# Overview¶

YottaDB is a multi-language NoSQL database. The daemonless database engine resides in the address space of the process, and can be accessed from any supported language. Functions in the supported languages can call one another to the extent that such calling is permitted by the Supported language implementations.

# Quick Start¶

## Local Installation¶

• Create a temporary directory and change to it, e.g.: mkdir /tmp/tmp ; cd /tmp/tmp
• Get the YottaDB installer: wget https://gitlab.com/YottaDB/DB/YDB/raw/master/sr_unix/ydbinstall.sh
• Make it executable: chmod +x ydbinstall.sh
• Run it with your choice of directory where you want it installed (omit the --verbose option for less output): sudo ./ydbinstall.sh --utf8 default --verbose. If you do not specify an installation directory with --installdir, the script installs YottaDB in /usr/local/lib/yottadb/r### where r### is the release, e.g., r122.
1. Choose a directory for your default environment and initialize it: export ydb_dir=$HOME/.yottadb ; . /usr/local/lib/yottadb/r122/ydb_env_set 2. Put your C program in the $ydb_dir directory, #include the file /usr/local/lib/yottadb/r122/libyottadb.h in your C program and compile it. As a sample program, you can download the wordfreq.c program, with a reference input file and corresponding reference output file and compile it with gcc -I $ydb_dist -L$ydb_dist -o wordfreq wordfreq.c -lyottadb.
3. Run your program and verify that the output matches the reference output. For example:
$cd$ydb_dir
$gcc -I$ydb_dist -L $ydb_dist -o wordfreq wordfreq.c -lyottadb$ ./wordfreq <wordfreq_input.txt >wordfreq_output.tmp
$diff wordfreq_output.tmp wordfreq_output.txt$


Note that the wordfreq.c program randomly uses local or global variables (see Local and Global Variables).

## Docker Container¶

You must have at least Docker 17.05 installed. Pre-built images are available at Docker Hub. The Docker image is built to provide sane defaults to begin exploring YottaDB. It is not meant for production usage.

To run a pre-built image: docker run --rm -it yottadb/yottadb to run the image but not persist any changes you make, and docker run -it yottadb/yottadb for persistent changes.

Volumes are supported by mounting the /data directory. To mount the local directory ydb-data into the container to save your database and routines locally and use them in the container, add an appropriate command line parameter before the yottadb/yottadb argument, e.g., docker run -it -v \pwd\/ydb-data:/data yottadb/yottadb

This creates a ydb-data directory in your current working directory. After the container is shutdown and removed, delete the directory if you want to remove all data created in the YottaDB container (such as your database and routines).

# Concepts¶

## Keys, Values, Nodes, Variables, and Subscripts¶

The fundamental core data structure provided by YottaDB is key-value tuples. For example, the following is a set of key value tuples:

["Capital","Belgium","Brussels"]
["Capital","Thailand","Bangkok"]
["Capital","USA","Washington, DC"]


Note that data in YottaDB is always ordered. [1] Even if you input data out of order, YottaDB always stores them in order. In the discussion below, data is therefore always shown in order. For example, the data below may well have been loaded by country.

 [1] The terms “collate”, “order”, and “sort” are equivalent.

Each of the above tuples is called a node. In an n-tuple, the first n-1 items can be thought of as the keys, and the last item is the value associated with the keys.

While YottaDB itself assigns no meaning to the data in each node, by convention, application maintainability is improved by using meaningful keys, for example:

["Capital","Belgium","Brussels"]
["Capital","Thailand","Bangkok"]
["Capital","USA","Washington, DC"]
["Population","Belgium",1367000]
["Population","Thailand",8414000]
["Population","USA",325737000]


As YottaDB assigns no inherent meaning to the keys or values, its key value structure lends itself to implementing Variety. [2] For example, if an application wishes to add historical census results under “Population”, the following is a perfectly valid set of tuples (source: United States Census):

["Capital","Belgium","Brussels"]
["Capital","Thailand","Bangkok"]
["Capital","USA","Washington, DC"]
["Population","Belgium",1367000]
["Population","Thailand",8414000]
["Population","USA",325737000]
["Population","USA",17900802,3929326]
["Population","USA",18000804,5308483]
…
["Population","USA",20100401,308745538]


In the above, 17900802 represents August 2, 1790, and an application would determine from the number of keys whether a node represents the current population or historical census data.

 [2] Variety is one of the three “V”s of “big data” — Velocity, Volume, and Variety. YottaDB handles all three very well.

In YottaDB, the first key is called a variable, and the remaining keys are called subscripts allowing for a representation both compact and familiar to a programmer, e.g., Capital("Belgium")="Brussels". The set of all nodes under a variable is called a tree (so in the example, there are two trees, one under Capital and the other under Population). The set of all nodes under a variable and a leading set of its subscripts is called a subtree (e.g., Population("USA") is a subtree of the Population tree). [3]

 [3] Of course, the ability to represent the data this way does not in any way detract from the ability to represent the same data another way with which you are comfortable, such as XML or JSON. However, note while any data that can be represented in JSON can be stored in a YottaDB tree not all trees that YottaDB is capable of storing can be represented in JSON, or at least, may require some encoding (for example, see JSON-M) - in order to be represented in JSON.

With this representation, the Population tree can be represented as follows:

Population("Belgium")=1367000
Population("Thailand")=8414000
Population("USA")=325737000
Population("USA",17900802)=3929326
Population("USA",18000804)=5308483
…
Population("USA",20100401)=308745538


YottaDB has functions for applications to traverse trees in both breadth-first and depth-first order.

If the application designers now wish to enhance the application to add historical dates for capitals, the Capital("Thailand") subtree might look like this (source: The Four Capitals of Thailand).

Capital("Thailand")="Bangkok"
Capital("Thailand",1238,1378)="Sukhothai"
Capital("Thailand",1350,1767)="Ayutthaya"
Capital("Thailand",1767,1782)="Thonburi"
Capital("Thailand",1782)="Bangkok"


### Variables vs. Subscripts vs. Values¶

When viewed as ["Capital","Belgium","Brussels"] each component is a string, and in an abstract sense they are all conceptually the same. When viewed as Capital("Belgium")="Brussels" differences become apparent:

• Variables are ASCII strings from 1 to 31 characters, the first of which is “%”, or a letter from “A” through “Z” and “a” through “z”. Subsequent characters are alphanumeric (“A” through “Z”, “a” through “z”, and “0” through “9”). Variable names are case-sensitive, and variables of a given type are always in ASCII order (i.e., “Capital” always precedes “Population”).
• Subscripts are sequences of bytes from 0 bytes (the null or empty string, “”) to 1048576 bytes (1MiB). When a subscript is a canonical number, YottaDB internally converts it to, and stores it as, a number. When ordering subscripts:
• Empty string subscripts precede all numeric subscripts. Note: YottaDB recommends against applications that use empty string subscripts. [4]
• Numeric subscripts precede string subscripts. Numeric subscripts are in numeric order.
• String subscripts follow numeric subscripts and collate in byte order. Where the natural byte order does not result in linguistically and culturally correct ordering of strings, YottaDB has a framework for an application to create and use custom collation routines.
 [4] The YottaDB code base includes code for a legacy subscript collation in which empty strings collate after numeric subscripts and before non-empty strings. This is supported only in M code for backward compatibility reasons, and is not supported for use with C or any other language. Any attempt to bypass protections and use this legacy collation with new code will almost certainly result in buggy applications that are hard to debug.

Like subscripts, values are sequences of bytes, except that ordering of values is not meaningful unlike ordering of subscripts. YottaDB automatically converts between numbers and strings, depending on the type of operand required by an operator or argument required by a function (see Numeric Considerations).

