malloc - Associating metadata in custom memory allocator for C89/C90 -

let's have following implementation custom memory allocator wrapping standard malloc , free library functions:

#include <stdlib.h> #include <stddef.h>  struct metadata {         void *private;         size_t len;         char data[1]; };  void *custom_malloc(size_t len) {         struct metadata *mtdata;          mtdata = malloc(offsetof(struct metadata, data) + len);          if (mtdata == null)                 return null;          mtdata->private = null; /* or pointer implementation data */         mtdata->len = len;          return mtdata->data; }  void custom_free(void *ptr) {         struct metadata *mtdata;         char *bytes;          bytes = ptr;         mtdata = (struct metadata *)(bytes - offsetof(struct metadata, data));          /* stuff mtdata->private , mtdata->len */          free(mtdata); } 

is implementation valid (for c89/c90)?

ultimately, goal allocate contiguous memory len bytes requested user, associate them metadata. since not know length @ compile-time, data member declared array of char, which, understanding, can used alias object.

i unsure alignment issues because while pointer returned malloc guaranteed satisfy alignment requirements of pointers object type, data member of struct metadata might not after adding offsetof.

if not valid, there way achieve want (that is, associating metadata allocated memory, can accessed in custom_free function through pointer)?

i don't mind if metadata located contiguously before or after user data, nor in separate location (such obtained separate call malloc). need able access somehow through ptr user data passed custom_free.