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Connection handles

Kdb+ communicates with the console, stdout, stderr, file system, and other processes through connection handles.

There are three permanent system handles:

0  console
1  stdout
2  stderr

File and process handles are created by hopen and destroyed by hclose.

hopen, hclose, hsym

Write

Syntax:

    h  x
neg[h] x

where h is a handle, writes x to its target as described below and returns itself.

A handle is an int atom but is variadic. Syntactically, it can be an int atom or a unary function.

q)1           / one is one
1
q)1 "abc\n"   / or stdout
abc
1

A handle is an applicable value. It (and its negation) can be applied to an argument and iterated.

Console

Where h is 0 and x is a string or parse tree, evaluates x in the main thread and returns the result.

q)0 "1 \"hello\""   /string
hello1

q)0 (+;2;2)         /parse tree
4

eval
Parse trees

File, stdout, stderr

Where h is stdout, stderr, or a file handle

  • h x appends string x to the file
  • neg[h] x where x is a
    • string, appends x,"\n"
    • list of strings, appends x,'"\n" to the file.
q)a:1 "quick brown fox\n"
quick brown fox
q)a
1

q)a:-1 ("quick";"brown";"fox")
quick
brown
fox
q)a
-1

q)f:`:tmp.txt
q)hopen f
3i
q)3 "quick brown fox"
3
q)-3 ("quick";"brown";"fox")
-3
q)hclose 3
q)read0 f
"quick brown foxquick"
"brown"
"fox"

q)\ls data
ls: data: No such file or directory
'os
  [0]  \ls data
       ^
q)h:hopen `:data/new
q)h                        /handle is an integer
3i
q)type h                   /atom
-6h
q)h "now is the time"      /but can be applied as a unary
3i
q)/and iterated
q)h each (" for all good men";" to come to the aid of the party")
3 3i
q)hclose h
q)read0 `:data/new         /hopen created file path
"now is the time for all good men to come to the aid of the party"

File system

Process

  • h x sends string x as a sync request (get)
  • neg[h] x sends string x as an async request (set)

Interprocess communication

Read

Console

Reading from the console with read0 permits interactive input.

q)s:{1 x;read0 0}"Next track: "
Next track: Bewlay Brothers
q)s
"Bewlay Brothers"

File

read0, File Text
read1, File Binary

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