This means that if an application were to store the current capital of Thailand as Capital("Thailand","current")="Bangkok" instead of Capital("Thailand")="Bangkok", the above subtree would have the following order:

Capital("Thailand",1238,1378)="Sukhothai"
Capital("Thailand",1350,1767)="Ayutthaya"
Capital("Thailand",1767,1782)="Thonburi"
Capital("Thailand",1782)="Bangkok"
Capital("Thailand","current")="Bangkok"


## Local and Global Variables¶

YottaDB is a database, and data in a database must persist and be shared. The variables discussed above are specific to an application process (i.e., are not shared).

• Local variables reside in process memory, are specific to an application process, are not shared between processes, and do not persist beyond the lifetime of a process. [5]
• Global variables reside in databases, are shared between processes, and persist beyond the lifetime of any individual process.
 [5] In other words, what YottaDB calls a local variable, the C programming language calls a global variable. There is no C counterpart to a YottaDB global variable.

Syntactically, local and global variables look alike, with global variable names having a caret (“^”) preceding their names. Unlike the local variables above, the global variables below are shared between processes and are persistent.

^Population("Belgium")=1367000
^Population("Thailand")=8414000
^Population("USA")=325737000


Even though they may appear superficially similar, a local variable is distinct from a global variable of the same name. Thus ^X can have the value 1 and X can at the same time have the value "The quick brown fox jumps over the lazy dog." For maintainability YottaDB strongly recommends that applications use different names for local and global variables, except in the special case where a local variable is an in-process cached copy of a corresponding global variable.

## Global Directories¶

To application software, files in a file system provide persistence. This means that global variables must be stored in files for persistence. A global directory file provides a process with a mapping from the name of every possible global variable name to one or more regions. A database is a set of regions, which in turn map to database files. Global directories are created and maintained by a utility program, which is discussed at length in Chapter 4 Global Directory Editor of the YottaDB Administration and Operations Guide and is outside the purview of this document.

The name of the global directory file required to access a global variable such as ^Capital, is provided to the process at startup by the environment variable ydb_gbldir.

In addition to the implicit global directory an application may wish to use alternate global directory names. For example, consider an application that wishes to provide an option to display names in other languages while defaulting to English. This can be accomplished by having different versions of the global variable ^Capital for different languages, and having a global directory for each language. A global variable such as ^Population would be mapped to the same database file for all languages, but a global variable such as ^Capital would be mapped to a database file with language-specific entries. So a default global directory Default.gld mapping a ^Capital to a database file with English names can be specified in the environment variable ydb_gbldir but a different global directory file, e.g., ThaiNames.gld can have the same mapping for a global variable such as ^Population but a different database file for ^Capital. The intrinsic special variable $zgbldir can be set to a global directory name to change the mapping from one global directory to another. Thus, we can have: $zgbldir="ThaiNames.gld"
^Capital("Thailand")="กรุ่งเทพฯ"
^Capital("Thailand",1238,1378)="สุโขทัย"
^Capital("Thailand",1350,1767)="อยุธยา"
^Capital("Thailand",1767,1782)="ธนบุรี"
^Capital("Thailand",1782)="กรุ่งเทพฯ"


### Client/Server Operation¶

In common usage, database files reside on the same computer system as that running application code. However, as described in Chapter 13 GT.CM Client/Server of the Administration and Operations Guide, database files can reside on a computer system different from that running application code. This mapping of global variables to regions that map to remote files is also performed using global directories, and is transparent to application code except that YottaDB client/server operation does not support transaction processing. This means that within a ydb_tp_s() call, application code on a client machine is not permitted to access a database region that resides in a file on a remote server. Furthermore, there are configurations that impliticly invoke transaction processing logic, such as distributing a global variable over multiple database regions, or a trigger invocation (see Chapter 14 Triggers of the YottaDB M Programmers Guide). Operations that invoke implicit transaction processing are not supported for global variables that reside on remote database files.

## Intrinsic Special Variables¶

$zgbldir is the name of the current global directory file; any global variable reference that does not explicitly specify a global directory uses$zgbldir. For example, an application can set an intrinsic special variable $zgbldir="ThaiNames.gld" to use the ThaiNames.gld mapping. At process startup, YottaDB initializes $zgbldir from the environment variable value $ydb_gbldir. ###$zmaxtptime¶

$zmaxtptime provides a limit in seconds for the time that a transaction can be open (see Transaction Processing). $zmaxtptime is initialized at process startup from the environment variable ydb_maxtptime, with values greater than 60 seconds truncated to 60 seconds. In the unlikely event that an application legitimately needs a timeout greater than 60 seconds, use ydb_set_s() to set it.

### $zstatus¶ $zstatus provides additional details of the last error. Application code can retrieve $zstatus using ydb_get_s(). $zstatus consists of several comma-separated substrings.

• The first is an error number.
• The second is always "(SimpleAPI)".
• The remainder is more detailed information about the error, and may contain commas within.

## Transaction Processing¶

YottaDB provides a mechanism for an application to implement ACID (Atomic, Consistent, Isolated, Durable) transactions, ensuring strict serialization of transactions, using optimistic concurrency control.

Here is a simplified view [6] of YottaDB’s implementation of optimistic concurrency control:

• Each database file header has a field of the next transaction number for updates in that database.
• The block header of each database block in a database file has the transaction number when that block was last updated.
• When a process is inside a transaction, it keeps track of every database block it has read, and the transaction number of that block when read. Other processes are free to update the database during this time.
• The process retains updates in its memory, without committing them to the database, so that its own logic sees the updates, but no other process does. As every block that the process wishes to write must also be read, tracking the transaction numbers of blocks read suffices to track them for blocks to be written.
• To commit a transaction, a process checks whether any block it has read has been updated since it was read. If none has, the process commits the transaction to the database, incrementing the file header fields of each updated database file for the next transaction.
• If even one block has been updated, the process discards its work, and starts over. If after three attempts, it is still unable to commit the transaction, it executes the transaction logic on the fourth attempt with updates by all other processes blocked so that the transaction at commit time will not encounter database changes made by other processes.
 [6] At the high level at which optimistic concurrency control is described here, a single logical database update (which can span multiple blocks and even multiple regions) is a transaction that contains a single update.

In YottaDB’s API for transaction processing, an application packages the logic for a transaction into a function with one parameter, passing the function and its parameter as parameters to the ydb_tp_s() function. YottaDB then calls that function.

