malloc.c

   1 #include <stdlib.h>
   2 #include <sys/mman.h>
   3
   4 #define PGSIZE          4096
   5 #define PGMASK          (PGSIZE - 1)
   6 #define MSETMAX         4096
   7 #define MSETLEN         (1 << 15)
   8
   9 struct mset {
  10         int refs;
  11         int size;
  12 };
  13
  14 static struct mset *pool;
  15
  16 static int mk_pool(void)
  17 {
  18         if (pool && !pool->refs) {
  19                 pool->size = sizeof(*pool);
  20                 return 0;
  21         }
  22         pool = mmap(NULL, MSETLEN, PROT_READ | PROT_WRITE,
  23                                 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  24         if (pool == MAP_FAILED) {
  25                 pool = NULL;
  26                 return 1;
  27         }
  28         pool->size = sizeof(*pool);
  29         pool->refs = 0;
  30         return 0;
  31 }
  32
  33 void *malloc(long n)
  34 {
  35         void *m;
  36         if (n >= MSETMAX) {
  37                 m = mmap(NULL, n + PGSIZE, PROT_READ | PROT_WRITE,
  38                                 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  39                 if (m == MAP_FAILED)
  40                         return NULL;
  41                 *(long *) m = n + PGSIZE;       /* store length in the first page */
  42                 return m + PGSIZE;
  43         }
  44         if (!pool || MSETLEN - pool->size < n + sizeof(void *))
  45                 if (mk_pool())
  46                         return NULL;
  47         m = (void *) pool + pool->size;
  48         *(void **) m = pool;                    /* the address of the owning mset */
  49         pool->refs++;
  50         pool->size += (n + sizeof(void *) + 7) & ~7;
  51         if (!((pool + pool->size + sizeof(void *)) & PGMASK))
  52                 pool->size += sizeof(long);
  53         return m + sizeof(void *);
  54 }
  55
  56 void free(void *v)
  57 {
  58         if (!v)
  59                 return;
  60         if (v & PGMASK) {
  61                 struct mset *mset = *(void **) (v - sizeof(void *));
  62                 mset->refs--;
  63                 if (!mset->refs && mset != pool)
  64                         munmap(mset, mset->size);
  65         } else {
  66                 munmap(v - PGSIZE, *(long *) (v - PGSIZE));
  67         }
  68 }