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, 6 insertions, 6 deletions

diff --git a/src/or/or.h b/src/or/or.h
index 2d1fbef59c..8aef2d0233 100644
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -857,8 +857,8 @@ typedef struct cell_t {
 typedef struct var_cell_t {
   uint8_t command;
   circid_t circ_id;
-  uint16_t payload_len;
-  uint8_t payload[1];
+  uint16_t payload_len; /**< The actual length of <b>payload</b>. */
+  uint8_t payload[FLEXIBLE_ARRAY_MEMBER];
 } var_cell_t;
 
 /** A cell as packed for writing to the network. */
@@ -1660,11 +1660,11 @@ typedef struct short_policy_t {
   unsigned int is_accept : 1;
   /** The actual number of values in 'entries'. */
   unsigned int n_entries : 31;
-  /** An array of (probably more than 1!) short_policy_entry_t values,
-   * each descriping a range of ports that this policy accepts or rejects
-   * (depending on the value of is_accept).
+  /** An array of 0 or more short_policy_entry_t values, each descriping a
+   * range of ports that this policy accepts or rejects (depending on the
+   * value of is_accept).
    */
-  short_policy_entry_t entries[1];
+  short_policy_entry_t entries[FLEXIBLE_ARRAY_MEMBER];
 } short_policy_t;
 
 /** A microdescriptor is the smallest amount of information needed to build a