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.\"
.IX Title "ici::doc::pod3::zco 3"
.TH ici::doc::pod3::zco 3 "2016-07-07" "perl v5.24.1" "ICI library functions"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
zco \- library for manipulating zero\-copy objects
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    #include "zco.h"
\&
\&    typedef enum
\&    {
\&        ZcoFileSource = 1,
\&        ZcoSdrSource = 2,
\&        ZcoZcoSource = 3
\&    } ZcoMedium;
\&
\&    typedef void (*ZcoCallback);
\&
\&    [see description for available functions]
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\*(L"Zero-copy objects\*(R" (ZCOs) are abstract data access representations
designed to minimize I/O in the encapsulation of application source
data within one or more layers of communication protocol structure.  ZCOs
are constructed within the heap space of an \s-1SDR\s0 to which implementations
of all layers of the stack must have access.  Each \s-1ZCO\s0 contains information
enabling access to the source data objects, together with (a) a linked list
of zero or more \*(L"extents\*(R" that reference portions of these source data
objects and (b) linked lists of protocol header and trailer capsules that
have been explicitly attached to the \s-1ZCO\s0 since its creation.  The
concatenation of the headers (in ascending stack sequence), source data
object extents, and trailers (in descending stack sequence) is what is to
be transmitted or has been received.
.PP
Each source data object may be either a file (identified by pathname
stored in a \*(L"file reference\*(R" object in \s-1SDR\s0 heap) or an array of bytes
in \s-1SDR\s0 heap space (identified by \s-1SDR\s0 address).  Each protocol header
or trailer capsule indicates the length and the address (within \s-1SDR\s0
heap space) of a single protocol header or trailer at some layer of the
stack.  Note that for some purposes the source data object for a newly
added extent of a \s-1ZCO\s0 may be specified indirectly, by reference to an
extent of an existing \s-1ZCO.\s0
.PP
The extents of multiple ZCOs may reference the same files and/or \s-1SDR\s0 source
data objects.  The source data objects are reference-counted to ensure
that they are deleted automatically when (and only when) all \s-1ZCO\s0 extents
that reference them have been deleted.
.PP
Note that the safety of shared access to a \s-1ZCO\s0 is protected by the
fact that the \s-1ZCO\s0 resides in \s-1SDR\s0 and therefore cannot be modified
other than in the course of an \s-1SDR\s0 transaction, which serializes
access.  Moreover, extraction of data from a \s-1ZCO\s0 may entail the reading
of file-based source data extents, which may cause file progress to
be updated in one or more file reference objects in the \s-1SDR\s0 heap.  For
this reason, all \s-1ZCO \s0\*(L"transmit\*(R" and \*(L"receive\*(R" functions must be performed
within \s-1SDR\s0 transactions.
.PP
Note also that \s-1ZCO\s0 can more broadly be used as a general-purpose
reference counting system for non-volatile data objects, where a
need for such a system is identified.
.PP
The total volume of file system space that may be occupied by file-sourced
\&\s-1ZCO\s0 extents and the total volume of \s-1SDR\s0 heap space that may be occupied by
heap-sourced \s-1ZCO\s0 extents are system configuration parameters that may be set
by \s-1ZCO\s0 library functions.  Those limits are enforced when extents are appended
to ZCOs: total \s-1ZCO\s0 file space occupancy and total \s-1ZCO\s0 heap occupancy are
updated continuously as ZCOs are created and destroyed, and the formation
of a new extent is prohibited when the length of the extent exceeds the
difference between the applicable limit and the corresponding current
occupancy total.
.IP "void zco_register_callback(ZcoCallback notify)" 4
.IX Item "void zco_register_callback(ZcoCallback notify)"
This function registers the \*(L"callback\*(R" function that the \s-1ZCO\s0 system will
invoke every time a \s-1ZCO\s0 is destroyed, making \s-1ZCO\s0 file and/or heap space
available for the formation of new \s-1ZCO\s0 extents.  This mechanism can be
used, for example, to notify tasks that are waiting for \s-1ZCO\s0 space to be
made available so that they can resume some communication protocol
procedure.
