On 11/6/2020 2:33 PM, JJ wrote:
> On Wed, 4 Nov 2020 13:53:28 +0100, R.Wieser wrote:
>> Hello all,
>>
>> I've got a small program which rewrites the contents of a textfile. I'm
>> using "fopen" for both the source as well as the target file, reading lines
>> from the first and writing them to the second.
>>
>> The problem is that when both have the same name the file is destroyed. :-((
>>
>> To fix that I've been looking for an "exclusive read" open mode (so that
>> opening the targetfile would error-out). The thing is that Windows doesn't
>> seem to offer it :
>>
>> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/fopen-wfopen
>>
>> It does offer an "x" modifier, but that one only works on writing.
>>
>> I've also found a reference to "flock", but that doesn't seem to be part of
>> the CRTDLL.DLL on XPsp3 .
>>
>> tl;dr:
>> How do I use "fopen" (or a similar crtdll function) to get an exclusive read
>> lock ?
>>
>> Regards,
>> Rudy Wieser
>
> Not possible using `fopen()`. Use `fsopen()` instead. `fopen()` is simply
> not share aware.
>
> `fopen()` uses `_SH_DENYNO` sharing flag no matter what. That flag
> translates to Windows' `FILE_SHARE_READ` and `FILE_SHARE_WRITE`.
>

AFAIK you don't need a read lock for file access (see a database course.)

holly

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holyghost,

> AFAIK you don't need a read lock for file access (see a database course.)

:-) A database locks the record when writing to it, so that when reading
there is no way that you will get half the old and half the new record.

But I suggest you read my problem specification first. Its not about the
read and write actions themselves.

Regards,
Rudy Wieser