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C API Reference

C client for kdb+


K object

The C API provides access to the fundamental data K object of kdb+ and methods of manipulating it. The K object is a pointer to a k0 struct, a tagged union, and most of the API manipulates this pointer.

It is defined as

typedef struct k0 *K;

More detailed information can be found in C API header file k.h. The C API defines some types to improve uniformity of the API.

type type letter
unsigned char G
16-bit int H
32-bit int I
64-bit int J
32-bit float E
64-bit double F
char C
char* S
void V
16-byte array U

Accessing members of the K object

K object properties for object x

The members which are common to all variant types are t, u, and r. The field n is common to all variant types which have a length. These may be dereferenced as usual in the C language:

accessor description
x->t type of K object. (signed char)
x->u attribute. 0 means no attributes. (C)
x->r reference count. Modify only via r1(x), r0(x). (I)
x->n number of elements in a list. (J)

Atom accessors for object x

The fields of the variant types which represent an atom (sometimes called a scalar) are:

kdb+ type accessor derived types
byte x->g (G) boolean, char
short x->h (H)
int x->i (I) month, date, minute, second, time
long x->j (J) timestamp, timespan
real x->e (E)
float x->f (F) datetime
symbol x->s (S) error
table x->k (K)

List accessors

To simplify accessing the members for list variant, the following multiple helper macros are provided, to be used as, kG(x), for example.

q type name interface list accessor function
mixed list K* kK(x)
boolean G* kG(x)
guid U* kU(x)
byte G* kG(x)
short H* kH(x)
int I* kI(x)
long J* kJ(x)
real E* kE(x)
float F* kF(x)
char C* kC(x)
symbol S* kS(x)
timestamp J* kJ(x)
month I* kI(x)
date I* kI(x) (days from 2000.01.01)
datetime F* kF(x) (days from 2000.01.01)
timespan J* kJ(x) (nanoseconds)
minute I* kI(x)
second I* kI(x)
time I* kI(x) (milliseconds)
dictionary kK(x)[0] for keys and kK(x)[1] for values

Reference counting

Q uses reference counting to manage object lifetimes. You are said to own a reference if you have created it with r1 or received it with an object returned to a call by a C API function. You are responsible for destroying the reference you own with r0 when you are finished with the object.

Ownership of a reference to an object passed as parameter can be taken from you by some of the C API functions. Other C API functions, and all functions from dynamically-linked modules, do not take ownership of references to their parameters; they have to create a new reference to any object they wish to retain or return.

If ownership of a reference has been taken from you, you are no longer responsible for it and should not destroy it. To retain an owned reference to an object, create a new reference to it prior to the call.

Error handling

C API functions marked as requiring ee return 0 on error. You should either propagate the error further by returning 0 or handle it by calling ee and handling the resulting object.

Note that you can only return an error object at a top level from a C function called from q.


Q has a rich type system. The type is indicated by a type number and many of these numbers have a constant defined around 0. Positive numbers are used for types which have a length, and the negative of these represent the scalar type.

For example, KB is the type for a vector of booleans, and the negative, -KB is for an atom of type boolean.

Some types do not have a constant. For example, mixed list has type 0, and error has a type -128.

constant associated type value
KB boolean 1
UU guid 2
KG byte 4
KH short 5
KI int 6
KJ long 7
KE real 8
KF float 9
KC char 10
KS symbol 11
KP timestamp 12
KM month 13
KD date 14
KZ datetime 15
KN timespan 16
KU minute 17
KV second 18
KT time 19
XT table 98
XD dictionary 99

Some numeric constants are defined and have special meaning – indicating null or positive infinity for that type.

constant value description
nh 0xFFFF8000 short null
wh 0x7FFF short infinity
ni 0x80000000 int null
wi 0x7FFFFFFF int infinity
nj 0x8000000000000000 long null
wj 0x7FFFFFFFFFFFFFFF long infinity
nf log(-1.0) on Windows or (0/0.0) on Linux floating point null
wf -log(0.0) in Windows or (1/0.0) on Linux floating point infinity

Functions by category


function constructs function constructs function constructs
ka atom kj long ktj timespan
kb boolean knk list ktn vector
kc char knt keyed table ku guid
kd date kp char array kz datetime
ke real kpn char array vaknk va_list version of knk
kf float ks symbol xD dictionary
kg byte kt time xT table
kh short ktd simple table
ki int ktj timestamp


function joins function joins
ja raw value to list js interned string to symbol vector
jk K object to list jv K list to first of same type

When appending to a list, if the capacity of the list is insufficient to accommodate the new data, the list is reallocated with the contents of x updated. The new data is always appended, unless the reallocation causes an out-of-memory condition which is then fatal; these functions never return NULL. The reallocation of the list will cause the initial list’s reference count to be decremented. The target list passed to join functions should not have an attribute, and the caller should consider that modifications to that target object will be visible to all references to that object unless a reallocation occurred.