.IP "void zco_unregister_callback( )" 4
.IX Item "void zco_unregister_callback( )"
This function simply unregisters the currently registered callback function
for \s-1ZCO\s0 destruction.
.IP "Object zco_create_file_ref(Sdr sdr, char *pathName, char *cleanupScript)" 4
.IX Item "Object zco_create_file_ref(Sdr sdr, char *pathName, char *cleanupScript)"
Creates and returns a new file reference object, which can be used as the
source data extent location for creating a \s-1ZCO\s0 whose source data object is
the file identified by \fIpathName\fR.  \fIcleanupScript\fR, if not \s-1NULL,\s0 is invoked
at the moment the last \s-1ZCO\s0 that cites this file reference is destroyed
[normally upon delivery either down to the \*(L"\s-1ZCO\s0 transition layer\*(R" of the
protocol stack or up to a ZCO-capable application].  A zero-length string
is interpreted as implicit direction to delete the referenced file when
the file reference object is destroyed.  Maximum length of \fIcleanupScript\fR
is 255.  Returns \s-1SDR\s0 location of file reference object on success, 0 on any
error.
.IP "Object zco_revise_file_ref(Sdr sdr, Object fileRef, char *pathName, char *cleanupScript)" 4
.IX Item "Object zco_revise_file_ref(Sdr sdr, Object fileRef, char *pathName, char *cleanupScript)"
Changes the \fIpathName\fR and \fIcleanupScript\fR of the indicated file
reference.  The new values of these fields are validated as for
\&\fIzco_create_file_ref()\fR.  Returns 0 on success, \-1 on any error.
.IP "char *zco_file_ref_path(Sdr sdr, Object fileRef, char *buffer, int buflen)" 4
.IX Item "char *zco_file_ref_path(Sdr sdr, Object fileRef, char *buffer, int buflen)"
Retrieves the pathName associated with \fIfileRef\fR and stores it in \fIbuffer\fR,
truncating it to fit (as indicated by \fIbuflen\fR) and NULL-terminating it.  On
success, returns \fIbuffer\fR; returns \s-1NULL\s0 on any error.
.IP "int zco_file_ref_xmit_eof(Sdr sdr, Object fileRef)" 4
.IX Item "int zco_file_ref_xmit_eof(Sdr sdr, Object fileRef)"
Returns 1 if the last octet of the referenced file (as determined at the
time the file reference object was created) has been read by \s-1ZCO\s0 via a
reader with file offset tracking turned on.  Otherwise returns zero.
.IP "void zco_destroy_file_ref(Sdr sdr, Object fileRef)" 4
.IX Item "void zco_destroy_file_ref(Sdr sdr, Object fileRef)"
If the file reference object residing at location \fIfileRef\fR within
the indicated Sdr is no longer in use (no longer referenced by any \s-1ZCO\s0),
destroys this file reference object immediately.  Otherwise, flags this
file reference object for destruction as soon as the last reference to
it is removed.
.IP "vast zco_get_file_occupancy(Sdr sdr)" 4
.IX Item "vast zco_get_file_occupancy(Sdr sdr)"
Returns the total number of file system space bytes occupied by ZCOs created
in this Sdr.
.IP "void zco_set_max_file_occupancy(Sdr sdr, vast occupancy)" 4
.IX Item "void zco_set_max_file_occupancy(Sdr sdr, vast occupancy)"
Declares the total number of file system space bytes that may be occupied by
ZCOs created in this Sdr.
.IP "vast zco_get_max_file_occupancy(Sdr sdr)" 4
.IX Item "vast zco_get_max_file_occupancy(Sdr sdr)"
Returns the total number of file system space bytes that may be occupied by
ZCOs created in this Sdr.
.IP "int zco_enough_file_space(Sdr sdr, vast length)" 4
.IX Item "int zco_enough_file_space(Sdr sdr, vast length)"
Returns 1 if the total remaining file system space available for ZCOs in this
Sdr is greater than \fIlength\fR.  Returns 0 otherwise.