• If the function returns a YDB_OK, YottaDB attempts to commit the transaction. If it is unable to commit as described above, or if the called function returns a YDB_TP_RESTART return code, it calls the function again.
• If the function returns a YDB_TP_ROLLBACK, ydb_tp_s() returns to its caller with that return code after discarding the uncommitted database updates and releasing any locks acquired within the transaction.
• To protect applications against poorly coded transactions, if a transaction takes longer than the number of seconds specified by the intrinsic special variable $zmaxtptime, YottaDB aborts the transaction and the ydb_tp_s() function returns the YDB_ERR_TPTIMEOUT error. ## Locks¶ YottaDB locks are a fast, lightweight tool for multiple processes to coordinate their work. An analogy with the physical world may help to explain the functionality. When it is locked, the lock on a door prevents you from going through it. In contrast, a traffic light does not stop you from driving through a street intersection: it works because drivers by convention stop when their light is red and drive when it is green. YottaDB locks are more akin to traffic lights than door locks. Each lock has a name: as lock names have the same syntax as local or global variable names, Population, ^Capital, and ^Capital("Thailand",1350,1767) are all valid lock names. Features of YottaDB locks include: • Locks are exclusive: one and only one process can acquire a lock with the resource name. For example, if process P1 acquires lock Population("USA"), process P2 cannot simultaneously acquire that lock. However, P2 can acquire lock Population("Canada") at the same time that process P1 acquires Population("USA"). • Locks are hierarchical: a process that has a lock at a higher level blocks locks at lower levels and vice versa. For example, if a process P0 must wait for processes P1, P2, … to complete, each of P1, P2, … can acquire lock Process(pid). P0’s subsequent attempt to acquire lock Process is blocked till processes P1, P2, … complete. • Locks include counters: a process that acquires ^Capital("Belgium") can acquire that lock again, incrementing its count to 2. This simplifies application code logic: for example, a routine in application code that requires ^Capital("Belgium") can simply incrementally acquire that lock without needing to test whether a higher level routine has already acquired it. More importantly, when it completes its work, the routine can decrementally release the lock without concern for whether or not a higher level routine needs that lock. When the count goes from 1 to 0, the lock becomes available for acquisition by another process. • Locks are robust: while normal process exit releases locks held by that process, if a process holding a lock exits abnormally without releasing it, another process that needs the lock, and finding it held by a non-existent process will automatically scavenge the lock. Although YottaDB lock names are the same as local and global variable names, YottaDB imposes no connection between a lock name and the same variable name. By convention, and for application maintainability, it is good practice to use lock names associated with the variables to which application code requires exclusive access, e.g., use a lock called ^Population to protect or restrict access to a global variable called ^Population. [7]  [7] Since a process always has exclusive access to its local variables, access to them never needs protection from a lock. So, it would be reasonable to use a lock Population to restrict access to the global variable ^Population. Since YottaDB lock acquisitions are always timed for languages other than M, it is not in principle possible for applications to deadlock on YottaDB locks. Consequently defensive application code must always validate the return code of calls to acquire locks. As a practical matter, it is possible to set timeouts that are long enough that users may perceive applications to be hung. ### Locks and Transaction Processing¶ Transaction Processing and Locks solve overlapping though not congruent use cases. For example, consider application code to transfer$100 from a customer’s savings account to that same customer’s savings account, which would likely include the requirement that business transactions on an account must be serializable. This can be implemented by acquiring a lock on that customer (with an application coded so that other accesses to that customer are blocked till the lock is released) or by executing the transfer inside a YottaDB transaction (which provides ACID properties). Unless the application logic or data force pathological transaction restarts that cannot be eliminated or worked around, transaction processing’s optimistic concurrency control typically results in better application throughput than the pessimistic concurrency control that locks imply.

In general, we recommend using either transaction processing or locks, and not mixing them. However, there may be business logic that requires the use of locks for some logic, but otherwise permits the use of transaction processing. If an application must mix them, the following rules apply:

• A lock that a process acquires prior to starting a transaction cannot be released inside the transaction - it can only be released after the transaction is committed or rolled back. Locks acquired inside a transaction can be released either inside the transaction, or after the transaction is committed or rolled back.

# Programming in C¶

## Symbolic Constants¶

The libyottadb.h file defines several symbolic constants, which are one of the following types:

• Function Return Codes, which in turn are one of:
• Normal Return Codes
• Error Return Codes
• Limits
• Other

Symbolic constants all fit within the range of a C int.

### Function Return Codes¶

Return codes from calls to YottaDB are usually of type int and occasionally other types. Normal return codes are non-negative (greater than or equal to zero); error return codes are negative.

#### Normal Return Codes¶

Symbolic constants for normal return codes have YDB_ prefixes other than YDB_ERR_.

YDB_LOCK_TIMEOUT — This return code from lock acquisition functions indicates that the specified timeout was reached without the requested locks being acquired.

YDB_OK — This the standard return code of all functions following successful execution.

YDB_TP_RESTART — Return code to YottaDB from an application function that implements a transaction to indicate that it wishes YottaDB to restart the transaction, or by a YottaDB function invoked within a transaction to its caller that the database engine has detected that it will be unable to commit the transaction and will need to restart. Application code designed to be executed within a transaction should be written to recognize this return code and in turn perform any cleanup required and return to the YottaDB ydb_tp_s() invocation from which it was called. See Transaction Processing for a discussion of restarts.

YDB_TP_ROLLBACK — Return code to YottaDB from an application function that implements a transaction, and in turn returned to the caller indicating that the transaction was not committed.

#### Error Return Codes¶

Symbolic constants for error codes returned by calls to YottaDB are prefixed with YDB_ERR_ and are all less than zero. The symbolic constants below are not a complete list of all error messages that YottaDB functions can return — error return codes can indicate system errors and database errors, not just application errors. A process that receives a negative return code, including one not listed here, can call ydb_get_s() to get the value of $zstatus. Error messages can be raised by the YottaDB runtime system or by the underlying operating system. Remember that the error codes returned by YottaDB functions are the negated numeric values of the error codes above. YDB_ERR_CALLINAFTEREXIT – A YottaDB function was called after ydb_exit() was called. YDB_ERR_FATALERROR1 – A fatal error occurred. The process is generating a core dump and terminating. As a process cannot receive a fatal error code, this error appears in the syslog. YDB_ERR_FATALERROR2 – A fatal error occurred. The process is terminating without generating a core dump. As a process cannot receive a fatal error code, this error appears in the syslog. YDB_ERR_GVUNDEF — No value exists at a requested global variable node. YDB_ERR_INVNAMECOUNT – A namecount parameter has an invalid value. YDB_ERR_INSUFFSUBS — A call to ydb_node_next_s() or ydb_node_previous_s() did not provide enough parameters for the return values. YDB_ERR_INVSTRLEN — A buffer provided by the caller is not long enough for a string to be returned, or the length of a string passed as a parameter exceeds YDB_MAX_STR. In the event the return code is YDB_ERR_INVSTRLEN and if *xyz is a ydb_buffer_t structure whose xyz->len_alloc indicates insufficient space, then xyz->len_used is set to the size required of a sufficiently large buffer. In this case the len_used field of a ydb_buffer_t structure is greater than the len_alloc field, and the caller is responsible for correcting the xyz->len_used field. YDB_ERR_INVSVN — A special variable name provided by the caller is invalid. YDB_ERR_INVVARNAME — A variable name provided by the caller is invalid. YDB_ERR_KEY2BIG — The length of a global variable name and subscripts exceeds the limit configured for the database region to which it is mapped. YDB_ERR_LVUNDEF — No value exists at a requested local variable node. YDB_ERR_MAXNRSUBSCRIPTS — The number of subscripts specified in the call exceeds YDB_MAX_SUBS. YDB_ERR_MINNRSUBSCRIPTS – The number of subscripts cannot be negative. YDB_ERR_NAMECOUNT2HI – The number of variable names specified to ydb_delete_excl_s() or ydb_tp_s() exceeded the YDB_MAX_NAMES. YDB_NOTOKydb_file_name_to_id() was called with a NULL pointer to a filename. YDB_ERR_NUMOFLOW — A ydb_incr_s() operation resulted in a numeric overflow. YDB_ERR_PARAMINVALID — A parameter provided by the caller is invalid. YDB_ERR_SIMPLEAPINEST – An attempt was made to nest Simple API calls, which cannot be nested. YDB_ERR_SUBSARRAYNULL – The subs_used parameter of a function is greater than zero, but the subsarray parameter is a NULL pointer. YDB_ERR_SVNOSET — the application inappropriately attempted to modify the value of an intrinsic special variable such as an attempt to modify $trestart using ydb_set_s().

YDB_ERR_TIME2LONG – This return code indicates that a value greater than YDB_MAX_TIME_NSEC was specified for a time duration.

YDB_ERR_TPTIMEOUT — This return code from ydb_tp_s() indicates that the transaction took too long to commit.

YDB_ERR_UNIMPLOP — An operation that is not supported for an intrinsic special variable – of the Simple API functions only ydb_get_s() and ydb_set_s() are supported – was attempted on an intrinsic special variable.

YDB_ERR_VARNAME2LONG – A variable name length exceeds YottaDB’s limit.

### Limits¶

Symbolic constants for limits are prefixed with YDB_MAX_ or YDB_MIN_.

YDB_MAX_IDENT — The maximum space in bytes required to store a complete variable name, not including the preceding caret for a global variable. Therefore, when allocating space for a string to hold a global variable name, add 1 for the caret.

YDB_MAX_NAMES – The maximum number of variable names that can be passed to ydb_delete_excl_s() or ydb_tp_s().

YDB_MAX_STR — The maximum length of a string (or blob) in bytes. A caller to ydb_get_s() whose *ret_value parameter provides a buffer of YDB_MAX_STR will never get a YDB_ERR_INVSTRLEN error.

