libutil: add function to concatenate a format string in place

Add function util_concatf() that appends the result of a format string
expansion to the end of an existing string while taking care of the
required memory allocations.

Usage example:

  char *str = NULL;

  util_concatf(&str, "list:");
  for (int i = 1; i <= 3; i++)
    util_concatf(&str, "%spart%d", (i > 1 ? "," : ""), i);
  printf("%s\n", str); /* list:part1,part2,part3 */

Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

libutil/util_libc.c

@@ -139,6 +139,26 @@ char *util_strcat_realloc(char *str1, const char *str2)
 	return buf;
 }
 
+/**
+ *  Concatenate a string with the result of a format string expansion
+ *
+ *  @param[in, out]  str1  Pointer to pointer to first string
+ *  @param[in]       fmt   Format string for generation of the second string
+ *  @param[in]       ...   Parameters for format string
+ */
+void util_concatf(char **str1, const char *fmt, ...)
+{
+	va_list args;
+	char *str2;
+
+	va_start(args, fmt);
+	util_vasprintf(&str2, fmt, args);
+	va_end(args);
+
+	*str1 = util_strcat_realloc(*str1, str2);
+	free(str2);
+}
+
 /**
  * Convert string to uppercase
  *