New memory management macro 'snew_plus'.

This formalises my occasional habit of using a single malloc to make a
block that contains a header structure and a data buffer that a field
of the structure will point to, allowing it to be freed in one go
later. Previously I had to do this by hand, losing the type-checking
advantages of snew; now I've written an snew-style macro to do the
job, plus an accessor macro to cleanly get the auxiliary buffer
pointer afterwards, and switched existing instances of the pattern
over to using that.

puttymem.h

@@ -49,4 +49,19 @@ void safefree(void *);
     ((type *)snrealloc(sizeof((type *)0 == (ptr)) ? (ptr) : (ptr),      \
                        (n), sizeof(type)))
 
+/*
+ * For cases where you want to allocate a struct plus a subsidiary
+ * data buffer in one step, this macro lets you add a constant to the
+ * amount malloced.
+ *
+ * Since the return value is already cast to the struct type, a
+ * pointer to that many bytes of extra data can be conveniently
+ * obtained by simply adding 1 to the returned pointer!
+ * snew_plus_get_aux is a handy macro that does that and casts the
+ * result to void *, so you can assign it straight to wherever you
+ * wanted it.
+ */
+#define snew_plus(type, extra) ((type *)snmalloc(1, sizeof(type) + (extra)))
+#define snew_plus_get_aux(ptr) ((void *)((ptr) + 1))
+
 #endif

sshshare.c

@@ -932,17 +932,14 @@ static void share_closing(Plug plug, const char *error_msg, int error_code,
 static void share_xchannel_add_message(
     struct share_xchannel *xc, int type, const void *data, int len)
 {
-    unsigned char *block;
     struct share_xchannel_message *msg;
 
     /*
-     * Be a little tricksy here by allocating a single memory block
-     * containing both the 'struct share_xchannel_message' and the
-     * actual data. Simplifies freeing it later.
+     * Allocate the 'struct share_xchannel_message' and the actual
+     * data in one unit.
      */
-    block = smalloc(sizeof(struct share_xchannel_message) + len);
-    msg = (struct share_xchannel_message *)block;
-    msg->data = block + sizeof(struct share_xchannel_message);
+    msg = snew_plus(struct share_xchannel_message, len);
+    msg->data = snew_plus_get_aux(msg);
     msg->datalen = len;
     msg->type = type;
     memcpy(msg->data, data, len);

unix/gtkfont.c

@@ -258,7 +258,6 @@ struct xlfd_decomposed {
 
 static struct xlfd_decomposed *xlfd_decompose(const char *xlfd)
 {
-    void *mem;
     char *p, *components[14];
     struct xlfd_decomposed *dec;
     int i;
@@ -266,15 +265,14 @@ static struct xlfd_decomposed *xlfd_decompose(const char *xlfd)
     if (!xlfd)
         return NULL;
 
-    mem = smalloc(sizeof(struct xlfd_decomposed) + strlen(xlfd) + 1);
-    p = ((char *)mem) + sizeof(struct xlfd_decomposed);
+    dec = snew_plus(struct xlfd_decomposed, strlen(xlfd) + 1);
+    p = snew_plus_get_aux(dec);
     strcpy(p, xlfd);
-    dec = (struct xlfd_decomposed *)mem;
 
     for (i = 0; i < 14; i++) {
         if (*p != '-') {
             /* Malformed XLFD: not enough '-' */
-            sfree(mem);
+            sfree(dec);
             return NULL;
         }
         *p++ = '\0';
@@ -283,7 +281,7 @@ static struct xlfd_decomposed *xlfd_decompose(const char *xlfd)
     }
     if (*p) {
         /* Malformed XLFD: too many '-' */
-        sfree(mem);
+        sfree(dec);
         return NULL;
     }