YDB_MAX_SUBS — The maximum number of subscripts for a local or global variable.

YDB_MAX_TIME_NSEC — The maximum value in nanoseconds that an application can instruct libyottab to wait, e.g., until the process is able to acquire locks it needs before timing out, or for ydb_hiber_start().

YDB_MAX_YDBERR – The absolute (positive) value of any YottaDB function error return code. If the absolute value of an error return code is greater than YDB_MAX_YDBERR, then it is an error code from elsewhere, e.g., e.g. errno. Also, see YDB_IS_YDBERR().

YDB_MIN_YDBERR - The absolute (positive) value of any YottaDB function error return code. If the absolute value of an error return code is less than YDB_MIN_YDBERR, then it is an error code from elsewhere, e.g., e.g. errno. Also, see YDB_IS_YDBERR().

Symbolic constants for the severities of message numbers in return codes and $zstatus are prefixed with YDB_SEVERITY_. YDB_SEVERITY_ERROR – The number corresponds to an error from which the process can recover. YDB_SEVERITY_FATAL – The number corresponds to an error that terminated the process. YDB_SEVERITY_INFORMATION – The number corresponds to an informational message. YDB_SEVERITY_SUCCESS – The number corresponds to the successful completion of a requested operation. YDB_SEVERITY_WARNING – The number corresponds to a warning, i.e., it indicates a possible problem. ### Other¶ Other symbolic constants have a prefix of YDB_. YDB_DEL_NODE and YDB_DEL_TREE — As values of the deltype parameter, these values indicate to ydb_delete_s() whether to delete an entire subtree or just the node at the root, leaving the subtree intact. YDB_NODE_END — In the event a call to ydb_node_next_s() or ydb_node_previous_s() wishes to report that there no further nodes, the *ret_subs_used parameter is set to this value. Application code should make no assumption about this constant other than that it is negative (<0). ## Data Structures & Type Definitions¶ ydb_buffer_t is a descriptor for a string [8] value, and consists of the following fields: • buf_addr — pointer to an unsigned char, the starting address of a string. • len_alloc and len_used — fields of type unsigned int where: • len_alloc is the number of bytes allocated to store the string, • len_used is the length in bytes of the currently stored string, and • len_alloclen_used except when a YDB_ERR_INVSTRLEN occurs.  [8] Strings in YottaDB are arbitrary sequences of bytes that are not null-terminated. Other languages may refer to them as binary data or blobs. ydb_string_t is a descriptor for a string provided for compatibility with existing code, and consists of the following fields: • address — pointer to an unsigned char, the starting address of a string. • length — the length of the string starting at the address field. ydb_tpfnptr_t is a pointer to a function with one parameter, a pointer, and which returns an integer, defined thus: typedef int (*ydb_tpfnptr_t)(void *tpfnparm);  ## Macros¶ YDB_ASSERT(x) – Conditionally include this macro in code for debugging and testing purposes. If x is non-zero, it prints an error message on stderr and generates a core file by calling ydb_fork_n_core(). YDB_BUFFER_IS_SAME(buffer1, buffer2) – Use this macro to test whether the memory locations (strings) pointed to by two ydb_buffer_t structures have the same content, returning FALSE (0) if they differ and a non-zero value if the contents are identical. YDB_COPY_BUFFER_TO_BUFFER(source, destination, done) – Use this macro to copy the memory locations (strings) pointed to by source to the memory locations pointed to by destination and set: • destination->len_used to source->len_used; and • done to TRUE if destination->len_allocsource->len_used and the underlying memcpy() completed successfully, and FALSE otherwise. YDB_COPY_LITERAL_TO_BUFFER(literal, buffer, done) - Use this macro to copy a literal string to previously allocated memory referenced by a ydb_buffer_t structure (for example, to set an initial subscript to sequence through nodes). It sets: • buffer->len_used to the size of the literal; and • done to TRUE if buffer->len_alloc ≥ the size of the literal excluding its terminating null byte and the underlying memcpy() completed successfully, and FALSE otherwise. YDB_COPY_STRING_TO_BUFFER(string, buffer, done) – Use this macro to copy a null-terminated string to previously allocated memory referenced by a ydb_buffer_t structure. This macro requires the code to also #include <string.h>. It sets: • buffer->len_used to the size of the copied string; and • done to TRUE if buffer->len_alloc ≥ the size of the string to be copied and the underlying memcpy() completed successfully, and FALSE otherwise. YDB_LITERAL_TO_BUFFER(literal, buffer) – Use this macro to set a ydb_buffer_t structure to refer to a literal (such as a variable name). With a string literal, and a buffer pointer to a ydb_buffer_t structure, set: • buffer->buf_addr to the address of literal; and • buffer->len_used and buffer->len_alloc to the length of literal excluding the terminating null byte. YDB_IS_YDBERR(msgnum) – returns TRUE if the absolute value of msgnum lies between YDB_MIN_YDBERR and YDB_MAX_YDBERR. YDB_SEVERITY(msgnum, severity) – The error return code from a function indicates both the nature of an error as well as its severity. For message msgnum, the variable severity is set to one of the YDB_SEVERITY_* symbolic constants. YDB_SEVERITY() is only meaningful for error return codes and not other numbers. Use YDB_IS_YDBERR() to determine whether a return code is a YottaDB error return code. YottaDB functions are divided into: • Simple API — a core set of functions that provides easy-to-use access to the major features of YottaDB. • Comprehensive API — a more elaborate set of functions for specialized or optimized access to additional functionality within libyottadb.so that YottaDB itself uses. The Comprehensive API is a project for the future. • Utility Functions — Functions useful to a C application using YottaDB. ## Simple API¶ As all subscripts and node data passed to YottaDB using the Simple API are strings, use the sprintf() and atoi()/strtoul() family of functions to convert between numeric values and strings which are canonical numbers. To allow the YottaDB Simple API functions to handle a variable tree whose nodes have varying numbers of subscripts, the actual number of subscripts is itself passed as a parameter. In the prototypes of functions, parameters of the form: • ydb_buffer_t *varname refers to the name of a variable; • int subs_used and int *subs_used refer to an actual number of subscripts; and • ydb_buffer_t *subsarray refers to an array of ydb_buffer_t structures used to pass subscripts whose actual number is defined by subs_used or *subs_used parameters. To pass an intrinsic special variable, or unsubscripted local or global variable, subs_used should be zero and *subsarray should be NULL. Caveat: Specifying a subs_used that exceeds the actual number of parameters passed in *subsarray will almost certainly result in an unpleasant bug that is difficult to troubleshoot. Function names specific to the YottaDB Simple API end in _s. ### ydb_data_s()¶ int ydb_data_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, unsigned int *ret_value);  In the location pointed to by ret_value, ydb_data_s() returns the following information about the local or global variable node identified by *varname, subs_used and *subsarray. • 0 — There is neither a value nor a subtree, i.e., it is undefined. • 1 — There is a value, but no subtree • 10 — There is no value, but there is a subtree. • 11 — There are both a value and a subtree. It is an error to call ydb_data_s() on an intrinsic special variable; doing so results in the YDB_ERR_UNIMPLOP error. ydb_data_s() returns YDB_OK or an error return code. The error YDB_ERR_PARAMINVALID is returned when • ret_value is NULL • len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript, in subsarray. ### ydb_delete_s()¶ int ydb_delete_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, int deltype);  Deletes nodes in the local or global variable tree or subtree specified. A value of YDB_DEL_NODE or YDB_DEL_TREE for deltype specifies whether to delete just the node at the root, leaving the (sub)tree intact, or to delete the node as well as the (sub)tree. Intrinsic special variables cannot be deleted. ydb_delete_s() returns YDB_OK, a YDB_ERR_UNIMPLOP if deltype is neither YDB_DEL_NODE nor YDB_DEL_TREE, YDB_ERR_PARAMINVALID is returned when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray, or another error return code. ### ydb_delete_excl_s()¶ int ydb_delete_excl_s(int namecount, ydb_buffer_t *varnames);  ydb_delete_excl_s() deletes the trees of all local variables except those in the *varnames array. It is an error for *varnames to include a global or intrinsic special variable. In the special case where namecount is zero, ydb_delete_excl_s() deletes all local variables. If your application mixes M and non M code, and you wish to use ydb_delete_excl_s() to delete local variables that are aliases, formal parameters, or actual parameters passed by reference, make sure you understand what (sub)trees are being deleted. This warning does not apply to applications that do not include M code. ydb_delete_excl_s() returns YDB_OK, YDB_ERR_NAMECOUNT2HI if more than YDB_MAX_NAMES are specified, or another error return code. YDB_ERR_PARAMINVALID is returned when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one variable name in “code:varnames. Note that specifying a larger value for namecount than the number of variable names actually provided in *varnames can result in a buffer overflow. ### ydb_get_s()¶ int ydb_get_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, ydb_buffer_t *ret_value);  To the location pointed to by ret_value->buf_addr, ydb_get_s() copies the value of the specified node or intrinsic special variable, setting ret_value->len_used on both normal and error returns (the latter case as long as the data exists). Return values are: • YDB_OK for a normal return; • YDB_ERR_GVUNDEF, YDB_ERR_INVSVN, or YDB_ERR_LVUNDEF as appropriate if no such variable or node exists; • YDB_ERR_INVSTRLEN if ret_value->len_alloc is insufficient for the value at the node; • YDB_ERR_PARAMINVALID when ret_value is NULL or ret_value->buf_addr is NULL and the return value has a non-zero len_used; or len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray; or • another applicable error return code. Notes: • In the unlikely event an application wishes to know the length of the value at a node, but not access the data, it can call ydb_get_s() and provide an output buffer (retvalue->len_alloc) with a length of zero, since even in the case of a YDB_ERR_INVSTRLEN error, retvalue->len_used is set. • Within a transaction implemented by ydb_tp_s() application code observes stable data at global variable nodes because YottaDB transaction processing ensures ACID properties. • Outside a transaction, a global variable node can potentially be changed by another, concurrent, process between the time that a process calls ydb_data_s() to ascertain the existence of the data and a subsequent call to ydb_get_s() to get that data. A caller of ydb_get_s() to access a global variable node should code in anticipation of a potential YDB_ERR_GVUNDEF. ### ydb_incr_s()¶ int ydb_incr_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, ydb_buffer_t *increment, ydb_buffer_t *ret_value);  ydb_incr_s() atomically: • converts the value in the specified node to a number if it is not a canonical number, using a zero value if the node does not exist; • increments it by the value specified by *increment, converting the value to a number if it is not a canonical number, defaulting to 1 if the parameter is NULL; and • storing the value as a canonical number in *ret_value. Return values: • The normal return value is YDB_OK. • If the atomic increment results in a numeric overflow, the function returns a YDB_ERR_NUMOFLOW error; in this case, the value in the node is untouched and that in *ret_value is unreliable. • YDB_ERR_INVSTRLEN if ret_value->len_alloc is insufficient for the result. As with ydb_get_s(), in this case ret_value->len_used is set to the required length. • YDB_ERR_PARAMINVALID when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray or increment. • Other errors return the corresponding error return code. Notes: • Intrinsic special variables cannot be atomically incremented, and an attempt to do so returns the YDB_ERR_UNIMPLOP error. ### ydb_lock_s()¶ int ydb_lock_s(unsigned long long timeout_nsec, int namecount[, [ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray], ...]);  namecount is the number of variable names in the call. Release any locks held by the process, and attempt to acquire all the requested locks. Except in the case of an error or a YDB_LOCK_TIMEOUT return value, the release is unconditional. On return, the function will have acquired all requested locks or none of them. If no locks are requested (namecount is zero), the function releases all locks and returns YDB_OK. timeout_nsec specifies a time in nanoseconds that the function waits to acquire the requested locks. If it is not able to acquire all requested locks, it acquires no locks, returning with a YDB_LOCK_TIMEOUT return value. If timeout_nsec is zero, the function makes exactly one attempt to acquire the locks, and if it is unable to, it returns YDB_LOCK_TIMEOUT. If all requested locks are successfully acquired, the function returns YDB_OK. If the requested timeout_nsec exceeds YDB_MAX_TIME_NSEC, the function immediately returns YDB_ERR_TIME2LONG. YDB_ERR_PARAMINVALID is returned when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray or variable name in varname. In other cases, the function returns an error return code. ### ydb_lock_decr_s()¶ int ydb_lock_decr_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray);  Decrements the count of the specified lock held by the process. As noted in the Concepts section, a lock whose count goes from 1 to 0 is released. A lock whose name is specified, but which the process does not hold, is ignored. As releasing a lock cannot fail, the function returns YDB_OK, unless there is an error such as an invalid name that results in the return of an error code such as YDB_ERR_INVVARNAME. Errors result in an appropriate error return code. YDB_ERR_PARAMINVALID is returned when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray. ### ydb_lock_incr_s()¶ int ydb_lock_incr_s(unsigned long long timeout_nsec, ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray);  Without releasing any locks held by the process, attempt to acquire the requested lock incrementing it if already held. timeout_nsec specifies a time in nanoseconds that the function waits to acquire the requested lock. If it is not able to acquire the lock, it returns with a YDB_LOCK_TIMEOUT return value. If timeout_nsec is zero, the function makes exactly one attempt to acquire the lock, and if unable to, it returns YDB_LOCK_TIMEOUT. If the requested lock is successfully acquired, the function returns YDB_OK. If the requested timeout_nsec exceeds YDB_MAX_TIME_NSEC, the function immediately returns YDB_ERR_TIME2LONG. Errors result in an appropriate error return code. YDB_ERR_PARAMINVALID is returned when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray. ### ydb_node_next_s()¶ int ydb_node_next_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, int *ret_subs_used, ydb_buffer_t *ret_subsarray);  ydb_node_next_s() facilitates depth-first traversal of a local or global variable tree. As the number of subscripts can differ between the input node of the call and the output node reported by the call *ret_subs_used is an input as well as an output parameter: • On input, *ret_subs_used specifies the number of elements allocated for returning the subscripts of the next node. • On normal output (YDB_OK return code), *ret_subs_used contains the actual number of subscripts returned or is YDB_NODE_END. See below for error return codes. Return values of ydb_node_next_s() are: • YDB_OK with the next node, if there is one, changing *ret_subs_used and *ret_subsarray parameters to those of the next node. If there is no next node (i.e., the input node is the last), *ret_subs_used on output is YDB_NODE_END. • YDB_ERR_INSUFFSUBS if *ret_subs_used specifies insufficient parameters to return the subscript. In this case *ret_subs_used reports the actual number of subscripts required. • YDB_ERR_INVSTRLEN if one of the ydb_buffer_t structures pointed to by *ret_subsarray does not have enough space for the subscript. In this case, *ret_subs_used is the index into the *ret_subsarray array with the error, and the len_used field of that structure specifies the size required. • YDB_ERR_PARAMINVALID when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray. • Another error return code, in which case the application should consider the values of *ret_subs_used and the *ret_subsarray to be undefined. ### ydb_node_previous_s()¶ int ydb_node_previous_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, int *ret_subs_used, ydb_buffer_t *ret_subsarray);  Analogous to ydb_node_next(s), ydb_node_previous_s() facilitates reverse breadth-first traversal of a local or global variable tree, except that ydb_node_previous_s() searches for and reports the predecessor node. Unlike ydb_node_next_s(), *ret_subs_used can be zero if an expected previous node is the unsubscripted root. Return values of ydb_node_previous_s() are: • YDB_OK with the previous node, if there is one, changing *ret_subs_used and *ret_subsarray parameters to those of the previous node. If there is no previous node (i.e., the input node is the first), *ret_subs_used on output is YDB_NODE_END. • YDB_ERR_INSUFFSUBS if *ret_subs_used specifies insufficient parameters to return the subscript. In this case *ret_subs_used reports the actual number of subscripts required. • YDB_ERR_INVSTRLEN if one of the ydb_buffer_t structures pointed to by *ret_subsarray does not have enough space for the subscript. In this case, *ret_subs_used is the index into the *ret_subsarray array with the error, and the len_used field of that structure specifies the size required. • YDB_ERR_PARAMINVALID when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray. • Another error return code, in which case the application should consider the values of *ret_subs_used and the *ret_subsarray to be undefined. ### ydb_set_s()¶ int ydb_set_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, ydb_buffer_t *value);  Copies the value->len_used bytes at value->buf_addr as the value of the specified node or intrinsic special variable specified. A NULL value parameter is treated as equivalent to one that points to a ydb_buffer_t specifying an empty string. Return values are: • YDB_OK for a normal return; • YDB_ERR_INVSVN if no such intrinsic special variable exists; • YDB_ERR_PARAMINVALID when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray or increment; or • another applicable error return code. ### ydb_str2zwr_s()¶ int ydb_str2zwr_s(ydb_buffer_t *str, ydb_buffer_t *zwr);  In the buffer referenced by *zwr, ydb_str2zwr_s() provides the zwrite formatted version of the string pointed to by *str, returning: • YDB_OK; • YDB_ERR_INVSTRLEN if the *zwr buffer is not long enough; • YDB_ERR_PARAMINVALID if zwr is NULL or zwr->buf_addr is NULL and the return value has a non-zero len_used; or • another applicable error return code. ### ydb_subscript_next_s()¶ int ydb_subscript_next_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, ydb_buffer_t *ret_value);  ydb_subscript_next_s() provides a primitive for implementing breadth-first traversal of a tree by searching for the next subscript at the level specified by subs_used, i.e., the next subscript after the one referred to by subsarray[subs_used-1].buf_addr. A node need not exist at the subscripted variable name provided as input to the function. If subsarray[subs_used-1].len_used is zero, ret_value->buf_addr points to first node at that level with a subscript that is not the empty string. ydb_subscript_next_s() returns YDB_OK or an error return code. On return from ydb_subscript_next_s() with a YDB_OK, if ret_value->len_used is non-zero, ret_value->buf_addr points to the value of the next subscript. If it is zero, it means that there is no node greater than the input node at that level. In the special case where subs_used is zero, ret_value->buf_addr points to the next local or global variable name. The error YDB_ERR_PARAMINVALID is returned when • ret_value is NULL; • ret_value->buf_addr is NULL and the return value has a non-zero len_used; • len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray. ### ydb_subscript_previous_s()¶ int ydb_subscript_previous_s(ydb_buffer_t *varname, int subs_used, ydb_buffer_t *subsarray, ydb_buffer_t *ret_value);  ydb_subscript_previous_s() provides a primitive for implementing reverse breadth-first traversal of a tree by searching for the previous subscript at the level specified by subs_used. i.e. the subscript preceding the one referred to by subsarray[subs_used-1].buf_addr. A node need not exist at the subscripted variable name provided as input to the function. If subsarray[subs_used-1].len_used is zero, ret_value->buf_addr points to last node at that level with a subscript that is not the empty string. ydb_subscript_previous_s() returns YDB_OK or an error return code. On return from ydb_subscript_previous_s(), if ret_value->len_used is non-zero, ret_value->buf_addr points to the value of the previous subscript. If it is zero, it means that there is no node less than the input node at that level. Notes: • If an application uses empty strings as subscripts, a subsequent call to ydb_data_s() is required to determine whether the first subscript has been reached or whether the first subscript is a node with the empty string as a subscript. • In the special case where subs_used is zero, ret_value->buf_addr points to the previous local or global variable name. The error YDB_ERR_PARAMINVALID is returned when • ret_value is NULL; or • ret_value->buf_addr is NULL and the return value has a non-zero len_used; or • len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one subscript in subsarray. ### ydb_tp_s()¶ int ydb_tp_s(ydb_tpfnptr_t tpfn, void *tpfnparm, const char *transid, int namecount, ydb_buffer_t *varnames);  ydb_tp_s() calls the function pointed to by tpfn passing it tpfnparm as a parameter. As discussed under Transaction Processing, the function should use the intrinsic special variable $trestart to manage any externally visible action (which YottaDB recommends against, but which may be unavoidable). The function pointed to by tpfn should return one of the following:

