| KMEM(9) | Kernel Developer's Manual | KMEM(9) | 
kmem —
#include <sys/kmem.h>
void *
  
  kmem_alloc(size_t
    size, km_flag_t
    kmflags);
void *
  
  kmem_zalloc(size_t
    size, km_flag_t
    kmflags);
void
  
  kmem_free(void
    *p, size_t
  size);
void *
  
  kmem_intr_alloc(size_t
    size, km_flag_t
    kmflags);
void *
  
  kmem_intr_zalloc(size_t
    size, km_flag_t
    kmflags);
void
  
  kmem_intr_free(void
    *p, size_t
  size);
char *
  
  kmem_asprintf(const
    char *fmt,
  ...);
char *
  
  kmem_strdupsize(const
    char *str, size_t
    *size, km_flag_t
    kmflags);
char *
  
  kmem_strdup(const
    char *str, km_flag_t
    kmflags);
char *
  
  kmem_strndup(const
    char *str, size_t
    manxlen, km_flag_t
    kmflags);
void
  
  kmem_strfree(char
    *str);
void *
  
  kmem_tmpbuf_alloc(size_t
    size, void
    *stackbuf, size_t
    stackbufsize, km_flag_t
    kmflags);
void
  
  kmem_tmpbuf_free(void
    *p, size_t size,
    void *stackbuf);
  
  options KMEM_SIZE
kmem_alloc() allocates kernel wired memory. It takes the
  following arguments.
KM_SLEEPKM_SLEEP is specified,
          then the allocation cannot fail.KM_NOSLEEPNULL if there
          is not enough memory available. It should only be used when failure to
          allocate will not have harmful, user-visible effects.
        
        KM_NOSLEEP is strongly
          discouraged as it can create transient, hard to debug failures that
          occur when the system is under memory pressure.In situations where it is not possible to sleep, for example because locks are held by the caller, the code path should be restructured to allow the allocation to be made in another place.
The contents of allocated memory are uninitialized.
Unlike Solaris, kmem_alloc(0, flags) is illegal.
kmem_zalloc() is the equivalent of
    kmem_alloc(), except that it initializes the memory
    to zero.
kmem_asprintf() functions as the well
    known asprintf() function, but allocates memory
    using kmem_alloc(). This routine can sleep during
    allocation. The size of the allocated area is the length of the returned
    character string, plus one (for the NUL terminator). This must be taken into
    consideration when freeing the returned area with
    kmem_free().
kmem_free() frees kernel wired memory
    allocated by kmem_alloc() or
    kmem_zalloc() so that it can be used for other
    purposes. It takes the following arguments.
kmem_alloc() or
      kmem_zalloc().kmem_alloc() or
      kmem_zalloc() when the memory was allocated.Freeing NULL is illegal.
kmem_intr_alloc(),
    kmem_intr_zalloc() and
    kmem_intr_free() are the equivalents of the above
    kmem routines which can be called from the interrupt context. These routines
    are for the special cases. Normally,
    pool_cache(9) should be
    used for memory allocation from interrupt context.
The kmem_strdupsize() function is a
    utility function that can be used to copy the string in the
    str argument to a new buffer allocated using
    kmem_alloc() and optionally return the size of the
    allocation (the length of the string plus the trailing
    NUL) in the size argument if
    that is not NULL.
The kmem_strdup() function is a simplified
    version of kmem_strdupsize() that does not return
    the size of the allocation.
The kmem_strndup() function is variation
    of kmem_strdup() that copies at most
    maxlen characters from the string
    str always NUL terminating the copied string.
The kmem_strfree() function can be used to
    free a NUL terminated string computing the length of
    the string using strlen(3) and
    adding one for the NUL and then using
    kmem_free().
The kmem_tmpbuf_alloc() function is a
    utility function for allocating memory for temporary use, where allocation
    on the stack is desirable, but only up to a certain size. If the requested
    size fits within the specified stack buffer, the stack buffer is returned.
    Otherwise, memory is allocated with kmem_alloc().
    The kmem_tmpbuf_free() function compares the result
    of a previous call to kmem_tmpbuf_alloc() and frees
    the memory using kmem_free() if it is not the
    specified stack buffer.
KM_SLEEP allocations while holding mutexes or
  reader/writer locks is discouraged, as the caller can sleep for an unbounded
  amount of time in order to satisfy the allocation. This can in turn block
  other threads that wish to acquire locks held by the caller. It should be
  noted that kmem_free() may also block.
For some locks this is permissible or even unavoidable. For others, particularly locks that may be taken from soft interrupt context, it is a serious problem. As a general rule it is better not to allow this type of situation to develop. One way to circumvent the problem is to make allocations speculative and part of a retryable sequence. For example:
  retry:
        /* speculative unlocked check */
        if (need to allocate) {
                new_item = kmem_alloc(sizeof(*new_item), KM_SLEEP);
        } else {
                new_item = NULL;
        }
        mutex_enter(lock);
        /* check while holding lock for true status */
        if (need to allocate) {
                if (new_item == NULL) {
                        mutex_exit(lock);
                        goto retry;
                }
                consume(new_item);
                new_item = NULL;
        }
        mutex_exit(lock);
        if (new_item != NULL) {
                /* did not use it after all */
                kmem_free(new_item, sizeof(*new_item));
        }
KMEM_SIZE option ensure the
  size given in kmem_free() matches the actual allocated
  size. On kmem_alloc(), the kernel will allocate an
  additional contiguous kmem page of eight bytes in the buffer, will register
  the allocated size in the first kmem page of that buffer, and will return a
  pointer to the second kmem page in that same buffer. When freeing, the kernel
  reads the first page, and compares the size registered with the one given in
  kmem_free(). Any mismatch triggers a panic.
KMEM_SIZE is enabled by default on
    DIAGNOSTIC.
kmem_alloc(),
  kmem_asprintf(),
  kmem_intr_alloc(),
  kmem_intr_zalloc(),
  kmem_strdupsize(), and
  kmem_zalloc() return a pointer to allocated memory.
  Otherwise, NULL is returned.
kmem subsystem is implemented within the file
  sys/kern/subr_kmem.c.
kmem_alloc(),
  kmem_asprintf(), kmem_free(),
  kmem_strdupsize(),
  kmem_strfree(), and
  kmem_zalloc() functions cannot be used from interrupt
  context, from a soft interrupt, or from a callout. Use
  pool_cache(9) in these
  situations.
kmem_alloc() is
  uninitialized, it can contain security-sensitive data left by its previous
  user. It is the caller's responsibility not to expose it to the world.
| January 24, 2021 | NetBSD 10.0 |