Other functions

function action function action
b9 serialize r0 decrement ref count
d9 deserialize r1 increment ref count
dj date to integer sd0 remove callback
dl dynamic link sd0x remove callback
dot apply sd1 function on event loop
ee capture error setm toggle symbol lock
k evaluate sn intern chars from string
krr signal C error ss intern null-terminated string
m9 release memory vak va_list version of k
okx verify IPC message ymd encode q date
orr signal system error

Standalone applications

function action
kclose disconnect from host
khp connect to host without credentials
khpu connect to host without timeout
khpun connect to host


Unless otherwise specified, no function accepting K objects should be passed NULL.

Functions by name

In the following descriptions, functions are tagged as follows.

tag indicates the function
c.o is also available in c.o
own takes ownership of a reference
ee requires ee for error handling

b9 – serialize

Signature: K b9(I mode, K x)
Tags: c.o ee

Uses q IPC and mode capabilities level, where mode is:

value effect
-1 valid for V3.0+ for serializing/deserializing within the same process
0 unenumerate, block serialization of timespan and timestamp (for working with versions prior to V2.6)
1 retain enumerations, allow serialization of timespan and timestamp: Useful for passing data between threads
2 unenumerate, allow serialization of timespan and timestamp
3 unenumerate, compress, allow serialization of timespan and timestamp

On success, returns a byte-array K object with serialized representation. On error, NULL is returned; use ee to retrieve error string.

d9 – deserialize

Signature: K d9(K x)
Tags: c.o ee

The byte array x is not modified.

On success, returns deserialized K object. On error, NULL is returned; use ee to retrieve the error string.

dj – date to number

Signature: I dj(I date)
Tags: c.o

Converts a q date to a yyyymmdd integer.

ymd – numbers to date

Signature: K dl(V* f, I n)

Function takes a C function that would take n K objects as arguments and returns a K object. Shared library only.

Returns a q function.

dot – apply

Signature: K dot(K x, K y)
Tags: ee

The same as the q function Apply, i.e. .[x;y]. Shared library only.

On success, returns a K object with the result of the . application. On error, NULL is returned. See ee for result-handling example.

ee – error string

Signature: K ee(K)
Tags: c.o

Capture (and reset) error string into usual error object, e.g.

K x=ee(dot(a,b));if(xt==-128)printf("error %s\n", x->s);

Since V3.5 2017.02.16, V3.4 2017.03.13

Handling errors

If a function returns type K and has the option to return NULL, the user should wrap the call with ee, and check for the error result, also considering that the error string pointer (x->s) may also be NULL. e.g.

c ​K x=ee(dot(a,b));if(xt==-128)printf("error %s\n", x->s?x->s:"");

Otherwise the error status within the interpreter may still set set, resulting in the error being signalled incorrectly elsewhere in kdb+.

Calling ee(…) has the side effect of clearing the interpreter’s error status for the NULL result path.

ja – join value

Signature: K ja(K* x, V*)
Tags: c.o

Appends a raw value to a list. x points to a K object, which may be reallocated during the function. The contents of x, i.e. *x, will be updated in case of reallocation.

Returns a pointer to the (potentially reallocated) K object.

jk – join K object

Signature: K jk(K* x, K y)
Tags: c.o own

Appends another K object to a mixed list. Takes ownership of a reference to its argument y.

Returns a pointer to the (potentially reallocated) K object.

js – join string

Signature: K js(K* x, S s)
Tags: c.o

Appends an interned string s to a symbol list.

Returns a pointer to the (potentially reallocated) K object.

jv – join K lists

Signature: K jv(K* x, K y)
Tags: c.o

Append a K list y to K list x.

Returns a pointer to the (potentially reallocated) K object.

k – evaluate

Signature: K k(I handle, const S s, …)
Tags: own

Evaluates s. Optional parameters are either local (shared library only) or remote. The last argument must be NULL.

Takes ownership of references to its arguments.

Behavior depends on the value of handle.