• YDB_OK — application logic indicates that the transaction can be committed (the YottaDB engine may still decide that a restart is required to ensure ACID transaction properties) as discussed under Transaction Processing.
• YDB_TP_RESTART — application logic indicates that the transaction should restart.
• YDB_TP_ROLLBACK — application logic indicates that the transaction should not be committed.
• YDB_ERR_PARAMINVALID when len_alloc < len_used or the len_used is non-zero and buf_addr is NULL in at least one variable name in varnames.
• An error return code returned by a YottaDB function called by the function pointed to by tpfn.

transid is a string, up to the first 8 bytes of which are recorded in the commit record of journal files for database regions participating in the transaction. If not NULL or the empty string, a case-insensitive value of "BA" or "BATCH" indicates that at transaction commit, YottaDB need not ensure Durability (it always ensures Atomicity, Consistency, and Isolation). Use of this value may improve latency and throughput for those applications where an alternative mechanism (such as a checkpoint) provides acceptable Durability. If a transaction that is not flagged as "BATCH" follows one or more transactions so flagged, Durability of the later transaction ensures Durability of the the earlier "BATCH" transaction(s).

If namecount>0, varnames[i] where 0≤i<namecount specifies local variable names whose values are restored to their original values when the transaction is restarted. In the special case where namecount=1 and varnames[0] provides the value "*", all local variables are restored on a restart. It is an error for a varnames to include a global or intrinsic special variable.

A top level ydb_tp_s() can return YDB_OK, YDB_TP_ROLLBACK, YDB_ERR_TPTIMEOUT (see Transaction Processing), or an error return code, including YDB_ERR_NAMECOUNT2HI. A ydb_tp_s() call that is within another transaction can also return YDB_TP_RESTART to its caller. [9]

 [9] An enclosing transaction can result not just from another ydb_tp_s() higher in the stack, but also from an M tstart command as well as a database trigger resulting from a ydb_delete_s(), or ydb_set_s().

### ydb_zwr2str_s()¶

int ydb_zwr2str_s(ydb_buffer_t *zwr, ydb_buffer_t *str);


In the buffer referenced by *str, ydb_zwr2str_s() provides the string described by the zwrite formatted string pointed to by *zwr, returning

• YDB_OK (with str->len_used set to zero if the zwrite formatted string has an error);
• YDB_ERR_INVSTRLEN error if the *str buffer is not long enough;
• YDB_ERR_PARAMINVALID either if the *str buffer is NULL or the return value contains a non-zero len_used and the str->buf_addr is NULL.

## Comprehensive API¶

The Comprehensive API is a project for the future.

## Utility Functions¶

Utility functions are functions that are not core to YottaDB functionality, but which are useful to application code.

### ydb_child_init()¶

int ydb_child_init(void *param)


As the YottaDB engine resides in the address space of the process, child processes must call ydb_child_init() to re-initialize data structures after a fork() or equivalent in other languages (e.g., os.fork() in Python).

Notes:

• A child process that fails to call ydb_child_init() after a fork() can cause structural damage to database files, as well as other possible side-effects.
• After a fork(), a parent process should not exit until the child process has executed ydb_child_init(). One way to implement this would be for the parent to set a node such as ^Proc(ppid)=1 where ppid is the parent’s pid, and for the child to set it to zero or to delete the node. A parent process that wishes to fork() a number of child processes can use ydb_incr_s() to increment a node such as ^Proc(ppid) and each child can decrement it after executing ydb_child_init(). When the value at the node is zero, the parent process knows that it is safe for it to exit.

