Streams: Difference between revisions
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A ‘stream’ is a serial flow of data. It is not the same as a [[Files|file]] – files can be addressed in a ‘random’ manner – but files are not infrequently read or written as serial streams. Indeed it is often said of [https://en.wikipedia.org/wiki/Unix Unix-family] operating systems that “[https://en.wikipedia.org/wiki/Everything_is_a_file everything is a file]”. | A ‘stream’ is a serial flow of data. It is not the same as a [[Files|file]] – files can be addressed in a ‘random’ manner – but files are not infrequently read or written as serial streams. Indeed it is often said of [https://en.wikipedia.org/wiki/Unix Unix-family] operating systems that “[https://en.wikipedia.org/wiki/Everything_is_a_file everything is a file]”. | ||
In this case, the stream is an unstructured, serial succession of bytes. Input from the stream will give the next byte – if and <em>when</em> there is one – and the byte is removed from the input sequence in the process. Output to a stream will output – assuming no ‘back-pressure’ from something (such as writing a file) | In this case, the stream is an unstructured, serial succession of bytes. Input from the stream will give the next byte – if and <em>when</em> there is one – and the byte is removed from the input sequence in the process. Output to a stream will output – assuming no ‘back-pressure’ from something (such as writing a file) being slow further downstream; again the byte is then ‘gone’. | ||
being slow further downstream; again the byte is then ‘gone’. | |||
I/O devices can also be treated as streams. Your keyboard provides an | I/O devices can also be treated as streams. Your keyboard provides an input stream, for example. | ||
input stream, for example. | |||
=== Unix Standard Streams === | === Unix Standard Streams === | ||
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*Standard error (<code>stderr</code>) – usually also defaults to display. | *Standard error (<code>stderr</code>) – usually also defaults to display. | ||
Processes can ‘get’ and ‘put’ characters to | Processes can ‘get’ and ‘put’ characters to these streams in various ways. They are typically the default source/sink of input/output in system applications. You should be quite used to using them by now! | ||
these streams in various ways. They are typically the default | |||
source/sink of input/output in system applications. | |||
You should be quite used to using them by now! | |||
=== Redirection === | === Redirection === | ||
Unix can redirect a stream to a file or, via a [[Pipes|pipe]], to | Unix can redirect a stream to a file or, via a [[Pipes|pipe]], to another process. From a Unix command line try something like: | ||
another process. From a Unix command line try something like: | |||
<blockquote> | <blockquote> | ||
<code>ls | wc > clutter</code> | <code>ls | wc > clutter</code> | ||
</blockquote> | </blockquote> | ||
That example redirects the <code>stdout</code> from the <code>ls</code> process to the | That example redirects the <code>stdout</code> from the <code>ls</code> process to the <code>stdin</code> of <code>wc</code>, and <code>wc</code>'s <code>stdout</code> to a file. This sort of thing is often jolly useful! | ||
<code>stdin</code> of <code>wc</code>, and <code>wc</code>'s <code>stdout</code> to a file. | |||
This sort of thing is often jolly useful! | |||
=== Streams as files === | === Streams as files === | ||
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</syntaxhighlight> | </syntaxhighlight> | ||
---- | ---- | ||
{{PageGraph}} | {{PageGraph}} | ||
{{Category|IO}} | {{Category|IO}} | ||
{{Category|User}} | {{Category|User}} | ||
Latest revision as of 10:03, 5 August 2019
| Depends on | Queues • Shell |
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A ‘stream’ is a serial flow of data. It is not the same as a file – files can be addressed in a ‘random’ manner – but files are not infrequently read or written as serial streams. Indeed it is often said of Unix-family operating systems that “everything is a file”.
In this case, the stream is an unstructured, serial succession of bytes. Input from the stream will give the next byte – if and when there is one – and the byte is removed from the input sequence in the process. Output to a stream will output – assuming no ‘back-pressure’ from something (such as writing a file) being slow further downstream; again the byte is then ‘gone’.
I/O devices can also be treated as streams. Your keyboard provides an input stream, for example.
Unix Standard Streams
All Unix processes are provided with three standard streams:
- Standard input (
stdin) – usually defaults to keyboard. - Standard output (
stdout) – usually defaults to display. - Standard error (
stderr) – usually also defaults to display.
Processes can ‘get’ and ‘put’ characters to these streams in various ways. They are typically the default source/sink of input/output in system applications. You should be quite used to using them by now!
Redirection
Unix can redirect a stream to a file or, via a pipe, to another process. From a Unix command line try something like:
ls | wc > clutter
That example redirects the stdout from the ls process to the stdin of wc, and wc's stdout to a file. This sort of thing is often jolly useful!
Streams as files
Standard streams can often be both implicit or explicit when programming.
Example
The C statement:
printf("Hello World!\n");
can also be written as:
fprintf(stdout, "Hello World!\n");
in a similar way to ‘printing’ to a file:
fprintf(file_handle, "Hello World!\n");
