EOF: Difference between revisions

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* Returns 0 until the end of a file. This avoids a file read error.
* Returns 0 until the end of a file. This avoids a file read error.
* Returns -1 (true) at the end of the file.
* Returns -1 (true) at the end of the file.
<!-- confusing statement; further details are required: * [[CHR$]](26) can be used to denote the end of a file. -->
 
* '''Note that [[GET]] can return invalid data at the end of a file.''' Read [[EOF]] after a GET operation to see if the end of the file has been reached and discard last read.
=== Notes ===
** This is not a problem when using [[EOF]] with HTTP connections with a variable length string, the string will always only contain valid data or be empty.
* In files opened with the [[INPUT (file mode)]] the '''EOF''' function returns '''true''' after any used input function reads a [[CHR$]](26) (Ctrl-Z) from the file, which denotes the "logical" end of a file. This is not necessarily equal to the "physical" end.
** Although this subtle behavior is not required nowadays, it is still here for the sake of compatibility. If you're interested in the historic cause of it see [https://devblogs.microsoft.com/oldnewthing/20040316-00/?p=40233 this Article].
** To be able to read those files completely use the [[BINARY|BINARY (file mode)]] instead, which is also much faster when used in conjunction with the regular [[INPUT (file statement)|INPUT]], [[LINE INPUT (file statement)|LINE INPUT]] and [[INPUT$]] functions.
* [[GET]] can return invalid data at the end of a file. Read '''EOF''' after a [[GET]] operation to see if the end of the file has been reached and discard the last read if required.
** This is not a problem when using [[GET]] with HTTP connections with a variable length string, the string will always only contain valid data or be empty.
 
 
{{PageExamples}}
;Example 1:Showing the difference between INPUT and BINARY file modes when Ctrl-Z is involved.
{{CodeStart}}
'Write a simple test file with Ctrl-Z in the middle.
{{Cl|OPEN}} "test.txt" {{Cl|FOR}} {{Cl|OUTPUT}} {{Cl|AS}} #1
{{Cl|PRINT}} #1, "Hello"; {{Cl|CHR$}}(26); "World!"
{{Cl|CLOSE}} #1
 
'Now read it back, but uhh, this gives us the "Hello"
'only because of the Ctrl-Z.
{{Cl|OPEN}} "test.txt" {{Cl|FOR}} {{Cl|INPUT}} {{Cl|AS}} #1
{{Cl|WHILE}} {{Cl|NOT}} {{Cl|EOF}}(1)
    {{Cl|PRINT}} {{Cl|INPUT$}}(1, 1);
{{Cl|WEND}}
{{Cl|CLOSE}} #1
 
{{Cl|PRINT}}: {{Cl|PRINT}}
 
'However, it works in the BINARY file mode.
{{Cl|OPEN}} "test.txt" {{Cl|FOR}} {{Cl|BINARY}} {{Cl|AS}} #1
{{Cl|WHILE}} {{Cl|NOT}} {{Cl|EOF}}(1)
    {{Cl|PRINT}} {{Cl|INPUT$}}(1, 1);
{{Cl|WEND}}
{{Cl|CLOSE}} #1
{{CodeEnd}}
{{OutputStart}}
Hello
 
Hello World!
{{OutputEnd}}




Revision as of 20:41, 28 January 2023

The EOF function indicates that the end of a file or HTTP response has been reached.


Syntax

endReached%% = EOF([#]fileNumber&)
endReached%% = EOF([#]httpHandle&)


Description

  • fileNumber& or httpHandle& is the number of the file or HTTP connected being read. # is not required.
    • fileNumber& is a file opened using OPEN.
    • httpHandle& is a HTTP connection opened using _OPENCLIENT.
  • Returns 0 until the end of a file. This avoids a file read error.
  • Returns -1 (true) at the end of the file.

Notes

  • In files opened with the INPUT (file mode) the EOF function returns true after any used input function reads a CHR$(26) (Ctrl-Z) from the file, which denotes the "logical" end of a file. This is not necessarily equal to the "physical" end.
    • Although this subtle behavior is not required nowadays, it is still here for the sake of compatibility. If you're interested in the historic cause of it see this Article.
    • To be able to read those files completely use the BINARY (file mode) instead, which is also much faster when used in conjunction with the regular INPUT, LINE INPUT and INPUT$ functions.
  • GET can return invalid data at the end of a file. Read EOF after a GET operation to see if the end of the file has been reached and discard the last read if required.
    • This is not a problem when using GET with HTTP connections with a variable length string, the string will always only contain valid data or be empty.


Examples

Example 1
Showing the difference between INPUT and BINARY file modes when Ctrl-Z is involved.
'Write a simple test file with Ctrl-Z in the middle.
OPEN "test.txt" FOR OUTPUT AS #1
PRINT #1, "Hello"; CHR$(26); "World!"
CLOSE #1

'Now read it back, but uhh, this gives us the "Hello"
'only because of the Ctrl-Z.
OPEN "test.txt" FOR INPUT AS #1
WHILE NOT EOF(1)
    PRINT INPUT$(1, 1);
WEND
CLOSE #1

PRINT: PRINT

'However, it works in the BINARY file mode.
OPEN "test.txt" FOR BINARY AS #1
WHILE NOT EOF(1)
    PRINT INPUT$(1, 1);
WEND
CLOSE #1

Hello

Hello World!


See also


Navigation:
Main Page with Articles and Tutorials
Keyword Reference - Alphabetical
Keyword Reference - By usage
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