Add support for truncatef(), formatted truncate() replacement

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

README.md

@@ -335,6 +335,13 @@ Generic Functions
   **Note:** Requires Linux v3.11, or later, will fall back to the old
     and unsafe `tmpfile()` on older systems.
 
+- `truncatef(length, fmt, ...)`
+
+  Like `truncate()`, but takes a formatted string as argument.  This
+  simplifies some operations when the filename otherwise have to be
+  composed from parts into a separate array before calling the real
+  function.
+
 - `which(cmd)`, `whichp(cmd)`
 
   C implementation of UNIX which(1).  Returns a malloc'ed string with the

src/Makefile.am

@@ -15,7 +15,7 @@ libite_la_SOURCES     = chomp.c		copyfile.c			\
 			strlcpy.c 	strlcat.c 	strtonum.c	\
 			strdupa.h	strndupa.h	strnlen.h	\
 			strmatch.c	systemf.c			\
-			telnet.c	tempfile.c			\
+			telnet.c	tempfile.c	truncatef.c	\
 			yorn.c		which.c				\
 			lite.h		strlite.h			\
 			queue.h		tree.h

src/lite.h

@@ -72,6 +72,7 @@ ssize_t copyfile   (const char *src, const char *dst, int len, int opt);
 int     movefile   (const char *src, const char *dst);
 int     fcopyfile  (FILE *src, FILE *dst);
 ssize_t fsendfile  (FILE *src, FILE *dst, size_t len);
+int     truncatef  (off_t length, const char *fmt, ...);
 
 int     ifconfig   (const char *ifname, const char *addr, const char *mask, int up);
 

src/truncatef.c

@@ -0,0 +1,65 @@
+/* Formatted truncate()
+ *
+ * Copyright (c) 2020  Joachim Nilsson <troglobit@gmail.com>
+ *
+ * Permission to use, copy, modify, and/or 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.
+ */
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+/**
+ * truncatef - Truncate a file based on the formatted string
+ * @length: Size in bytes to truncate file to, zero to empty it
+ * @fmt:  Formatted string to be composed into a pathname
+ *
+ * This is an extension to the truncate() family, lessening the burden
+ * of having to compose the filename from parts in a seprate buffer.
+ *
+ * Returns:
+
+ * Upon successful completion truncate() returns POSIX OK(0), otherwise,
+ * -1 is returned and errno is set to indicate the error.
+ */
+int truncatef(off_t length, const char *fmt, ...)
+{
+	va_list ap;
+	char *file;
+	int len;
+
+	va_start(ap, fmt);
+	len = vsnprintf(NULL, 0, fmt, ap);
+	va_end(ap);
+
+	file = alloca(len + 1);
+	if (!file) {
+		errno = ENOMEM;
+		return -1;
+	}
+
+	va_start(ap, fmt);
+	vsnprintf(file, len + 1, fmt, ap);
+	va_end(ap);
+
+	return truncate(file, length);
+}
+
+/**
+ * Local Variables:
+ *  indent-tabs-mode: t
+ *  c-file-style: "linux"
+ * End:
+ */