fopen modes

**Pawel Dziepak** · Nov 3 '08, 08:35 PM

Re: fopen modes

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Bill Cunningham wrote:

Is there a real difference in r+ and w+ ? I read in my man pages that r+
opened a file for reading and writing while w+ opened a file for reading and
writing and if the file didn't exist it would by created. If it existed it
would be truncated.

That's the difference. r+ doesn't truncate the file, it just opens it
and allows to read and write. fopen with w+ always returns a handle to
empty file with read and write privileges.

Pawel Dziepak
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**Lew Pitcher** · Nov 3 '08, 08:35 PM

Re: fopen modes

On November 3, 2008 15:14, in comp.lang.c, Bill Cunningham
(nospam@nspam.i nvalid) wrote:

Is there a real difference in r+ and w+ ? I read in my man pages that
r+
opened a file for reading and writing while w+ opened a file for reading
and writing and if the file didn't exist it would by created. If it
existed it would be truncated.

Bill, ask yourself this...
"What would fopen("MyFile", "r+") do/return if the file "MyFile" didn't
exist?

Now, ask yourself
"What would fopen("MyFile", "w+") do/return if the file "MyFile" didn't
exist?

Are your two answers the same? If not, why not?

(Hint: Does the documentation say that "r+" will create the file if it
doesn't exist?)

--
Lew Pitcher

Master Codewright & JOAT-in-training | Registered Linux User #112576
http://pitcher.digitalfreehold.ca/ | GPG public key available by request
---------- Slackware - Because I know what I'm doing. ------

**Bill Cunningham** · Nov 3 '08, 08:45 PM

Re: fopen modes

"Lew Pitcher" <lpitcher@teksa vvy.comwrote in message
news:c3f43$490f 5f27$4c0aa7c0$5 627@TEKSAVVY.CO M...

(Hint: Does the documentation say that "r+" will create the file if it
doesn't exist?)

Too my understanding of the docs that I read on my posix system, no the
file needs to already exist.

Bill

**Lew Pitcher** · Nov 3 '08, 08:45 PM

Re: fopen modes

On November 3, 2008 15:38, in comp.lang.c, Bill Cunningham
(nospam@nspam.i nvalid) wrote:

>
"Lew Pitcher" <lpitcher@teksa vvy.comwrote in message
news:c3f43$490f 5f27$4c0aa7c0$5 627@TEKSAVVY.CO M...
>

>(Hint: Does the documentation say that "r+" will create the file if it
>doesn't exist?)

>
Too my understanding of the docs that I read on my posix system, no
the
file needs to already exist.

OK, so you've got one difference:
With "r+", the file needs to exist for the fopen() to succeed, but
with "w+", the file does not need to exist.

And, from another post, you learned another difference:
With "r+", the newly opened file will not be truncated, but with "w+", the
file will be truncated.

So, I think that you've answered your own question of if "there a real
difference in r+ and w+".

--
Lew Pitcher

Master Codewright & JOAT-in-training | Registered Linux User #112576
http://pitcher.digitalfreehold.ca/ | GPG public key available by request
---------- Slackware - Because I know what I'm doing. ------

**Pawel Dziepak** · Nov 3 '08, 08:55 PM

Re: fopen modes

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Bill Cunningham wrote:

Too my understanding of the docs that I read on my posix system, no the
file needs to already exist.

Was there anywhere said that r+ creates file if it does not exist?
If you are not sure, you should read docs more carefully and/or just
write an example program that will show you what is the meaning of r+,
w+, etc. This is not the case that implementations are different than
specification.

Pawel Dziepak

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**John Bode** · Nov 3 '08, 10:05 PM

Re: fopen modes

On Nov 3, 2:14 pm, "Bill Cunningham" <nos...@nspam.i nvalidwrote:

Is there a real difference in r+ and w+ ? I read in my man pages that r+
opened a file for reading and writing while w+ opened a file for reading and
writing and if the file didn't exist it would by created. If it existed it
would be truncated.
>
Bill

r+ allows you to read and modify data already in the file, as well as
adding and modifying new data.

w+ discards data already in the file, and allows you to add and modify
new data.

a+ does not allow you to read or modify data already in the file, but
does allow you to add and modify new data.

If I had file that served as a database of addresses and phone
numbers, and I wanted to be able to read and edit existing records as
well as add new records, I would use r+. If I wanted to throw away
all existing records and start over with all new records, I would use w
+. If I wanted to add new records and keep the existing records
without changing them, I would use a+.

**Barry Schwarz** · Nov 4 '08, 12:15 AM

Re: fopen modes

On Mon, 3 Nov 2008 15:38:05 -0500, "Bill Cunningham"
<nospam@nspam.i nvalidwrote:

>
>"Lew Pitcher" <lpitcher@teksa vvy.comwrote in message
>news:c3f43$490 f5f27$4c0aa7c0$ 5627@TEKSAVVY.C OM...
>

>(Hint: Does the documentation say that "r+" will create the file if it
>doesn't exist?)

