Add stpeprintf()

[v]stpeprintf() are similar to [v]snprintf(3), but they allow chaining.
[v]snprintf(3) are very dangerous for catenating strings, since the
obvious ways to do it invoke Undefined Behavior, and the ways that avoid
UB are very error-prone.

Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

configure.ac

@@ -50,7 +50,7 @@ AC_CHECK_FUNCS(arc4random_buf futimes \
 	initgroups lckpwdf lutimes \
 	setgroups updwtmp updwtmpx innetgr \
 	getspnam_r \
-	memset_explicit explicit_bzero)
+	memset_explicit explicit_bzero stpeprintf)
 AC_SYS_LARGEFILE
 
 dnl Checks for typedefs, structures, and compiler characteristics.

lib/stpeprintf.h

@@ -0,0 +1,119 @@
+/*
+ * SPDX-FileCopyrightText:  2022 - 2023, Alejandro Colomar <alx@kernel.org>
+ *
+ * SPDX-License-Identifier:  BSD-3-Clause
+ */
+
+
+#ifndef SHADOW_INCLUDE_LIB_STPEPRINTF_H_
+#define SHADOW_INCLUDE_LIB_STPEPRINTF_H_
+
+
+#include <config.h>
+
+#if !defined(HAVE_STPEPRINTF)
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdio.h>
+
+#include "defines.h"
+
+
+format_attr(printf, 3, 4)
+inline char *stpeprintf(char *dst, char *end, const char *restrict fmt, ...);
+
+format_attr(printf, 3, 0)
+inline char *vstpeprintf(char *dst, char *end, const char *restrict fmt,
+    va_list ap);
+
+
+/*
+ * SYNOPSIS
+ *	[[gnu::format(printf, 3, 4)]]
+ *	char *_Nullable stpeprintf(char *_Nullable dst, char end[0],
+ *	                           const char *restrict fmt, ...);
+ *
+ *	[[gnu::format(printf, 3, 0)]]
+ *	char *_Nullable vstpeprintf(char *_Nullable dst, char end[0],
+ *	                           const char *restrict fmt, va_list ap);
+ *
+ *
+ * ARGUMENTS
+ *	dst	Destination buffer where to write a string.
+ *
+ *	end	Pointer to one after the last element of the buffer
+ *		pointed to by `dst`.  Usually, it should be calculated
+ *		as `dst + NITEMS(dst)`.
+ *
+ *	fmt	Format string
+ *
+ *	...
+ *	ap	Variadic argument list
+ *
+ * DESCRIPTION
+ *	These functions are very similar to [v]snprintf(3).
+ *
+ *	The destination buffer is limited by a pointer to its end --one
+ *	after its last element-- instead of a size.  This allows
+ *	chaining calls to it safely, unlike [v]snprintf(3), which is
+ *	difficult to chain without invoking Undefined Behavior.
+ *
+ * RETURN VALUE
+ *	dst + strlen(dst)
+ *		•  On success, these functions return a pointer to the
+ *		   terminating NUL byte.
+ *
+ *	end
+ *		•  If this call truncated the resulting string.
+ *		•  If `dst == end` (a previous chained call to these
+ *		   functions truncated).
+ *	NULL
+ *		•  If this function failed (see ERRORS).
+ *		•  If `dst == NULL` (a previous chained call to these
+ *		   functions failed).
+ *
+ * ERRORS
+ *	These functions may fail for the same reasons as vsnprintf(3).
+ */
+
+
+inline char *
+stpeprintf(char *dst, char *end, const char *restrict fmt, ...)
+{
+	char     *p;
+	va_list  ap;
+
+	va_start(ap, fmt);
+	p = vstpeprintf(dst, end, fmt, ap);
+	va_end(ap);
+
+	return p;
+}
+
+
+inline char *
+vstpeprintf(char *dst, char *end, const char *restrict fmt, va_list ap)
+{
+	int        len;
+	ptrdiff_t  size;
+
+	if (dst == end)
+		return end;
+	if (dst == NULL)
+		return NULL;
+
+	size = end - dst;
+	len = vsnprintf(dst, size, fmt, ap);
+
+	if (len == -1)
+		return NULL;
+	if (len >= size)
+		return end;
+
+	return dst + len;
+}
+
+
+#endif  // !HAVE_STPEPRINTF
+#endif  // include guard

libmisc/Makefile.am

@@ -61,6 +61,7 @@ libmisc_la_SOURCES = \
 	setugid.c \
 	setupenv.c \
 	shell.c \
+	stpeprintf.c \
 	strtoday.c \
 	sub.c \
 	sulog.c \

libmisc/stpeprintf.c

@@ -0,0 +1,25 @@
+/*
+ * SPDX-FileCopyrightText:  2022 - 2023, Alejandro Colomar <alx@kernel.org>
+ *
+ * SPDX-License-Identifier:  BSD-3-Clause
+ */
+
+
+#include <config.h>
+
+#if !defined(HAVE_STPEPRINTF)
+
+#ident "$Id$"
+
+#include "stpeprintf.h"
+
+#include <stdarg.h>
+
+
+extern inline char *stpeprintf(char *dst, char *end, const char *restrict fmt,
+    ...);
+extern inline char *vstpeprintf(char *dst, char *end, const char *restrict fmt,
+    va_list ap);
+
+
+#endif  // !HAVE_STPEPRINTF