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Hello all,
For some reasons, somewhere in a program, I'd like, if possible, to quickly
parse a whole file before rewinding it and letting the full analysis start.
My problem is that the FILE* I want do parse has been fopen'ed far away
from where I am and I don't know in which MODE my FILE* has been opened.
And additionally, my FILE* may not be a regular file, but a continuous
stream (pipe), in which case it is not rewindable.
So my program basically behaves like this:
void PreParsing( FILE *my_file )
{
/*
* Test if my FILE * is seekable
*/
if( fseek( my_file, 0, SEEK_SET ) == 0 ) {
/*
* If so, go on for the pre-parsing.
*/
//...
fread( buf, BUF_SIZE, 1, my_file );
//...
/*
* Rewind the file.
*/
if( fseek( my_file, 0, SEEK_SET ) != 0 ) {
// unexpected error since my FILE * was expected to be seekable.
}
}
}
* On both Linux 2.6.xx and Solaris 5.10,
when the my_file is a regular file that has been fopen'ed with "w" as mode,
- the first fseek returns 0 meaning success: my_file is seekable
- the fread returns 0 meaning that there was nothing to read: expected
since a file opened in "w" is truncated, hence is empty, and anyhow, cannot
be read!
- the second fseek returns 0 meaning success: my_file could be rewound.
when the my_file is a pipe that has been fopen'ed with "w" as mode,
- the first fseek returns -1 and errno is set to 29 (Illegal seek).
That's exactly what I expected since a pipe cannot be sought.
Ok. So, on Linux 2.6.xx and Solaris 5.10, my code behaves like I expected
for both regular files and pipes.
* On AIX 5.2,
when the my_file is a pipe that has been fopen'ed with "w" as mode,
- the first fseek returns -1 and errno is set to 29 (Illegal seek).
I'm still OK with that.
but...
when the my_file is a regular file that has been fopen'ed with "w" as mode,
- the first fseek returns 0 meaning success: my_file is seekable
- the fread returns 0 meaning that there was nothing to read: expected
since a file opened in "w" is truncated, hence is empty, and anyhow, cannot
be read!
- the second fseek returns -1 and sets errno to 9 (Bad file number)
I'm a little bit surprised by this error.
In fact, after having had a deeper look on that, it appears to me that a
fread attempt on a FILE * opened in "w" mode breaks it since any subsequent
operation (fseek, fwrite and even fclose !) fails with the error 9 (Bad
file number).
So, on AIX , my function fails to restore the FILE * state at its end.
So, I have a few questions:
- a fread on a FILE * opened in "w" will for sure return 0 item, but is it
really expected that it makes the given FILE * totally unusable even for
fclose !?! This behavior has been observed only on AIX. Linux and Solaris
works.
- given a FILE *, is there a better way to guess in which mode it has been
opened than attempting to read or write it and look at errors ?
- given a FILE *, is there a better way to guess if it can be sought than
attempting a fseek on it ?
PS: I Xposted my problem on both comp.lang.c and comp.unix.aix because I
have no idea whether this is an AIX specific problem or if the C norm
specifies explicitly that fread have unpredictable effects on a write-only
file descriptor. Anyhow, my goal is to find a portable solution that uses
as less platform specific stuff as possible.
Thanks for your advices.
Lénaïc ...still puzzled by AIX behavior...
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Nov 18 '08
15  3184 
Lénaïc Huard wrote:
Richard Tobin a écrit :
>The other aspect - seekability - is entirely reasonable, but I
don't think you can handle it in standard C. If you are (as you
appear to be) using Posix systems, you can stat the underlying
file descriptor and seek only if it is a regular file.
Sounds to be exactly what I was looking for. I'm still
wondering why I didn't think about stat before. Thanks!
>I seem to recall using at least one system where fseek()
succeeded even on pipes provided the seek was within the
existing stdio buffer, so just because one fseek() succeeds,
it doesn't mean others will.
Sounds to be an excellent reason for not testing the seekability
with fseek!
And it is entirely off-topic on c.l.c, so all such answers are
necessarily suspect. If you ask on comp.unix.progr ammer, however,
you will probably be on-target and get valid answers and criticisms
of those answers.
As an aside, if a file is opened in "w" mode, you can't do many
things to it. Read the C standard.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section. vi******@gmail. com wrote:
On Nov 19, 12:43 am, Mark McIntyre <markmcint...@T ROUSERSspamcop. net>
wrote:
<snip>
AFAIK reading from a write-only stream is undefined behaviour.
I've recently asked this (or remember asking it), but I believe I
received no answers.
I've also searched the standard for the answer, with no luck. Anyone?
It's undefined behavior by virture of the fact that all I/O takes place
as if by calls to fgetc() and fputc(), and their behavior is only
defined for input and output streams, respectively.
--
Larry Jones
Talk about someone easy to exploit! -- Calvin
On Nov 19, 8:50 pm, lawrence.jo...@ siemens.com wrote:
vipps...@gmail. com wrote:
On Nov 19, 12:43 am, Mark McIntyre <markmcint...@T ROUSERSspamcop. net>
wrote:
<snip>
AFAIK reading from a write-only stream is undefined behaviour.
I've recently asked this (or remember asking it), but I believe I
received no answers.
I've also searched the standard for the answer, with no luck. Anyone?
It's undefined behavior by virture of the fact that all I/O takes place
as if by calls to fgetc() and fputc(), and their behavior is only
defined for input and output streams, respectively.
Ah, I suspected something like that. Thanks!
Lénaïc Huard wrote:
So, I have a few questions:
- a fread on a FILE * opened in "w" will for sure return 0 item, but is it
really expected that it makes the given FILE * totally unusable even for
fclose !?! This behavior has been observed only on AIX. Linux and Solaris
works.
- given a FILE *, is there a better way to guess in which mode it has been
opened than attempting to read or write it and look at errors ?
Once you've eliminated the pipe possibility, dup() the descriptor and
fdopen() the duplicate in r-w mode. Close the old one.
You shouldn't be attempting to read from a stream opened for writing.
That makes no sense and getting undefined, inconsistent results seems
reasonable.
"Gary R. Hook" <gh***@no.spamm ers.netwrites:
Lénaïc Huard wrote:
>So, I have a few questions:
- a fread on a FILE * opened in "w" will for sure return 0 item,
but is it
really expected that it makes the given FILE * totally unusable even for
fclose !?! This behavior has been observed only on AIX. Linux and Solaris
works.
- given a FILE *, is there a better way to guess in which mode it has been
opened than attempting to read or write it and look at errors ?
There's no portable way to do it.
Once you've eliminated the pipe possibility, dup() the descriptor and
fdopen() the duplicate in r-w mode. Close the old one.
dup() and fdopen() are defined by POSIX. Using them is ok as long as
you don't mind limiting your code's portability to POSIX
implementations .
You shouldn't be attempting to read from a stream opened for
writing. That makes no sense and getting undefined, inconsistent
results seems reasonable.
Right, reading from a file opened in "w" mode invokes undefined
behavior.
--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
In article <%V************ *****@flpi149.f fdc.sbc.com>,
Gary R. Hook <gh***@no.spamm ers.netwrote:
>Once you've eliminated the pipe possibility, dup() the descriptor and
fdopen() the duplicate in r-w mode. Close the old one.
What is this supposed to do? If the underlying file descriptor is
opened for writing only, fdopen()ing it read-write won't do you
any good.
-- Richard
--
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