>
Too my understanding of the docs that I read on my posix system, no the
>file needs to already exist.

If you want a posix answer, ask in a posix group. If you want a C
answer, refer to C documentation since you already have K&R II.

--
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**Barry Schwarz** · Nov 4 '08, 12:15 AM

Re: fopen modes

On Mon, 03 Nov 2008 21:26:31 +0100, Pawel Dziepak
<pdziepak@quarn os.orgwrote:

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>
>Bill Cunningham wrote:

> Is there a real difference in r+ and w+ ? I read in my man pages that r+
>opened a file for reading and writing while w+ opened a file for reading and
>writing and if the file didn't exist it would by created. If it existed it
>would be truncated.

>
>That's the difference. r+ doesn't truncate the file, it just opens it
>and allows to read and write. fopen with w+ always returns a handle to

Standard C does not have file handles, or any other handles. If fopen
succeeds, it will return a FILE*. But not always. Even with w+,
fopen can fail and will return NULL.

>empty file with read and write privileges.
>
>Pawel Dziepak
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**ruckuus** · Nov 4 '08, 04:55 AM

Re: fopen modes

On Nov 4, 3:38 am, "Bill Cunningham" <nos...@nspam.i nvalidwrote:

"Lew Pitcher" <lpitc...@teksa vvy.comwrote in message
>
news:c3f43$490f 5f27$4c0aa7c0$5 627@TEKSAVVY.CO M...
>

(Hint: Does the documentation say that "r+" will create the file if it
doesn't exist?)

>
Too my understanding of the docs that I read on my posix system, no the
file needs to already exist.
>
Bill

what posix is your system ?
r+ will return 0 if the file is not exist taken it will return FILE*
w+ will always return FILE*, if file exist it will be truncated to 0

just simply write the code to demonstrate it. you can see it better.

**Pawel Dziepak** · Nov 4 '08, 02:15 PM

Re: fopen modes

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Barry Schwarz wrote:

>Standard C does not have file handles, or any other handles. If fopen
>succeeds, it will return a FILE*. But not always. Even with w+,
>fopen can fail and will return NULL.

According to Wikipedia [1] and other sources: struct FILE* is a file
handler.

[1] http://en.wikipedia.org/wiki/File_descriptor

Pawel Dziepak
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**Nick Keighley** · Nov 4 '08, 02:35 PM

Re: fopen modes

On 4 Nov, 14:13, Pawel Dziepak <pdzie...@quarn os.orgwrote:

Barry Schwarz wrote:

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could we lose the PGP stuff?

Standard C does not have file handles, or any other handles. If fopen
succeeds, it will return a FILE*. But not always. Even with w+,
fopen can fail and will return NULL.

>
According to Wikipedia [1] and other sources: struct FILE* is a file
handler.

ITYM "handle". Despite what wikipedia says this is not standard C
terminology

[1]http://en.wikipedia.or g/wiki/File_descriptor

--
Nick Keighley

**Pawel Dziepak** · Nov 4 '08, 02:45 PM

Re: fopen modes

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Nick Keighley worte:

ITYM "handle". Despite what wikipedia says this is not standard C
terminology

Of course, "handle".
7.19.5.3/8:
"The fopen function returns a pointer to the object controlling the stream."
C99 doesn't says that fopen returns a handle. However, there is no "file
handle" definition in the standard, so why don't call that "object
controlling the stream" a handle? I know that there is no clear
definition what a file handle is, but that doesn't mean that structure
FILE mustn't be called "a handle".

Pawel Dziepak

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**Chris Dollin** · Nov 4 '08, 03:15 PM

Re: fopen modes

Pawel Dziepak wrote:

"The fopen function returns a pointer to the object controlling the stream."
C99 doesn't says that fopen returns a handle. However, there is no "file
handle" definition in the standard, so why don't call that "object
controlling the stream" a handle? I know that there is no clear
definition what a file handle is, but that doesn't mean that structure
FILE mustn't be called "a handle".

Since /other/ things are called "file handles", and "a FILE*" is a way
to refer to a FILE* value, then calling a FILE* value a "file handle"
induces more confusion than is perhaps necessary.

Perhaps we should go to the other extreme and call it a "pedal";
I suspect I'll not be allowed to sustain that.

(Do we know that FILE is a struct at all? Clearly it's /likely/, even
/sensible/, to be a struct, but a hasty scan of a draft to hand didn't
actually commit.)

--
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there will be minimal destruction.' - Panic Room

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registered office: Berks RG12 1HN 690597 England

**Pawel Dziepak** · Nov 4 '08, 03:15 PM

Re: fopen modes

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Chris Dollin wrote:

Since /other/ things are called "file handles", and "a FILE*" is a way
to refer to a FILE* value, then calling a FILE* value a "file handle"
induces more confusion than is perhaps necessary.

Ok, I see your point and, now, I agree. I think that will end that part
of discussion.

Pawel Dziepak
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