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[cross-posted to comp.lang.c, comp.lang.c++]
Hello

I see there is now why to truncate a file (in C or C++)
and that I have to use platform-specific functions for
truncating files.

Anyone knows why ? I mean C/C++ evolved over many years now,
and still, people making _the_ standards never decided to
include such a function. Even in POSIX it was included only
in recent versions, mostly not fully supported yet.

Why is that ? They must think there is something wrong with it,
or that there are better ways to do it.

I mean I want to write a portable application, I keep
application data in a file, at some point the user
deletes something from the data my program presents
to him/her, and now I delete some data from the data file
and want to shrink the file. Like a database system,
after dropping some records or tables and compacting
(or vacuum) the database. That is a good example
of when a programmer legally needs to truncate a file.

Copying only remaining data to a new file that will then
replace the old one is not a solution for a large database,
and leaving the empty space in the file is also not a
solution for large files.

Thank you,
Timothy Madden

On Nov 10, 3:50 am, Timothy Madden <terminato...@gmail.com> wrote:
> [cross-posted to comp.lang.c, comp.lang.c++]
> Hello
>
> I see there is now why to truncate a file (in C or C++)
> and that I have to use platform-specific functions for
> truncating files.

There are ways.

int trunc(const char *file, size_t newsize) {
FILE *fp;
void *p = NULL;

if(newsize) {
fp = fopen(file, "rb");
if(fp == NULL) return 1;
p = malloc(newsize);
if(p == NULL) { fclose(fp); return 1; } /* errno may be lost and
the user will get a misleading error message */
if(fread(p, 1, newsize, fp) != newsize) { fclose(fp); return
1; } /* ditto */
fclose(fp);
}
fp = fopen(file, "wb");
if(fp == NULL) {
free(p);
return 1;
}
if(newsize) if(fwrite(p, 1, newsize, fp) != newsize) { free(p);
fclose(fp); return 1; } /* ditto */
free(p);
return fclose(fp) == EOF;
}



}

> Anyone knows why ? I mean C/C++ evolved over many years now,
> and still, people making _the_ standards never decided to
> include such a function. Even in POSIX it was included only
> in recent versions, mostly not fully supported yet.
>
> Why is that ? They must think there is something wrong with it,
> or that there are better ways to do it.

You're right. There is something wrong with it, that some platforms
that support C do not support file truncating (hell, some might not
even support file operations at all). There are better ways, to use a
more specific standard like POSIX.

Timothy Madden wrote:
>
> [cross-posted to comp.lang.c, comp.lang.c++]
>
> I see there is now why to truncate a file (in C or C++)
> and that I have to use platform-specific functions for
> truncating files.

That depends on file systems, so cannot be portable.

Do not cross-post to c.l.c++. The languages are different.

F'ups set.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.

CBFalconer wrote:
> Timothy Madden wrote:
> >
> > [cross-posted to comp.lang.c, comp.lang.c++]
> >
> > I see there is now why to truncate a file (in C or C++)
> > and that I have to use platform-specific functions for
> > truncating files.
>
> That depends on file systems, so cannot be portable.
>
> Do not cross-post to c.l.c++. The languages are different.

Cross-posting reinstated.

Don't overstate the differences between the languages. As far as this
issue is concerned they are very similar. All of the <stdio.h>
facilities are still present in C++, and the C++ standard mandates
connections between the behavior of the <iostream> library and the
behavior of the <stdio.h> library.

Both committees are committed to, among other priorities, avoiding
gratuitous incompatibilities between the two languages. Therefore, in
the unlikely event that either committee were inclined to add a file
truncation feature to their own language, the joint sub-committee
would seriously consider recommending that it also be added to the
other language in a compatible fashion.