  • handle>0, sends sync message to handle, to evaluate a string or function with parameters, and then blocks until a message of any type is received on handle. It can return NULL (indicating a network error) or a pointer to a K object. k(handle,(S)NULL) does not send a message, and blocks until a message of any type is received on handle. If that object has type -128, it indicates an error, accessible as a null-terminated string in r->s. When you have finished using this object, it should be freed by calling r0(r).

  • handle<0, this is for async messaging, and the return value can be either 0 (network error) or non-zero (success). This result should not be passed to r0.

  • handle==0 is valid only for a plugin, and executes against the kdb+ process in which it is loaded.

See more on message types. Note that a k call will block until a message is sent/received (handle!=0) or evaluated (handle=0).

ka – create atom

Signature: K ka(I t)
Tags: c.o

Creates an atom of type t.

kb – create boolean

Signature: K kb(I)
Tags: c.o

kc – create char

Signature: K kc(I)
Tags: c.o

Null: kc(" ")

kclose – disconnect

Signature: V kclose(I)

With the release of c.o with V2.6, c.o now tracks the connection type (pre V2.6, or V2.6+). Hence, to close the connection, you must call kclose (instead of close or closeSocket): this will clean up the connection tracking and close the socket.

Standalone apps only. Available only from the c/e libs and not as a shared library loaded into kdb+.

kd – create date

Signature: K kd(I)
Tags: c.o

Null: kd(ni)

ke – create real

Signature: K ke(F)
Tags: c.o

Null: ke(nf)

kf – create float

Signature: K kf(F)
Tags: c.o

Null: kf(nf)

kg – create byte

Signature: K kg(I)
Tags: c.o

kh – create short

Signature: K kh(I)
Tags: c.o

Null: kh(nh)

khp – connect anonymously

Signature: I khp(const S hostname, I port)

Standalone apps only. Available only from the c/e libs and not as a shared library loaded into kdb+.

khpu(hostname, port, "")

khpu – connect, no timeout

Signature: I khpu(const S hostname, I port, const S credentials)

Standalone apps only. Available only from the c/e libs and not as a shared library loaded into kdb+.

khpun(hostname, port, credentials, 0)

khpun – connect

Signature: I khpun(const S hostname, I port, const S credentials, I timeout)

Establish a connection to hostname on port providing credentials (username:password format) with timeout.

On success, returns positive file descriptor for established connection. On error, 0 or a negative value is returned.

code  error
  0   Authentication error
 -1   Connection error
 -2   Timeout error 

Standalone apps only. Available only from the c/e libs and not as a shared library loaded into kdb+.

khpunc – connect with capability

Signature: I khpunc(S hostname, I port, S credentials, I timeout, I capability)

capability is a bit field:

value  effect
  1    1 TB limit
  2    use TLS

A return value of -3 indicates the OpenSSL initialisation failed.

code  error
  0   Authentication error
 -1   Connection error
 -2   Timeout error 
 -3   OpenSSL initialisation failed


Unix domain socket

For khp, khpu, khpun, and khpunc a Unix domain socket may be requested via the IP address, e.g.

int handle=khpu("",5000,"user:password");

ki – create int

Signature: K ki(I)
Tags: c.o

Null: ki(ni)

kj – create long

Signature: K kj(J)
Tags: c.o

Null: kj(nj)

knk – create list

Signature: K knk(I n, …)
Tags: c.o

Create a mixed list.

Takes ownership of references to arguments.

knt – create keyed table

Signature: K knt(J n, K x)
Tags: c.o ee

Create a table keyed by n first columns if number of columns exceeds n.

Returns null if the argument x is not a table.

kp – create string

Signature: K kp(S x)
Tags: c.o

Create a char array from a string.

kpn – create fixed-length string

Signature: K kpn(S x, J n)
Tags: c.o

Create a char array from a string of length n.

krr – signal C error

Signature: K krr(const S)
Tags: c.o

Signal an error from your C code.

It is the user’s responsibility to ensure the string is valid for the expected lifetime of the error.

ks – create symbol

Signature: K ks(S x)
Tags: c.o

Null: ks("")

kt – create time

Signature: K kt(I x)
Tags: c.o

Create a time from a number of milliseconds since midnight.

Null: ki(ni)

ktd – create simple table

Signature: K ktd(K x)
Tags: c.o ee own

Create a simple table from a keyed table.

Takes ownership of a reference to its argument x.

ktj – create timestamp

Signature: K ktj(-KP, x)
Tags: c.o

Create a timestamp from a number of nanos since 2000.01.01.

