1 KMEM_ALLOC(9F)           Kernel Functions for Drivers           KMEM_ALLOC(9F)
   2 
   3 
   4 
   5 NAME
   6        kmem_alloc, kmem_zalloc, kmem_free - allocate kernel memory
   7 
   8 SYNOPSIS
   9        #include <sys/types.h>
  10        #include <sys/kmem.h>
  11 
  12 
  13 
  14        void *kmem_alloc(size_t size, int flag);
  15 
  16 
  17        void *kmem_zalloc(size_t size, int flag);
  18 
  19 
  20        void kmem_free(void *buf, size_t size);
  21 
  22 
  23 INTERFACE LEVEL
  24        Architecture independent level 1 (DDI/DKI).
  25 
  26 PARAMETERS
  27        size
  28                Number of bytes to allocate.
  29 
  30 
  31        flag
  32                Determines whether caller can sleep for memory. Possible flags
  33                are KM_SLEEP to allow sleeping until memory is available,
  34                KM_NOSLEEP to return NULL if memory is not available even after
  35                some reclamation attempts, and KM_NOSLEEP_LAZY to return NULL
  36                without reclamation attempts.  KM_NOSLEEP_LAZY is actually two
  37                flags combined: (KM_NOSLEEP | KM_NORMALPRI), the latter flag
  38                indicating not to attempt reclamation before giving up and
  39                returning NULL.  If any mention of KM_NOSLEEP appears in this
  40                man page by itself, it applies equally to KM_NOSLEEP_LAZY as
  41                well.
  42 
  43 
  44        buf
  45                Pointer to allocated memory.
  46 
  47 
  48 DESCRIPTION
  49        The kmem_alloc() function allocates size bytes of kernel memory and
  50        returns a pointer to the allocated memory. The allocated memory is at
  51        least double-word aligned, so it can hold any C data structure. No
  52        greater alignment can be assumed. flag determines whether the caller
  53        can sleep for memory.  KM_SLEEP allocations may sleep but are
  54        guaranteed to succeed.  KM_NOSLEEP and KM_NOSLEEP_LAZY allocations are
  55        guaranteed not to sleep but may fail (return NULL) if no memory is
  56        currently available. KM_NOSLEEP will first attempt to aggressively
  57        reclaim memory from otherwise unused blocks, while KM_NOSLEEP_LAZY will
  58        not attempt any reclamation. The initial contents of memory allocated
  59        using kmem_alloc() are random garbage.
  60 
  61 
  62        The kmem_zalloc() function is like kmem_alloc() but returns zero-filled
  63        memory.
  64 
  65 
  66        The kmem_free() function frees previously allocated kernel memory. The
  67        buffer address and size must exactly match the original allocation.
  68        Memory cannot be returned piecemeal.
  69 
  70 RETURN VALUES
  71        If successful, kmem_alloc() and kmem_zalloc() return a pointer to the
  72        allocated memory. If KM_NOSLEEP is set and memory cannot be allocated
  73        without sleeping, kmem_alloc() and kmem_zalloc() return NULL.
  74 
  75 CONTEXT
  76        The kmem_alloc() and kmem_zalloc() functions can be called from
  77        interrupt context only if the KM_NOSLEEP flag is set. They can be
  78        called from user context with any valid flag. The kmem_free() function
  79        can be called from from user, interrupt, or kernel context.
  80 
  81 SEE ALSO
  82        copyout(9F), freerbuf(9F), getrbuf(9F)
  83 
  84 
  85        Writing Device Drivers
  86 
  87 WARNINGS
  88        Memory allocated using kmem_alloc() is not paged. Available memory is
  89        therefore limited by the total physical memory on the system. It is
  90        also limited by the available kernel virtual address space, which is
  91        often the more restrictive constraint on large-memory configurations.
  92 
  93 
  94        Excessive use of kernel memory is likely to affect overall system
  95        performance.  Overcommitment of kernel memory will cause the system to
  96        hang or panic.
  97 
  98 
  99        Misuse of the kernel memory allocator, such as writing past the end of
 100        a buffer, using a buffer after freeing it, freeing a buffer twice, or
 101        freeing a null or invalid pointer, will corrupt the kernel heap and may
 102        cause the system to corrupt data or panic.
 103 
 104 
 105        The initial contents of memory allocated using kmem_alloc() are random
 106        garbage. This random garbage may include secure kernel data. Therefore,
 107        uninitialized kernel memory should be handled carefully. For example,
 108        never copyout(9F) a potentially uninitialized buffer.
 109 
 110 NOTES
 111        kmem_alloc(0, flag) always returns NULL, but if KM_SLEEP is set, this
 112        behavior is considered to be deprecated; the system may be configured
 113        to explicitly panic in this case in lieu of returning NULL.
 114        kmem_free(NULL, 0) is legal, however.
 115 
 116 
 117 
 118                                November 20, 2019                KMEM_ALLOC(9F)