--- name: "zco" section: "3" platform: "linux" package: "ion-doc" version: "3.2.1+dfsg-1.1" release: "Debian 9 (Stretch)" platforms: ["linux"] url: "https://new.carta.tech/zco" --- # zco(3) library for manipulating zero-copy objects ## NAME zco - library for manipulating zero-copy objects ## SYNOPSIS ``` #include "zco.h" typedef enum { ZcoFileSource = 1, ZcoSdrSource = 2, ZcoZcoSource = 3 } ZcoMedium; typedef void (*ZcoCallback); [see description for available functions] ``` ## DESCRIPTION "Zero-copy objects" (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 SDR to which implementations of all layers of the stack must have access. Each ZCO contains information enabling access to the source data objects, together with (a) a linked list of zero or more "extents" 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 ZCO 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. Each source data object may be either a file (identified by pathname stored in a "file reference" object in SDR heap) or an array of bytes in SDR heap space (identified by SDR address). Each protocol header or trailer capsule indicates the length and the address (within SDR 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 ZCO may be specified indirectly, by reference to an extent of an existing ZCO. The extents of multiple ZCOs may reference the same files and/or SDR source data objects. The source data objects are reference-counted to ensure that they are deleted automatically when (and only when) all ZCO extents that reference them have been deleted. Note that the safety of shared access to a ZCO is protected by the fact that the ZCO resides in SDR and therefore cannot be modified other than in the course of an SDR transaction, which serializes access. Moreover, extraction of data from a ZCO 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 SDR heap. For this reason, all ZCO "transmit" and "receive" functions must be performed within SDR transactions. Note also that ZCO 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. The total volume of file system space that may be occupied by file-sourced ZCO extents and the total volume of SDR heap space that may be occupied by heap-sourced ZCO extents are system configuration parameters that may be set by ZCO library functions. Those limits are enforced when extents are appended to ZCOs: total ZCO file space occupancy and total ZCO 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. - void zco\_register\_callback(ZcoCallback notify) This function registers the "callback" function that the ZCO system will invoke every time a ZCO is destroyed, making ZCO file and/or heap space available for the formation of new ZCO extents. This mechanism can be used, for example, to notify tasks that are waiting for ZCO space to be made available so that they can resume some communication protocol procedure. - void zco\_unregister\_callback( ) This function simply unregisters the currently registered callback function for ZCO destruction. - 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 ZCO whose source data object is the file identified by *pathName*. *cleanupScript*, if not NULL, is invoked at the moment the last ZCO that cites this file reference is destroyed \[normally upon delivery either down to the "ZCO transition layer" of the protocol stack or up to a ZCO-capable application\]. A zero-length string is interpreted as implicit direction to delete the referenced file when the file reference object is destroyed. Maximum length of *cleanupScript* is 255. Returns SDR location of file reference object on success, 0 on any error. - Object zco\_revise\_file\_ref(Sdr sdr, Object fileRef, char \*pathName, char \*cleanupScript) Changes the *pathName* and *cleanupScript* of the indicated file reference. The new values of these fields are validated as for *zco\_create\_file\_ref()*. Returns 0 on success, -1 on any error. - char \*zco\_file\_ref\_path(Sdr sdr, Object fileRef, char \*buffer, int buflen) Retrieves the pathName associated with *fileRef* and stores it in *buffer*, truncating it to fit (as indicated by *buflen*) and NULL-terminating it. On success, returns *buffer*; returns NULL on any error. - 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 ZCO via a reader with file offset tracking turned on. Otherwise returns zero. - void zco\_destroy\_file\_ref(Sdr sdr, Object fileRef) If the file reference object residing at location *fileRef* within the indicated Sdr is no longer in use (no longer referenced by any ZCO), destroys this file reference object immediately. Otherwise, flags this file reference object for destruction as soon as the last reference to it is removed. - vast zco\_get\_file\_occupancy(Sdr sdr) Returns the total number of file system space bytes occupied by ZCOs created in this Sdr. - 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. - 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. - 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 *length*. Returns 0 otherwise. - vast zco\_get\_heap\_occupancy(Sdr sdr) Returns the total number of SDR heap space bytes occupied by ZCOs created in this Sdr. - void zco\_set\_max\_heap\_occupancy(Sdr sdr, vast occupancy) Declares the total number of SDR heap space bytes that may be occupied by ZCOs created in this Sdr. - vast zco\_get\_max\_heap\_occupancy(Sdr sdr) Returns the total number of SDR heap space bytes that may be occupied by ZCOs created in this Sdr. - int zco\_enough\_heap\_space(Sdr sdr, vast length) Returns 1 if the total remaining SDR heap space available for ZCOs in this Sdr is greater than *length*. Returns 0 otherwise. - Object zco\_create(Sdr sdr, ZcoMedium firstExtentSourceMedium, Object firstExtentLocation, vast firstExtentOffset, vast firstExtentLength) Creates a new ZCO. *firstExtentLocation* and *firstExtentLength* must either both be zero (indicating that *zco\_append\_extent()* will