TEST_DO {
+        size_t result = cx_stream_bncopy(&source, &target,
+                                         (cx_read_func) cxBufferRead,
+                                         (cx_write_func) cxBufferWrite,
+                                         tmp, 4, 20);
+        CX_TEST_ASSERT(result == 20);
+        CX_TEST_ASSERT(target.size == 20);
+        CX_TEST_ASSERT(strcmp("This is a stream cop\0", tbuf) == 0);
+
+        result = cx_stream_bcopy(&source, &target,
+                                 (cx_read_func) cxBufferRead,
+                                 (cx_write_func) cxBufferWrite,
+                                 NULL, 16);
+
+        CX_TEST_ASSERT(result == 7);
+        CX_TEST_ASSERT(target.size == 27);
+        CX_TEST_ASSERT(strcmp("This is a stream copy test.\0", tbuf) == 0);
+    }
+
+    cxBufferDestroy(&source);
+    cxBufferDestroy(&target);
+}
+
+CX_TEST(test_stream_ncopy) {
+    CxBuffer source, target;
+    char sbuf[32], tbuf[32];
+    memset(tbuf, 0, 32);
+    cxBufferInit(&source, sbuf, 32, NULL, 0);
+    cxBufferInit(&target, tbuf, 32, NULL, 0);
+    cxBufferPutString(&source, "This is a stream copy test.");
+    cxBufferSeek(&source, 0, SEEK_SET);
+
+    CX_TEST_DO {
+        size_t result = cx_stream_ncopy(&source, &target,
+                                        (cx_read_func) cxBufferRead,
+                                        (cx_write_func) cxBufferWrite,
+                                        20);
+        CX_TEST_ASSERT(result == 20);
+        CX_TEST_ASSERT(target.size == 20);
+        CX_TEST_ASSERT(strcmp("This is a stream cop\0", tbuf) == 0);
+
+        result = cx_stream_copy(&source, &target,
+                                (cx_read_func) cxBufferRead,
+                                (cx_write_func) cxBufferWrite);
+
+        CX_TEST_ASSERT(result == 7);
+        CX_TEST_ASSERT(target.size == 27);
+        CX_TEST_ASSERT(strcmp("This is a stream copy test.\0", tbuf) == 0);
+    }
+
+    cxBufferDestroy(&source);
+    cxBufferDestroy(&target);
+}
+
+CX_TEST(test_forn) {
+    unsigned j;
+    j = 0;
+    CX_TEST_DO {
+        cx_for_n(i, 50) {
+            CX_TEST_ASSERT(i == j);
+            j++;
+        }
+    }
+}
+
+CX_TEST(test_swap_ptr) {
+    int i = 5;
+    int j = 8;
+    int *ip = &i;
+    int *jp = &j;
+    CX_TEST_DO {
+        cx_swap_ptr(ip, jp);
+        CX_TEST_ASSERT(ip == &j);
+        CX_TEST_ASSERT(jp == &i);
+    }
+}
+
+CX_TEST(test_szmul) {
+    size_t r;
+    int e;
+
+    CX_TEST_DO {
+        e = cx_szmul(5, 7, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 35);
+
+        size_t s = SIZE_MAX & ~3;
+
+        e = cx_szmul(s / 4, 2, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s / 2);
+
+        e = cx_szmul(2, s / 4, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s / 2);
+
+        e = cx_szmul(s / 4, 4, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s);
+
+        e = cx_szmul(4, s / 4, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s);
+
+        e = cx_szmul(s / 4, 5, &r);
+        CX_TEST_ASSERT(e != 0);
+
+        e = cx_szmul(5, s / 4, &r);
+        CX_TEST_ASSERT(e != 0);
+
+        e = cx_szmul(SIZE_MAX - 4, 0, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul(0, SIZE_MAX - 1, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul(SIZE_MAX, 0, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul(0, SIZE_MAX, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul(0, 0, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+    }
+}
+
+#ifdef CX_SZMUL_BUILTIN
+
+// also test the custom implementation
+#undef CX_SZMUL_BUILTIN
+
+#include "../src/szmul.c"
+
+#define CX_SZMUL_BUILTIN
+
+CX_TEST(test_szmul_impl) {
+    size_t r;
+    int e;
+
+    CX_TEST_DO {
+        e = cx_szmul_impl(5, 7, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 35);
+
+        size_t s = SIZE_MAX & ~3;
+
+        e = cx_szmul_impl(s / 4, 2, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s / 2);
+
+        e = cx_szmul_impl(2, s / 4, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s / 2);
+
+        e = cx_szmul_impl(s / 4, 4, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s);
+
+        e = cx_szmul_impl(4, s / 4, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == s);
+
+        e = cx_szmul_impl(s / 4, 5, &r);
+        CX_TEST_ASSERT(e != 0);
+
+        e = cx_szmul_impl(5, s / 4, &r);
+        CX_TEST_ASSERT(e != 0);
+
+        e = cx_szmul_impl(SIZE_MAX - 4, 0, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul_impl(0, SIZE_MAX - 1, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul_impl(SIZE_MAX, 0, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul_impl(0, SIZE_MAX, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+
+        e = cx_szmul_impl(0, 0, &r);
+        CX_TEST_ASSERT(e == 0);
+        CX_TEST_ASSERT(r == 0);
+    }
+}
+
+#endif // CX_SZMUL_BUILTIN
+
 
 CxTestSuite *cx_test_suite_utils(void) {
     CxTestSuite *suite = cx_test_suite_new("utils");
 
+    cx_test_register(suite, test_stream_bncopy);
+    cx_test_register(suite, test_stream_ncopy);
+    cx_test_register(suite, test_forn);
+    cx_test_register(suite, test_swap_ptr);
+    cx_test_register(suite, test_szmul);
+    cx_test_register(suite, test_szmul_impl);
+
     return suite;
 }