[PATCH v2 1/3] string: Add stpecpy()

[Alejandro Colomar]

This function is similar to stpcpy(3), but it tuncates the destination
string if it doesn't fit the buffer.  It's much simpler to use than
strscpy(9) or strlcpy(3), and slightly faster.

It also allows chaining with stpeprintf(3), which has the same interface
as stpecpy(3), but prints a formatted string (like snprintf(3)).

Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
 string/Makefile  |  1 +
 string/stpecpy.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 string/string.h  |  7 +++++++
 3 files changed, 53 insertions(+)
 create mode 100644 string/stpecpy.c

diff --git a/string/Makefile b/string/Makefile
index 938f528b8d..95e9ebce6d 100644
--- a/string/Makefile
+++ b/string/Makefile
@@ -73,6 +73,7 @@ routines := \
   sigabbrev_np \
   sigdescr_np \
   stpcpy \
+  stpecpy \
   stpncpy \
   strcasecmp \
   strcasecmp_l \
diff --git a/string/stpecpy.c b/string/stpecpy.c
new file mode 100644
index 0000000000..6884a949a0
--- /dev/null
+++ b/string/stpecpy.c
@@ -0,0 +1,45 @@
+/* Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library 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.
+
+   The GNU C Library 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 the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <stdbool.h>
+#include <string.h>
+
+char *
+stpecpy(char *dst, char *end, const char *restrict src)
+{
+	bool    trunc;
+	size_t  dsize, dlen, slen;
+
+	if (dst == end)
+		return end;
+	if (dst == NULL)  // Allow chaining with stpeprintf().
+		return NULL;
+	if (dst > end)
+		__builtin_unreachable();
+
+	dsize = end - dst;
+	slen = strnlen(src, dsize);
+	trunc = (slen == dsize);
+	dlen = slen - trunc;
+	dst[dlen] = '\0';
+
+	return mempcpy(dst, src, dlen) + trunc;
+}
diff --git a/string/string.h b/string/string.h
index 54dd8344de..966a8cb744 100644
--- a/string/string.h
+++ b/string/string.h
@@ -502,6 +502,13 @@ extern char *stpncpy (char *__restrict __dest,
 #endif
 
 #ifdef	__USE_GNU
+/* Copy the string SRC into a null-terminated string at DEST,
+   truncating if it would run after END.  Return a pointer to
+   the terminating null byte, or END if the string was truncated,
+   or NULL if DEST was NULL. */
+extern char *stpecpy (char *__dest, char *__end, const char *__restrict __src)
+     __THROW __nonnull ((2, 3));
+
 /* Compare S1 and S2 as strings holding name & indices/version numbers.  */
 extern int strverscmp (const char *__s1, const char *__s2)
      __THROW __attribute_pure__ __nonnull ((1, 2));
-- 
2.39.0