be used to insert the first source data extent later) or else both be non-zero. If *firstExtentLocation* is non-zero, then (a) *firstExtentLocation* must be the SDR location of a file reference object if *firstExtentSourceMedium* is ZcoFileSource and must otherwise be the SDR location of the source data itself, and (b) *firstExtentOffset* indicates how many leading bytes of the source data object should be skipped over when adding the initial source data extent to the new ZCO. On success, returns the SDR location of the new ZCO. Returns 0 if there is insufficient ZCO space for creation of the new ZCO; returns ((Object) -1) on any error. - 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 ZCO, as described for *zco\_create()*. Both the *location* and *length* of the source data must be non-zero. Returns *length* on success, 0 if there is insufficient ZCO space for creation of the new source data extent, -1 on any error. - int zco\_prepend\_header(Sdr sdr, Object zco, char \*header, vast length) - int zco\_append\_trailer(Sdr sdr, Object zco, char \*trailer, vast length) - void zco\_discard\_first\_header(Sdr sdr, Object zco) - void zco\_discard\_last\_trailer(Sdr sdr, Object zco) These functions attach and remove the ZCO's headers and trailers. *header* and *trailer* are assumed to be arrays of octets, not necessarily text. Attaching a header or trailer causes it to be written to the SDR. The prepend and append functions return 0 on success, -1 on any error. - void zco\_destroy(Sdr sdr, Object zco) Destroys the indicated Zco. This reduces the reference counts for all files and SDR objects referenced in the ZCO's extents, resulting in the freeing of SDR objects and (optionally) the deletion of files as those reference count drop to zero. - Object zco\_clone(Sdr sdr, Object zco, vast offset, vast length) Creates a new ZCO whose source data is a copy of a subset of the source data of the referenced ZCO. Portions of the source data extents of the original ZCO are copied as necessary, but no header or trailer capsules are copied. Returns SDR location of the new ZCO on success, 0 on any error. - vast zco\_clone\_source\_data(Sdr sdr, Object toZco, Object fromZco, vast offset, vast length) Appends to *toZco* a copy of a subset of the source data of *fromZCO*. Portions of the source data extents of *fromZCO* are copied as necessary. Returns *length* on success, -1 on any error. - vast zco\_length(Sdr sdr, Object zco) Returns length of entire ZCO, 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. - vast zco\_source\_data\_length(Sdr sdr, Object zco) Returns length of entire ZCO 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. - void zco\_start\_transmitting(Object zco, ZcoReader \*reader) Used by underlying protocol layer to start extraction of an outbound ZCO's bytes (both from header and trailer capsules and from source data extents) for "transmission" -- 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 ZCO object. Populates *reader*, which is used to keep track of "transmission" progress via this ZCO reference. Note that this function can be called multiple times to restart reading at the start of the ZCO. Note also that multiple ZcoReader objects may be used concurrently, by the same task or different tasks, to advance through the ZCO independently. - void zco\_track\_file\_offset(ZcoReader \*reader) Turns on file offset tracking for this reader. - vast zco\_transmit(Sdr sdr, ZcoReader \*reader, vast length, char \*buffer) Copies *length* as-yet-uncopied bytes of the total concatenated ZCO (referenced by *reader*) into *buffer*. If *buffer* is NULL, skips over *length* bytes without copying. Returns the number of bytes copied (or skipped) on success, 0 on any file access error, -1 on any other error. - void zco\_start\_receiving(Object zco, ZcoReader \*reader) Used by overlying protocol layer to start extraction of an inbound ZCO's bytes for "reception" -- 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 ZCO objects. Populates *reader*, which is used to keep track of "reception" progress via this ZCO reference and is required. - vast zco\_receive\_headers(Sdr sdr, ZcoReader \*reader, vast length, char \*buffer) Copies *length* as-yet-uncopied bytes of presumptive protocol header text from ZCO source data extents into *buffer*. If *buffer* is NULL, skips over *length* bytes without copying. Returns number of bytes copied (or skipped) on success, 0 on any file access error, -1 on any other error. - void zco\_delimit\_source(Sdr sdr, Object zco, vast offset, vast length) Sets the computed offset and length of actual source data in the ZCO, thereby implicitly establishing the total length of the ZCO's concatenated protocol headers as *offset* and the location of the ZCO's innermost protocol trailer as the sum of *offset* and *length*. Offset and length are typically determined from the information carried in received presumptive protocol header text. - vast zco\_receive\_source(Sdr sdr, ZcoReader \*reader, vast length, char \*buffer) Copies *length* as-yet-uncopied bytes of source data from ZCO extents into *buffer*. If *buffer* is NULL, skips over *length* bytes without copying. Returns number of bytes copied (or skipped) on success, 0 on any file access error, -1 on any other error. - vast zco\_receive\_trailers(Sdr sdr, ZcoReader \*reader, vast length, char \*buffer) Copies *length* as-yet-uncopied bytes of trailer data from ZCO extents into *buffer*. If *buffer* is NULL, skips over *length* bytes without copying. Returns number of bytes copied (or skipped) on success, 0 on any file access error, -1 on any other error. - 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 ZCO 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 ZCO to another node or before enqueuing it for reception by an overlying application or protocol layer. ## SEE ALSO [*sdr*(3)](/sdr/3.md)