.IP "vast zco_get_heap_occupancy(Sdr sdr)" 4
.IX Item "vast zco_get_heap_occupancy(Sdr sdr)"
Returns the total number of \s-1SDR\s0 heap space bytes occupied by ZCOs created
in this Sdr.
.IP "void zco_set_max_heap_occupancy(Sdr sdr, vast occupancy)" 4
.IX Item "void zco_set_max_heap_occupancy(Sdr sdr, vast occupancy)"
Declares the total number of \s-1SDR\s0 heap space bytes that may be occupied by
ZCOs created in this Sdr.
.IP "vast zco_get_max_heap_occupancy(Sdr sdr)" 4
.IX Item "vast zco_get_max_heap_occupancy(Sdr sdr)"
Returns the total number of \s-1SDR\s0 heap space bytes that may be occupied by
ZCOs created in this Sdr.
.IP "int zco_enough_heap_space(Sdr sdr, vast length)" 4
.IX Item "int zco_enough_heap_space(Sdr sdr, vast length)"
Returns 1 if the total remaining \s-1SDR\s0 heap space available for ZCOs in this Sdr
is greater than \fIlength\fR.  Returns 0 otherwise.
.IP "Object zco_create(Sdr sdr, ZcoMedium firstExtentSourceMedium, Object firstExtentLocation, vast firstExtentOffset, vast firstExtentLength)" 4
.IX Item "Object zco_create(Sdr sdr, ZcoMedium firstExtentSourceMedium, Object firstExtentLocation, vast firstExtentOffset, vast firstExtentLength)"
Creates a new \s-1ZCO.  \s0\fIfirstExtentLocation\fR and \fIfirstExtentLength\fR
must either both be zero (indicating that \fIzco_append_extent()\fR will be used to
insert the first source data extent later) or else both be non-zero.
If \fIfirstExtentLocation\fR is non-zero, then (a) \fIfirstExtentLocation\fR must
be the \s-1SDR\s0 location of a file reference object if \fIfirstExtentSourceMedium\fR is
ZcoFileSource and must otherwise be the \s-1SDR\s0 location of the source data itself,
and (b) \fIfirstExtentOffset\fR indicates how many leading bytes of the source
data object should be skipped over when adding the initial source data extent
to the new \s-1ZCO. \s0 On success, returns the \s-1SDR\s0 location of the new \s-1ZCO. \s0 Returns
0 if there is insufficient \s-1ZCO\s0 space for creation of the new \s-1ZCO\s0; returns
((Object) \-1) on any error.
.IP "int zco_append_extent(Sdr sdr, Object zco, ZcoMedium sourceMedium, Object location, vast offset, vast length)" 4
.IX Item "int zco_append_extent(Sdr sdr, Object zco, ZcoMedium sourceMedium, Object location, vast offset, vast length)"
Appends the indicated source data extent to the indicated \s-1ZCO,\s0 as described
for \fIzco_create()\fR.  Both the \fIlocation\fR and \fIlength\fR of the source data
must be non-zero.  Returns \fIlength\fR on success, 0 if there is insufficient
\&\s-1ZCO\s0 space for creation of the new source data extent, \-1 on any error.
.IP "int zco_prepend_header(Sdr sdr, Object zco, char *header, vast length)" 4
.IX Item "int zco_prepend_header(Sdr sdr, Object zco, char *header, vast length)"
.PD 0
.IP "int zco_append_trailer(Sdr sdr, Object zco, char *trailer, vast length)" 4
.IX Item "int zco_append_trailer(Sdr sdr, Object zco, char *trailer, vast length)"
.IP "void zco_discard_first_header(Sdr sdr, Object zco)" 4
.IX Item "void zco_discard_first_header(Sdr sdr, Object zco)"
.IP "void zco_discard_last_trailer(Sdr sdr, Object zco)" 4
.IX Item "void zco_discard_last_trailer(Sdr sdr, Object zco)"
.PD
These functions attach and remove the \s-1ZCO\s0's headers and trailers.  \fIheader\fR
and \fItrailer\fR are assumed to be arrays of octets, not necessarily text.  
