Tribblix: manual page: esballoc.9f

ESBALLOC(9F) Kernel Functions for Drivers ESBALLOC(9F)

NAME

esballoc, desballoc, esballoca, desballoca - allocate a message block
using a caller-supplied buffer

SYNOPSIS

#include <sys/stream.h>

mblk_t *esballoc(uchar_t *base, size_t size, uint_t pri,
frtn_t *fr_rtnp);

mblk_t *desballoc(uchar_t *base, size_t size, uint_t pri,
frtn_t *fr_rtnp);

#include <sys/strsubr.h>

mblk_t *esballoca(uchar_t *base, size_t size, uint_t pri,
frtn_t *fr_rtnp);

mblk_t *desballoca(uchar_t *base, size_t size, uint_t pri,
frtn_t *fr_rtnp);

mblk_t *esballoc_wait(uchar_t *base, size_t size, uint_t pri,
frtn_t *fr_rtnp);

mblk_t *esballoca_wait(uchar_t *base, size_t size, uint_t pri,
frtn_t *fr_rtnp);

INTERFACE LEVEL

esballoc(): Architecture independent level 1 (DDI/DKI)

esballoca(): illumos DDI specific (illumos DDI)
desballoc(): illumos DDI specific (illumos DDI)
desballoca(): illumos DDI specific (illumos DDI)

esballoc_wait(): Volatile (private DDI function)
esballoca_wait(): Volatile (private DDI function)

PARAMETERS

base
Address of caller-supplied data buffer.

size
Number of bytes in data buffer.

pri
Priority of the request (no longer used).

fr_rtnp
Free routine data structure.

DESCRIPTION

The esballoc(), esballoca(), desballoc() and desballoca() functions
operate identically to allocb(9F), except that the data buffer to
associate with the message is specified by the caller. The allocated
message will have both the b_wptr and b_rptr set to the supplied data
buffer starting at base. Only the buffer itself can be specified by
the caller. The message block and data block header are allocated as
if by allocb(9F).

When freeb(9F) is called to free the message, the driver's message-
freeing routine, referenced through the free_rtn(9S) structure, is
called with appropriate arguments to free the data buffer.

The free_rtn(9S) structure includes the following members:

void (*free_func)(); /* caller's freeing routine */
caddr_t free_arg; /* argument to free_func() */

Instead of requiring a specific number of arguments, the free_arg
field is defined of type caddr_t. This way, the driver can pass a
pointer to a structure if more than one argument is needed. Note
that the address of the free_rtn(9S) structure passed to esballoc()
is used when the returned mblk/dblk pair is freed, and must remain
valid until then.

If esballoc() or esballoca() was used, then free_func will be called
asynchronously at some point after the message is no longer
referenced. If desballoc() or desballoca() was used, then free_func
will be called synchronously by the thread releasing the final
reference. See freeb(9F).

The free_func routine must not sleep, and must not access any
dynamically allocated data structures that could be freed before or
during its execution. In addition, because messages allocated with
desballoc() or desballoca() are freed in the context of the caller,
free_func must not call another module's put procedure, or attempt to
acquire a private module lock which might be held by another thread
across a call to a STREAMS utility routine that could free a message
block. Finally, free_func routines specified using desballoc() or
desballoca() may run in interrupt context and thus must only use
synchronization primitives that include an interrupt priority
returned from ddi_intr_get_pri(9F) or ddi_intr_get_softint_pri(9F).
If any of these restrictions are not followed, the possibility of
lock recursion or deadlock exists.

The variants ending with 'a' (esballoca() or desballoca() add an
"extra" ref to the dblk returned, also setting an internal flag so
that freeb(9F) handles the extra ref. The point of this extra ref is
so that any streams code handling this data block knows that the data
should not be modified without taking a copy. This is used by
callers sending external buffers that must not be modified.

The variants ending with "_wait" do a sleeping allocation, where the
ordinary esballoc() functions can return NULL when the system is low
on free memory.

RETURN VALUES

On success, a pointer to the newly allocated message block is
returned. On failure, esballoc(), esballoca(), desballoc() and
desballoca() return NULL. The functions esballoc_wait() and
esballoca_wait() do not return errors, and instead sleep until memory
is available.

CONTEXT

The esballoc(), esballoca(), desballoc() and desballoca() functions
can be called from user, interrupt, or kernel context. The functions
esballoc_wait() and esballoca_wait() can block and should not be
called from interrupt context.

ATTRIBUTES