.TH ALLOCB 9
.SH NAME
allocb, iallocb, freeb, freeblist, BLEN, blocklen, concatblock, copyblock, trimblock, packblock, padblock, pullblock, pullupblock, adjustblock, checkb \- data block management
.SH SYNOPSIS
.ta \w'\fLBlock* 'u
.B
Block*	allocb(int size)
.PP
.B
Block*	iallocb(int size)
.PP
.B
void	freeb(Block *b)
.PP
.B
void	freeblist(Block *b)
.PP
.B
long	BLEN(Block *b)
.PP
.B
int	blocklen(Block *b)
.PP
.B
Block*	concatblock(Block *b)
.PP
.B
Block*	copyblock(Block *b, int n)
.PP
.B
Block*	trimblock(Block *b, int offset, int n)
.PP
.B
Block*	packblock(Block *b)
.PP
.B
Block*	padblock(Block *b, int n)
.PP
.B
int	pullblock(Block **bph, int n)
.PP
.B
Block*	pullupblock(Block *b, int n)
.PP
.B
Block*	adjustblock(Block *b, int n)
.PP
.B
void	checkb(Block *b, char *msg)
.SH DESCRIPTION
A
.B Block
provides a receptacle for data:
.IP
.EX
.DT
typedef
struct Block
{
    Block*  next;
    Block*  list;
    uchar*  rp;     /* first unconsumed byte */
    uchar*  wp;     /* first empty byte */
    uchar*  lim;    /* 1 past the end of the buffer */
    uchar*  base;   /* start of the buffer */
    void    (*free)(Block*);
    ulong   flag;
} Block;
.EE
.PP
Each
.B Block
has an associated buffer, located at
.BR base ,
and accessed via
.B wp
when filling the buffer, or
.B rp
when fetching data from it.
Each pointer should be incremented to reflect the amount of data written or read.
A
.B Block
is empty when
.B rp
reaches
.BR wp .
The pointer
.B lim
bounds the allocated space.
Some operations described below accept lists of
.BR Block s,
which are
chained via their
.B next
pointers, with a null pointer ending the list.
.B Blocks
are usually intended for a
.B Queue
(see
.IR qio (9)),
but can be used independently.
.PP
A
.B Block
and its buffer are normally allocated by one call to
.IR malloc (9)
and aligned on an 8 byte (\fLBY2V\fP) boundary.
Some devices with particular allocation constraints
(eg, requiring certain addresses for DMA) might allocate their own
.B Block
and buffer;
.B free
must then point to a function that can deallocate the specially allocated
.BR Block .
.PP
Many
.B Block
operations cannot be used in interrupt handlers
because they either
.IR sleep (9)
or raise an
.IR error (9).
Of operations that allocate blocks, only
.IR iallocb
is usable.
.PP
.I Allocb
allocates a
.B Block
of at least
.IR size
bytes.
The block
is initially empty:
.B rp
and
.B wp
point to the start of the data.
If it cannot allocate memory,
.I allocb
raises an
.IR error (9);
it cannot be used by an interrupt handler.
.PP
.IR Iallocb
is similar to
.IR allocb
but is intended for use by interrupt handlers,
and returns a null pointer if no memory is available.
It also limits its allocation to a quota allocated at system initialisation to interrupt-time buffering.
.PP
.I Freeb
frees a single
.B Block
(and its buffer).
.PP
.I Freeblist
frees the whole
list of blocks headed by
.IR b .
.PP
.I BLEN
returns the number of unread bytes in a single block
.IR b ;
it is implemented as a macro.
.PP
.I Blocklen
returns the number of bytes of unread data in the whole list of blocks headed by
.IR b .
.PP
.I Concatblock
returns
.I b
if it is not a list, and otherwise
returns a single
.B Block
containing all the data in the list of blocks
.IR b ,
which it frees.
.PP
.I Copyblock
by contrast returns a single
.B Block
containing a copy of the first
.I n
bytes of data in the block list
.IR b ,
padding with zeroes if the list contained less than
.I n
bytes.
The list
.I b
is unchanged.
.PP
.I Padblock
can pad a single
.B Block
at either end, to reserve space for protocol headers or trailers.
If
.IR n ≥ 0 ,
it inserts
.I n
bytes at the start of the block,
setting the read pointer
.B rp
to point to the new space.
If
.IR n < 0 ,
it adds
.I n
bytes at the end of the block,
leaving the write pointer
.B wp
pointing at the new space.
In both cases, it allocates a new
.B Block
if necessary, freeing the old, and
it always returns a pointer to the resulting
.BR Block .
.PP
.I Trimblock
trims the list
.I b
to contain no more than
.I n
bytes starting at
.I offset
bytes into the data of the original list.
It returns a new list, freeing unneeded parts of the old.
If no data remains, it returns a null pointer.
.PP
.I Packblock
examines each
.B Block
in the list
.IR b ,
reallocating any block in the list that has four times more available space than actual data.
It returns a pointer to the revised list.
.PP
.I Pullblock
discards up to
.I n
bytes from the start of the list headed by
.BI * bph \f1.\f0
Unneeded blocks are freed.
.I Pullblock
sets
.BI * bph
to point to the new list head
and returns the number of bytes discarded (which might be less than
.IR n ).
It is used by transport protocols to discard ack'd data at
the head of a retransmission queue.
.PP
.I Pullupblock
rearranges the data in the list of blocks
.I b
to ensure that there are at least
.I n
bytes of contiguous data in the first block,
and returns a pointer to the new list head.
It frees any blocks that it empties.
It returns a null pointer if there is not enough data in the list.
.PP
.I Adjustblock
ensures that the block
.I b
has at least
.I n
bytes of data, reallocating or padding with zero if necessary.
It returns a pointer to the new
.BR Block .
(If
.I n
is negative, it frees the block and returns a null pointer.)
.PP
.I Checkb
does some consistency checking of
the state of
.IR b ;
a
.IR panic (9)
results if things look grim.
It is intended for internal use by the queue I/O routines (see
.IR qio (9))
but could be used elsewhere.
.PP
The only functions that can be called at interrupt level are
.IR iallocb ,
.IR freeb ,
.IR freeblist ,
.IR BLEN ,
.IR blocklen ,
.IR trimblock
and
.IR pullupblock .
The others allocate memory and can potentially block.
.SH DIAGNOSTICS
Many functions directly or indirectly can raise an
.IR error (9),
and callers must therefore provide for proper error recovery
as described therein to prevent memory leaks and other bugs.
Except for
.IR iallocb ,
any functions that allocate new blocks or lists
are unsuitable for use by interrupt handlers.
.IR Iallocb
returns a null pointer when it runs out of memory.
.SH SOURCE
.B /sys/src/9/port/allocb.c
.SH SEE ALSO
.IR qio (9)