Add tests for stpncpy()

TESTED:
Tested on 10.4-10.5 ppc, 10.4-10.6 i386, 10.5-10.6 ppc (i386 Rosetta),
10.5-12.x x86_64, 11.x-14.x arm64.
Fails on <10.7 without added stpncpy(); passes with.

test/test_stpncpy.c

@@ -0,0 +1,103 @@
+/*
+ * Copyright (c) 2024 Frederick H. G. Wright II <[email protected]>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This provides rudimentary tests for stpncpy().  Given that
+ * the implementation is taken almost verbatim from Apple's code for 10.7+,
+ * exhaustive testing of corner cases shouldn't be necessary.  This just
+ * tests a few cases, primarily to verify that the function can be used at all
+ * (which is not true prior to 10.7 without this package), though the framework
+ * should be valid for all possible test cases.
+ */
+
+#include <assert.h>
+#include <errno.h>
+#include <string.h>
+
+const char *test_str = "The Quick Brown Fox";
+
+#define BUF_LEN 128  /* Generously longer than test_str */
+
+static int save_errno;
+static char *save_ret;
+static char dest[BUF_LEN], save_dest[BUF_LEN];
+
+/* Reference implementation of stpncpy() */
+static char *
+ref_stpncpy(char *dst, const char *src, size_t n)
+{
+  char *ret;
+
+  if (n <= 0) return dst;
+
+  while (n) {
+    if ((*dst++ = *src++) == 0) break;
+    --n;
+  }
+  ret = dst - 1;
+  while (n--) *dst++ = 0;
+  return *ret ? ret + 1 : ret;
+}
+
+/* Setup prior to test case */
+static void
+setup(void)
+{
+  errno = 0;
+  memset(dest, -1, BUF_LEN);
+}
+
+/* Save all results from first version */
+static void
+save(char *ret)
+{
+  save_errno = errno;
+  save_ret = ret;
+  memcpy(save_dest, dest, BUF_LEN);
+}
+
+/* Compare results from both versions */
+static void
+check(char *ret)
+{
+  assert(errno == save_errno);
+  assert(ret == save_ret);
+  assert(memcmp(save_dest, dest, BUF_LEN) == 0);
+}
+
+/* Run one test case */
+static void
+test_stpncpy(char *dst, const char *src, size_t n)
+{
+  setup();
+  save(ref_stpncpy(dst, src, n));
+  check(stpncpy(dst, src, n));
+}
+
+int
+main(int argc, char *argv[])
+{
+  int test_len = strlen(test_str);
+  int n;
+
+  (void) argc; (void) argv;
+
+  for (n = test_len - 2; n <= test_len + 2; ++n) {
+    test_stpncpy(&dest[1], test_str, n);
+  }
+
+  return 0;
+}