Null: ktj(-KP, nj)

ktj – create timespan

Signature: K ktj(-KN, x)
Tags: c.o

Create a timespan from a number of nanos since the beginning of the interval: midnight in the case of .z.n.

Null: ktj(-KN, nj)

ktn – create vector

Signature: K ktn(I type, J length)
Tags: c.o

ku – create guid

Signature: K ku(U)
Tags: c.o

Null: U g={0};ku(g)

kz – create datetime

Signature: K kz(F)
Tags: c.o

Create a datetime from the number of days since 2000.01.01. The fractional part is the time.

Null: kz(nf)

m9 – release memory

Signature: V m9(V)

Release the memory allocated for the thread’s pool.

Call m9() when the thread is about to complete, releasing the memory allocated for that thread’s pool.

okx – verify IPC message

Signature: I okx(K x)
Tags: c.o

Verify that the byte vector x is a valid IPC message.

Decompressed data only. x is not modified.

Returns 0 if not valid.

orr – signal system error

Signature: K orr(const S)
Tags: c.o

Similar to krr, this appends a system-error message to string S before passing it to krr.

r0 – decrement refcount

Signature: V r0(K)
Tags: c.o

Decrement an object‘s reference count.

If x->r is 0, x is unusable after the r0(x) call, and the memory pointed to by it may have been freed.

Start from nothing

Reference counting starts and ends with 0, not 1.

r1 – increment refcount

Signature: K r1(K)
Tags: c.o

Increment an object‘s reference count.

sd0 – remove callback

Signature: V sd0(I d)

Remove the callback on d and call kclose.

Returns null if the socket descriptor arg d is invalid or was not registered via sd1.

Shared library only.

sd0x – remove callback conditional

Signature: V sd0x(I d, I f)

Remove the callback on d and call kclose on d if f is 1.

Returns null if the socket descriptor arg d is invalid or was not registered via sd1.

Shared library only. Since V3.0 2013.04.04.

sd1 – set function on loop

Signature: K sd1(I d, f)

Put the function K f(I d){…} on the q main event loop given a socket d.

If d is negative, the socket is switched to non-blocking.

The function f should return NULL or a pointer to a K object, and its reference count will be decremented. (It is the return value of f that will be r0’d – and only if not null.)

Shared library only.

On success, returns int K object containing d. On error, NULL is returned, d is closed.

setm – toggle symbol lock

Signature: I setm(I m)

Set whether interning symbols uses a lock: m is either 0 or 1.

Returns the previously set value.

sn – intern chars

Signature: S sn(S, J n)
Tags: c.o

Intern n chars from a string.

Returns an interned string and should be used to store the string in a symbol vector.

ss – intern string

Signature: S ss(S)
Tags: c.o

Intern a null-terminated string.

Returns an interned string and should be used to store the string in a symbol vector.

sslInfo – SSL info

Signature: K sslInfo(K x)

A dictionary of settings similar to -26!x, or an error if SSL initialisation failed.

extern I khpunc(S hostname,I port,S usernamepassword,I timeout,I capability);
int handle=khpunc("remote host",5000,"user:password",timeout,2);
extern K sslInfo(K x); 
    K x=ee(sslInfo((K)0));
    printf("Init error %s\n",xt==-128?x->s:"unknown");

Returns null if there was an error initializing the OpenSSL lib.

vak, vaknk – va_list versions of k, knk

Signature: K vak(I,const S,va_list)
Signature: K vaknk(I,va_list)

where va_list is as defined in stdarg.h, included by k.h

These are va_list versions of the K k(I,const S,…) and K knk(I,…) functions, useful for writing variadic utility functions that can forward the K objects.

comp.lang.c: How can I write a function which takes a variable number of arguments and passes them to some other function (which takes a variable number of arguments)?

ver – release date

Signature: I ver()

Returns an int as yyyymmdd.

xD – create dictionary

Signature: K xD(K x, K y)
Tags: c.o own

Create a dictionary from two K objects.

Takes ownership of references to the arguments.

If y is null, will r0(x) and return null.

xT – table from dictionary

Signature: K xT(K x)
Tags: c.o ee own

Create a table from a dictionary object.

Will r0(x) and return null if it is unable to form a valid table from x.

Takes ownership of a reference to its argument x.

ymd – numbers to date

Signature: I ymd(year, month, day)
Tags: c.o

Encode a year/month/day as a q date, e.g. 0==ymd(2000, 1, 1)

dj – date to number