The void *param is reserved for future enhancements. As the initial release of YottaDB ignores it, we recommend using NULL. ydb_child_init() returns YDB_OK or an error return code.

### ydb_exit()¶

int ydb_exit(void)


When a caller no longer wishes to use YottaDB, a call to ydb_exit() cleans up the process connection/access to all databases and cleans up its data structures. Therafter, any attempt to call a YottaDB function produces a YDB_ERR_CALLINAFTEREXIT error.

Note that a typical application should not need to call ydb_exit(), but should instead just terminate with a call to exit() which will perform any cleanup needed by YottaDB.

### ydb_file_id_free()¶

int ydb_file_id_free(ydb_fileid_ptr_t fileid)


Releases the memory used by a fileid structure previously generated by ydb_file_name_to_id(). Calling the function twice for the same pointer, unless it has been returned a second time by a different ydb_file_name_to_id() is an application error with undefined consequences.

### ydb_file_is_identical()¶

int ydb_file_is_identical(ydb_fileid_ptr_t fileid1,
ydb_fileid_ptr_t fileid2)


Given two pointers to fileid structures (see ydb_file_name_to_id()), ydb_file_is_identical returns YDB_OK if the two fileid structures are the same file.

### ydb_file_name_to_id()¶

int ydb_file_name_to_id(ydb_string_t *filename,
ydb_fileid_ptr_t *fileid)


As a file is in principle reachable through different paths, and application code may need to check whether two paths do indeed lead to the same file, YottaDB provides a mechanism to do so. Provided with a path to a file, YottaDB creates an internal structure called a fileid that uniquely identifies the file if such a structure does not already exist for that file, and provides the caller with a pointer to that structure. The layout and contents of the fileid structure are opaque to the caller, which must not modify the pointer or the structure it points to.

When the fileid structure for a file is no longer needed, an application should call ydb_file_id_free() to release the structure and avoid a memory leak.

ydb_file_name_to_id() returns YDB_OK, YDB_NOTOK if the filename is NULL, or an error return code.

### ydb_fork_n_core()¶

void ydb_fork_n_core(void)


A core is a snapshot of a process, to help debug application code, for example to troubleshoot an out-of-design condition. When a process executes ydb_fork_n_core(), it forks. The child process sends itself a signal to generate a core and terminate. On termination of the child process, the parent process continues execution. Note that depending on the nature of the condition necessitating a core, an exit() may well be the right action for the parent process. An exit() call will drive YottaDB exit handlers to perform clean shutdown of databases and devices the process has open.

The content, location, and naming of cores is managed by the operating system – see man 5 core for details. We recommend that you set kernel.core_uses_pid to 1 to make it easier to identify and track cores. As cores will likely contain protected confidential information, you must ensure appropriate configuration and management of cores.

### ydb_free()¶

void ydb_free(void *ptr)


Releases memory previously allocated by ydb_malloc(). Passing ydb_free() a pointer not previously provided to the application by ydb_malloc() can result in unpredictable behavior. The signature of ydb_free() matches that of the POSIX free() call.

### ydb_hiber_start()¶

int ydb_hiber_start(unsigned long long sleep_nsec)


The process sleeps for the time in nanoseconds specified by sleep_nsec. If a value greater than YDB_MAX_TIME_NSEC is specified, ydb_hiber_start() immediately returns with a YDB_ERR_TIME2LONG error; otherwise it returns YDB_OK after the elapsed time.

### ydb_hiber_start_wait_any()¶

int ydb_hiber_start_wait_any(unsigned long long sleep_nsec)


The process sleeps for the time in nanoseconds specified by sleep_nsec or until it receives a signal. If a value greater than YDB_MAX_TIME_NSEC is specified, ydb_hiber_start() immediately returns with a YDB_ERR_TIME2LONG error; otherwise it returns YDB_OK after the elapsed time or when the wait is terminated by a signal.

### ydb_init()¶

int ydb_init(void)


ydb_init() initializes the YottaDB runtime environment. As YottaDB automatically initializes the runtime on the first call to its API or first M code invocation, there is usually no need to explicitly call ydb_init(). The exception is when an application wishes to set its own signal handlers (see Signals).

### ydb_malloc()¶

void *ydb_malloc(size_t size)


With a signature matching that of the POSIX malloc() call, ydb_malloc() returns an address to a block of memory of the requested size, or NULL if it is unable to satisfy the request. ydb_malloc() uses a buddy system, and provides debugging functionality under the control of the environment variable ydb_dbglvl whose values are a mask as described in gtmdbglvl.h.

int ydb_stdout_stderr_adjust(void)


The function checks whether stdout (file descriptor 1) and stderr (file descriptor 2) are the same file, and if so, routes writes to stderr to stdout instead. This ensures that output appears in the order in which it was written; otherwise owing to IO buffering, output can appear in an order different from that in which it was written. Application code which mixes C and M code, and which explicitly redirects stdout or stderr (e.g., using dup2()), should call this function as soon as possible after the redirection. ydb_stdout_stderr_adjust() returns YDB_OK.

int ydb_thread_is_main(void)


Returns YDB_OK if the thread is the main thread of the process, and another value if the thread is not.

### ydb_timer_cancel()¶

void ydb_timer_cancel(intptr_t timer_id)


Cancel a timer identified by timer_id and previously started with ydb_timer_start().

### ydb_timer_start()¶

typedef void (*ydb_funcptr_retvoid_t)(intptr_t timer_id,
unsigned int handler_data_len,
char *handler_data);
int ydb_timer_start(intptr_t timer_id,
unsigned long long limit_nsec,
ydb_funcptr_retvoid_t handler,
unsigned int handler_data_len
char *handler_data);


Starts a timer. Unless canceled, when the timer expires, ydb_timer_start() invokes a handler function, providing that function with input data.

timer_id is an identifier for the the timer. It is the responsibility of application code to ensure that timer_id is different from those of any other active / pending timers.

limit_nsec is the minimum number of nanoseconds before the timer expires and invokes the handler function. Owing to overhead and system load, the actual time will almost always be greater than this value.

handler is a pointer to the function to be called when the timer expires.

handler_data is a pointer to the data to be passed to handler and handler_data_len is the length of the data at *handler_data. Note that the data it points to must be on the heap rather than on the stack, as the stack frame may no longer be valid when the timer expires.

If the requested timeout_nsec exceeds YDB_MAX_TIME_NSEC, the function returns YDB_ERR_TIME2LONG; otherwise it returns YDB_OK.

# Programming in M¶

YottaDB includes a complete implementation of the M programming language (also known as MUMPS - see The Heritage and Legacy of M (MUMPS) – and the Future of YottaDB )) that mostly conforms to ISO/IEC 11756:1999. The YottaDB M Programmers Guide documents programming YottaDB in M and is not duplicated here.

# Programming Notes (Avoiding Common Pitfalls)¶

As YottaDB is likely different from other data management and persistence engines you have used, this section provides information about features of YottaDB intended to help you avoid common pitfalls.

## Numeric Considerations¶

To ensure the accuracy of financial calculations, [10] YottaDB internally stores numbers as, and performs arithmetic using, a scaled packed decimal representation with 18 significant decimal digits, with optimizations for values within a certain subset of its full range. YottaDB efficiently converts between strings and numbers.

 [10] For example, since a number such as .01 is not exactly representable as a binary or hexadecimal floating point number adding a list of currency values using floating point arithmetic does not guarantee that the result will be correct to the penny, which is a requirement for financial calculations.

When passed a string that is a canonical number for use as a subscript, YottaDB automatically converts it to a number. This automatic internal conversion is immaterial for applications:

• that simply store and retrieve data associated with subscripts, potentially testing for the existence of nodes; or
• whose subscripts are all numeric, and should be collated in numeric order.

This automatic internal conversion is material to applications that use:

• numeric subscripts and expect the subscripts to be sorted in lexical order rather than numeric order; or
• mixed numeric and non-numeric subscripts, including subscripts that are not canonical numbers.