Attaching a header or trailer causes it to be written to the \s-1SDR. \s0 The
prepend and append functions return 0 on success, \-1 on any error.
.IP "void zco_destroy(Sdr sdr, Object zco)" 4
.IX Item "void zco_destroy(Sdr sdr, Object zco)"
Destroys the indicated Zco.  This reduces the reference counts for all
files and \s-1SDR\s0 objects referenced in the \s-1ZCO\s0's extents, resulting in the
freeing of \s-1SDR\s0 objects and (optionally) the deletion of files as those
reference count drop to zero.
.IP "Object zco_clone(Sdr sdr, Object zco, vast offset, vast length)" 4
.IX Item "Object zco_clone(Sdr sdr, Object zco, vast offset, vast length)"
Creates a new \s-1ZCO\s0 whose source data is a copy of a subset of the source
data of the referenced \s-1ZCO. \s0 Portions of the source data extents of the
original \s-1ZCO\s0 are copied as necessary, but no header or trailer capsules are
copied.  Returns \s-1SDR\s0 location of the new \s-1ZCO\s0 on success, 0 on any error.
.IP "vast zco_clone_source_data(Sdr sdr, Object toZco, Object fromZco, vast offset, vast length)" 4
.IX Item "vast zco_clone_source_data(Sdr sdr, Object toZco, Object fromZco, vast offset, vast length)"
Appends to \fItoZco\fR a copy of a subset of the source data of \fIfromZCO\fR.
Portions of the source data extents of \fIfromZCO\fR are copied as necessary.
Returns \fIlength\fR on success, \-1 on any error.
.IP "vast zco_length(Sdr sdr, Object zco)" 4
.IX Item "vast zco_length(Sdr sdr, Object zco)"
Returns length of entire \s-1ZCO,\s0 including all headers and trailers and
all source data extents.  This is the size of the object that would be
formed by concatenating the text of all headers, trailers, and source
data extents into a single serialized object.
.IP "vast zco_source_data_length(Sdr sdr, Object zco)" 4
.IX Item "vast zco_source_data_length(Sdr sdr, Object zco)"
Returns length of entire \s-1ZCO\s0 minus the lengths of all attached header and
trailer capsules.  This is the size of the object that would be formed by
concatenating the text of all source data extents (including those that
are presumed to contain header or trailer text attached elsewhere) into
a single serialized object.
.IP "void zco_start_transmitting(Object zco, ZcoReader *reader)" 4
.IX Item "void zco_start_transmitting(Object zco, ZcoReader *reader)"
Used by underlying protocol layer to start extraction of an outbound \s-1ZCO\s0's
bytes (both from header and trailer capsules and from source data extents) for
\&\*(L"transmission\*(R" \*(-- i.e., the copying of bytes into a memory buffer for
delivery to some non-ZCO-aware protocol implementation.  Initializes
reading at the first byte of the total concatenated \s-1ZCO\s0 object.  Populates
\&\fIreader\fR, which is used to keep track of \*(L"transmission\*(R" progress via this
\&\s-1ZCO\s0 reference.
.Sp
Note that this function can be called multiple times to restart reading at
the start of the \s-1ZCO. \s0 Note also that multiple ZcoReader objects may be used
concurrently, by the same task or different tasks, to advance through the
\&\s-1ZCO\s0 independently.
.IP "void zco_track_file_offset(ZcoReader *reader)" 4
.IX Item "void zco_track_file_offset(ZcoReader *reader)"
Turns on file offset tracking for this reader.
