Use autoconf's FLEXIBLE_ARRAY_MEMBER for unspecified-length arrays

C99 allows a syntax for structures whose last element is of
unspecified length:
   struct s {
     int elt1;
     ...
     char last_element[];
   };

Recent (last-5-years) autoconf versions provide an
AC_C_FLEXIBLE_ARRAY_MEMBER test that defines FLEXIBLE_ARRAY_MEMBER
to either no tokens (if you have c99 flexible array support) or to 1
(if you don't).  At that point you just use offsetof
[STRUCT_OFFSET() for us] to see where last_element begins, and
allocate your structures like:

   struct s {
     int elt1;
     ...
     char last_element[FLEXIBLE_ARRAY_MEMBER];
   };

   tor_malloc(STRUCT_OFFSET(struct s, last_element) +
                                   n_elements*sizeof(char));

The advantages are:

   1) It's easier to see which structures and elements are of
      unspecified length.
   2) The compiler and related checking tools can also see which
      structures and elements are of unspecified length, in case they
      wants to try weird bounds-checking tricks or something.
   3) The compiler can warn us if we do something dumb, like try
      to stack-allocate a flexible-length structure.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/or/buffers.c b/src/or/buffers.c
index 9f393b9874..48acf505ef 100644
--- a/src/or/buffers.c
+++ b/src/or/buffers.c
@@ -68,8 +68,8 @@ typedef struct chunk_t {
   size_t datalen; /**< The number of bytes stored in this chunk */
   size_t memlen; /**< The number of usable bytes of storage in <b>mem</b>. */
   char *data; /**< A pointer to the first byte of data stored in <b>mem</b>. */
-  char mem[1]; /**< The actual memory used for storage in this chunk. May be
-                * more than one byte long. */
+  char mem[FLEXIBLE_ARRAY_MEMBER]; /**< The actual memory used for storage in
+                * this chunk. */
 } chunk_t;
 
 #define CHUNK_HEADER_LEN STRUCT_OFFSET(chunk_t, mem[0])