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Streams

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Streams[]

gives a list of all currently open streams.

Streams["file"]

lists only the open streams for the named file.

Examples (8)

Every input below was run against the current Mathilda build and its output recorded.

Basic Examples (6)

In[1]:= a = OpenWrite["/tmp/mathilda_io_a.txt"]
Out[1]= OutputStream["/tmp/mathilda_io_a.txt", 1]

In[2]:= b = OpenWrite["/tmp/mathilda_io_b.txt"]
Out[2]= OutputStream["/tmp/mathilda_io_b.txt", 2]

All open streams

In[3]:= Streams[]
Out[3]= {OutputStream["/tmp/mathilda_io_a.txt", 1], OutputStream["/tmp/mathilda_io_b.txt", 2]}

Just the streams for one file

In[4]:= Streams["/tmp/mathilda_io_a.txt"]
Out[4]= {OutputStream["/tmp/mathilda_io_a.txt", 1]}

In[5]:= Close[a]
Out[5]= "/tmp/mathilda_io_a.txt"

In[6]:= Close[b]
Out[6]= "/tmp/mathilda_io_b.txt"

Applications (2)

The open stream is listed

In[7]:= str = OpenWrite["/tmp/mathilda_streams.txt"]; MemberQ[Streams[], str]
Out[7]= True

After Close, none for that file

In[8]:= Close[str]; Streams["/tmp/mathilda_streams.txt"]
Out[8]= {}

Implementation notes

Algorithm. builtin_streams walks the process-global stream registry and builds a List of the live stream objects. With one argument it reads a file-name filter (stream_filename_arg) and keeps only slots whose name matches it; with none it keeps every open slot. For each surviving slot it emits stream_make_object (InputStream["file", id] or OutputStream["file", id] by the slot's is_output flag) into a malloc'd array that is grown as needed, then wraps it in List[...].

Data structures ā€” the stream layer. Reads the same process-global Stream registry described under OpenRead; the result is a snapshot of the slots whose id != 0, so it shrinks as Close frees slots and is empty at program start and once everything is closed. Each entry is the inert object itself, so it can be passed straight back to Read/Write/Close.

Complexity / limits. O(n) over the registry, n the number of open slots. The id inside each object is an internal handle whose value depends on how many streams have been opened, so code should test membership rather than match a printed form. ATTR_PROTECTED.

Attributes: Protected.

References

See also: InputStream, OutputStream

Notes & additional examples

Notes

Streams[] returns the list of currently open InputStream/OutputStream objects; Streams["file"] lists only those open for the named file. Each entry is the same inert object OpenRead/OpenWrite handed back, so it can be passed straight to Read, Write or Close.

The list reflects the live registry, so it shrinks as streams are closed and is empty once everything is closed (and at program start). The integer in each object is an internal handle whose exact value depends on how many streams have been opened in the session, so membership (MemberQ[Streams[], str]) is the robust way to test for a stream rather than matching its printed form.