.IP "vast zco_transmit(Sdr sdr, ZcoReader *reader, vast length, char *buffer)" 4
.IX Item "vast zco_transmit(Sdr sdr, ZcoReader *reader, vast length, char *buffer)"
Copies \fIlength\fR as-yet-uncopied bytes of the total concatenated \s-1ZCO
\&\s0(referenced by \fIreader\fR) into \fIbuffer\fR.  If \fIbuffer\fR is \s-1NULL,\s0 skips
over \fIlength\fR bytes without copying.  Returns the number of bytes copied
(or skipped) on success, 0 on any file access error, \-1 on any other error.
.IP "void zco_start_receiving(Object zco, ZcoReader *reader)" 4
.IX Item "void zco_start_receiving(Object zco, ZcoReader *reader)"
Used by overlying protocol layer to start extraction of an inbound \s-1ZCO\s0's
bytes for \*(L"reception\*(R" \*(-- i.e., the copying of bytes into a memory buffer
for delivery to a protocol header parser, to a protocol trailer parser,
or to the ultimate recipient (application).  Initializes reading of
headers, source data, and trailers at the first byte of the concatenated
\&\s-1ZCO\s0 objects.  Populates \fIreader\fR, which is used to keep track of \*(L"reception\*(R"
progress via this \s-1ZCO\s0 reference and is required.
.IP "vast zco_receive_headers(Sdr sdr, ZcoReader *reader, vast length, char *buffer)" 4
.IX Item "vast zco_receive_headers(Sdr sdr, ZcoReader *reader, vast length, char *buffer)"
Copies \fIlength\fR as-yet-uncopied bytes of presumptive protocol header text
from \s-1ZCO\s0 source data extents into \fIbuffer\fR.  If \fIbuffer\fR is \s-1NULL,\s0 skips
over \fIlength\fR bytes without copying.  Returns number of bytes copied (or
skipped) on success, 0 on any file access error, \-1 on any other error.
.IP "void zco_delimit_source(Sdr sdr, Object zco, vast offset, vast length)" 4
.IX Item "void zco_delimit_source(Sdr sdr, Object zco, vast offset, vast length)"
Sets the computed offset and length of actual source data in the \s-1ZCO,\s0
thereby implicitly establishing the total length of the \s-1ZCO\s0's concatenated
protocol headers as \fIoffset\fR and the location of the \s-1ZCO\s0's innermost
protocol trailer as the sum of \fIoffset\fR and \fIlength\fR.  Offset and length
are typically determined from the information carried in received presumptive
protocol header text.
.IP "vast zco_receive_source(Sdr sdr, ZcoReader *reader, vast length, char *buffer)" 4
.IX Item "vast zco_receive_source(Sdr sdr, ZcoReader *reader, vast length, char *buffer)"
Copies \fIlength\fR as-yet-uncopied bytes of source data from \s-1ZCO\s0 extents into
\&\fIbuffer\fR.  If \fIbuffer\fR is \s-1NULL,\s0 skips over \fIlength\fR bytes without
copying.  Returns number of bytes copied (or skipped) on success, 0 on any
file access error, \-1 on any other error.
.IP "vast zco_receive_trailers(Sdr sdr, ZcoReader *reader, vast length, char *buffer)" 4
.IX Item "vast zco_receive_trailers(Sdr sdr, ZcoReader *reader, vast length, char *buffer)"
Copies \fIlength\fR as-yet-uncopied bytes of trailer data from \s-1ZCO\s0 extents into
\&\fIbuffer\fR.  If \fIbuffer\fR is \s-1NULL,\s0 skips over \fIlength\fR bytes without copying.
Returns number of bytes copied (or skipped) on success, 0 on any file access
error, \-1 on any other error.
.IP "void zco_strip(Sdr sdr, Object zco)" 4
.IX Item "void zco_strip(Sdr sdr, Object zco)"
Deletes all source data extents that contain only header or trailer data and
adjusts the offsets and/or lengths of all remaining extents to exclude any
known header or trailer data.  This function is useful when handling a \s-1ZCO\s0
that was received from an underlying protocol layer rather than from an
overlying application or protocol layer; use it before starting the
transmission of the \s-1ZCO\s0 to another node or before enqueuing it for
reception by an overlying application or protocol layer.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIsdr\fR\|(3)