在 C 中不使用 malloc 将局部变量保存在堆上（使用 sbrk 或 brk）

Question

I have a struct as follows:

struct 
  __attribute__((__packed__)) // compiler directive, avoid "gcc" padding bytes to struct
  meta_data {
    size_t size;             // 8 bytes (in 64-bit OS)
    char free;               // 1 byte ('f' or 'o')
};

The size of the struct is 9 bytes.

I want to use sbrk or brk to allocate 9 bytes on the heap and save a struct on the heap, and return a pointer to the dynamically allocated memory block on heap.

Currently I have something like this:

void *add(size_t size) {
  void* returnPointer;
  returnPointer = sbrk(9);
  struct meta_data newMetadata;
  newMetadata.size = size;
  newMetadata.free = 'o';
  return returnPointer;
}

This function should be able to be used multiple times to add more metadata blocks on the heap.

However, in this case, when the function ends, the local variable newMetadata declared inside the function is also terminated.

How can I keep it on the heap and not lose access to the newMetadata created?

Note: I am not allowed to use malloc for this.

Answer 1

The usual method is to mimic what some malloc packages do. That is, put the meta data at a lower address than what is returned as the "data" pointer.

Here's your code refactored to do that:

#include <unistd.h>

struct meta_data {
    size_t size;                        // 8 bytes (in 64-bit OS)
    char free;                          // 1 byte ('f' or 'o')
// compiler directive, avoid "gcc" padding bytes to struct
} __attribute__ ((__packed__));

void *
add(size_t size)
{
    struct meta_data *meta;
    void *returnPointer;

    meta = sbrk(sizeof(struct meta_data) + size);
    meta->size = size;
    meta->free = 'o';

    returnPointer = meta;
    returnPointer += sizeof(struct meta_data);

    return returnPointer;
}

Note that most malloc replacements will guarantee that returnPointer is "aligned for any purpose", which usually means aligned to an 8 byte boundary (ie the lower 3 bits of returnPointer are 0).

The above code doesn't do that, so it's something you can add [later].