Applications that are affected by automatic internal conversion should prefix their subscripts with a character such as “x” which ensures that subscripts are not canonical numbers.

### Canonical Numbers¶

Conceptually, a canonical number is a string from the Latin character set that represents a decimal number in a standard, concise, form.

1. Any string of decimal digits, optionally preceded by a minus sign (“-“), the first of which is not “0” (except for the number zero itself), that represents an integer of no more than 18 significant digits.
• The following are canonical numbers: “-1”, “0”, “3”, “10”, “99999999999999999999”, “999999999999999999990”. Note that the last string has only 18 significant digits even though it is 19 characters long.
• The following are not canonical numbers: “+1” (starts with “+”), “00” (has an extra leading zero), “999999999999999999999” (19 significant digits), “-0” (the canonical representation of 0 is “0”).
2. Any string of decimal digits, optionally preceded by a minus sign that includes one decimal point (“.”), the first and last of which are not “0”, that represents a number of no more than 18 significant digits.
• The following are canonical numbers: “-.1”, “.3”, “.99999999999999999999”.
• The following are not canonical numbers “+.1” (starts with “+”), “0.3” (first digit is “0”), “.999999999999999999990” (last digit is “0”), “.999999999999999999999” (more than 18 significant digits).
3. Any of the above two forms followed by “E” (upper case only) followed by a canonical integer in the range -43 to 47 such that the magnitude of the resulting number is between 1E-43 through .1E47.

## Zwrite Format¶

Strings used as subscripts and as values can include unprintable bytes, for example control characters or binary data. YottaDB’s zwrite format is an encoding in printable ASCII of any sequence of bytes. Unlike formats such as Base64, the zwrite format attempts to preserve readability of printable ASCII characters. Note that a zwrite formatted string is always longer than the original string (at the very least, it has enclosing quotes).

## Signals¶

As YottaDB includes a real-time database engine that resides within the address space of a process, applications that use signals must not interfere with database operation.

When the YottaDB database engine initializes on the first call into the API, it initializes signal handling as follows:

• SIGALRM – YottaDB uses this signal extensively and sets its own signal handler for SIGALRM. Application code should not use SIGALRM, and must never replace YottaDB’s handler. YottaDB provides an API for applications that need timing functionality (see Utility Functions).
• SIGCHLD (formerly SIGCLD) – Set to SIG_DFL for the default action.
• SIGTSTP, SIGTTIN, and SIGTTOU – As suspending a real-time database engine at an inopportune moment is undesirable, YottaDB sets a signal handler for these signals that defers process suspension until the engine is in a state where it is safe to suspend.
• SIGCONT - YottaDB sets a handler that continues a suspended process, and nothing if the process is not suspended.
• SIGINT – If the top level invocation of the process is the mumps executable, the handler is the YottaDB Ctrl-C handler for M. Otherwise, if the handler is SIG_DFL, it is replaced by the YottaDB Ctrl-C handler for M, and if it is something else, YottaDB does not change it.
• SIGUSR1 – As YottaDB uses this signal to asynchronously execute the M code in the \$zinterrupt intrinsic special variable, it sets an appropriate handler. If non-M code is currently active when the process receives a SIGUSR1, the handler defers the signal till such time as M code is active. If an application uses no M code whatsoever, and does not intend to, it can change the SIGUSR1 handler after the first call to YottaDB. If an application has, or in the future may have, M code, it is best to leave the YottaDB handler in place.
• SIGUSR2 – As YottaDB processes other than the servers for client/server operation do not use SIGUSR2, YottaDB sets a SIG_IGN handler. SIGUSR2 is available for applications to use. To do so, set a handler after the first call to YottaDB.
• SIGQUIT – YottaDB sets a handler to terminate the process without generating a core dump.
• SIGABRT, SIGBUS, SIGFPE, SIGILL, SIGIOT, SIGSEGV, SIGTERM, and SIGTRAP – These signals are fatal, and the YottaDB handler terminates the process with a core dump. See the discussion about core dumps in the description of ydb_fork_n_core(). Although YottaDB normally cleans up processes’ interaction with databases on exit, these signals can indicate that the process is in a bad state and that its code and data cannot be trusted. The process therefore does not attempt to clean up before exit. After a fatal signal, no YottaDB functions can be called except ydb_exit(). In the event an application must use its own handler for one of these signals, it must either save YottaDB’s handler, and drive it before process termination or call ydb_exit() prior to process exit. [11]
• Handlers for all signals other than those mentioned above are set to SIG_IGN. If an application sets a signal hander for anther signal, it must ensure that ydb_exit() is explicitly called prior to process exit. An application can set its own signal handler after the first YottaDB API call.
 [11] Other YottaDB processes will attempt to automatically clean up after a process terminates abnormally. However, this is not guaranteed. Also, if the abnormally terminating process is the last process accessing a database file, there are no remaining processes to attempt a cleanup. Avoid using these signals to terminate processes unless you know what you are doing.

As database operations such as ydb_set_s() set timers, subsequent system calls can terminate prematurely with an EINTR. Such system calls should be wrapped to restart them when this occurs. An example from the file eintr_wrappers.h demonstrates how YottaDB itself is coded to handle system calls that terminate prematurely with an EINTR:

#define FGETS_FILE(BUF, LEN, FP, RC)                            \
{                                                               \
do                                                      \
{                                                       \
FGETS(BUF, LEN, FP, RC);                        \
} while (NULL == RC && !feof(FP) && ferror(FP) && EINTR == errno);      \
}


If YottaDB is used within a process with other code that cannot co-exist, or be made to co-exist, with YottaDB, for example, by safely saving and restoring handlers, separate the logic into multiple processes or use a client/server database configuration to place application logic and the database engine in separate processes (see Client/Server Operation).

To reiterate because of its importance: never replace YottaDB’s SIGALRM handler.

## Forking¶

There are two considerations when executing fork().

• Before a process that performs buffered IO executes fork(), it should execute fflush(). Otherwise, the child process will inherit unflushed buffers from the parent, which the child process will flush when it executes an fflush(). This is a general programming admonition, not specific to YottaDB except to the extent that M code within a parent process may have executed write commands which are still buffered when C code within the same process calls fork().
• After a fork(), the child process must immediately execute ydb_child_init() (see discussion at ydb_child_init()).

As YottaDB reserves the right to make the runtime system multi-threaded at a future date, you should ensure that your application code does not rely on the single-threadedness of the YottaDB runtime system. Also, while local variables are shared by all threads that call into YottaDB, this behavior may or may not continue if and when YottaDB makes the runtime system multi-threaded in the future.

## Timers and Timeouts¶

Although the Simple API uses nanosecond resolution to specify all time intervals, in practice underlying functions may have more granular resolutions (microseconds or milliseconds). Furthermore, even with a microsecond or millisecond resolution, the accuracy is always determined by the underlying hardware and operating system, as well as factors such as system load.

## Memory Allocation¶

Memory allocated by ydb_malloc() must be explicitly freed by ydb_free(). ydb_exit() does not free memory, and any memory allocated but not freed prior to ydb_exit() is released only on process exit.

## Syslog¶

Issues that pertain to the application and on which application code can take reasonable action are reported to the application (YDB_ERR_GVUNDEF being an example) and issues that pertain to operations and which application code cannot take reasonable action but which operations staff can (like running low on filesystem space, which are not discussed here, as this is a Programmers Guide) are reported to the syslog. In the event that a syslog does not exist (e.g., in default Docker containers), a process’ syslog messages go to its stderr.

YottaDB uses the existence of /dev/log as an indicator of the existence of a syslog.

## IO¶

Although YottaDB does not prohibit it, we recommend against performing IO to the same device from M and non-M code in a process unless you know exactly what you are doing and have the expertise to debug unexpected behavior. Owing to differences in buffering, and in the case of interactive sessions, setting terminal characteristics, performing IO to the same device from both M and non-M code will likely result in hard to troubleshoot race conditions and other behavior.