Re: Opening a file for update with creation if necessary
Ara.T.Howard_at_noaa.gov
Date: 12/14/04
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Date: Mon, 13 Dec 2004 18:57:39 -0700
On Mon, 13 Dec 2004, Tom Anderson wrote:
> On Mon, 13 Dec 2004, Måns Rullgård wrote:
>
>> Tom Anderson <twic@urchin.earth.li> writes:
>>
>>> On Sun, 12 Dec 2004, Bjørn Augestad wrote:
>>>
>>>> Tom Anderson wrote:
>>>>
>>>>> On Sat, 11 Dec 2004, Bjørn Augestad wrote:
>>>>>
>>>>>> Tom Anderson wrote:
>>>>>>
>>>>>>> I want to open a file for updating - mode r+. However, i'd like it to
>>>>>>> be created if it doesn't exist, as with mode w. Can i do this with
>>>>>>> fopen, or am i going to have to use open (with flags O_RDWR |
>>>>>>> O_CREAT) and fdopen?
>>>>>>
>>>>>> Check out fopen() with mode a+, possibly with an additional call to
>>>>>> fseek/rewind/fsetpos to move to the start of the file if needed.
>>>>>
>>>>> Won't that force all writes to go to the end of the file? The standard (or
>>>>> at least a standard) seems to think so:
>>>>>
>>>>> http://www.opengroup.org/onlinepubs/007908799/xsh/fopen.html
>>>>>
>>>>> Although i'm pretty sure Linux, at least, doesn't do this.
>>>>
>>>> You're right, from the man page:
>>>>
>>>> Opening a file with append mode (a as the first character in the mode
>>>> argument) causes all subsequent writes to the file to be forced to the
>>>> then current end-of-file, regardless of intervening calls to fseek().
>>>
>>> Which OS was that on? The man page on OS X (ie FreeBSD) says something
>>> similar; the one on Linux (Debian 2.4.28) says "Open for writing. The file
>>> is created if it does not exist. The stream is positioned at the end of
>>> the file." - nothing about forcing writes to EOF. Those crazy Linux
>>> hippies!
>>
>> The man page on my Linux system has this paragraph:
>>
>> Opening a file in append mode (a as the first character of
>> mode) causes all subsequent write operations to this stream to
>> occur at end-of-file, as if preceded by an
>> fseek(stream,0,SEEK_END);
>> call.
>
> Hmm. The man page for fopen on mine has nothing at all like that; perhaps
> we have different versions of glibc or something. The man page for open
> says:
>
> The file is opened in append mode. Before each
> write, the file pointer is positioned at the end of
> the file, as if with lseek.
>
> And when i actually get of my arse and try it, i see that fopening with
> mode "a" does indeed get you the 'correct' force-to-end behaviour. ISTR
> reading something about Linux not doing this; that might be an old version
> or something. So it goes.
this seems to work
jib:~/eg/c > cat a.c
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int
main (argc, argv)
int argc;
char **argv;
{
char *path, *word;
FILE *fs;
fpos_t pos;
path = argv[1] == NULL ? "foobar" : argv[1];
word = argv[2] == NULL ? "foobar" : argv[2];
fs = fopen (path, "a+");
ftruncate (fileno (fs), 0);
rewind (fs);
fprintf (fs, "%s\n", word);
fclose (fs);
return (EXIT_SUCCESS);
}
jib:~/eg/c > gcc a.c
jib:~/eg/c > ./a.out created zero && cat created
zero
jib:~/eg/c > ./a.out created one && cat created
one
jib:~/eg/c > ./a.out created two && cat created
two
jib:~/eg/c > ./a.out created forty-two && cat created
forty-two
if i am reading the man pages corectly this is how is SHOULD work everywhere -
once you've truncated the file and rewound append mode is essentially as in "w"
mode. perhaps this breaks down if you start doing random seeks - but it can be
made to work it seems.
regards.
-a
-- =============================================================================== | EMAIL :: Ara [dot] T [dot] Howard [at] noaa [dot] gov | PHONE :: 303.497.6469 | When you do something, you should burn yourself completely, like a good | bonfire, leaving no trace of yourself. --Shunryu Suzuki ===============================================================================
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