Checking if a file exists

[Army1987]

Is this a good way to check wheter a file already exists?

#include <stdio.h>
#include <stdlib.h>
int ask(const char *prompt);
typedef char filename[FILENAME_MAX];

int main(int argc, char *argv[])
{
FILE *in, *out;
filename in_name, out_name;
int overwrite = 0;
/* get filenames */
out = fopen(out_name, "r")
if (out != NULL) {
fclose(out);
printf("File %s already exists. ", out_name);
if (ask("Do you want to overwrite it?") == 1)
overwrite++;
else
exit(EXIT_FAILU RE);
}
/* etc... */
}

--
#include <stdio.h>
#include <stdlib.h>
int main(void) /* Don't try this at home */ {
const size_t dim = 256; int i;
for (i=0; malloc(dim); i++) /*nothing*/ ;
printf("You're done! %zu\n", i*dim);
puts("\n\n--Army1987"); return 0;
}

[Richard Heathfield]

Army1987 said:

Is this a good way to check wheter a file already exists?

#include <stdio.h>
#include <stdlib.h>
int ask(const char *prompt);
typedef char filename[FILENAME_MAX];

int main(int argc, char *argv[])
{
FILE *in, *out;
filename in_name, out_name;
int overwrite = 0;
/* get filenames */
out = fopen(out_name, "r")
if (out != NULL) {
fclose(out);
printf("File %s already exists. ", out_name);
if (ask("Do you want to overwrite it?") == 1)
overwrite++;
else
exit(EXIT_FAILU RE);
}
/* etc... */
}

It's about as good as you're going to get under ISO C, but it's not
reliable. The open might fail for other reasons than the non-existence
of the file (e.g. disk error, no read permission, etc). And there is no
guarantee that the file will continue to exist after the fclose, or
that it will continue not to exist (if indeed it did not exist) after
the fopen failure.

Abstract this functionality into your "non-portable stuff" module, and
consult your implementation' s documentation for details of how to solve
this problem (if indeed there is a solution) on the platform that it
targets.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[Jens Thoms Toerring]

Army1987 <pl********@for .itwrote:

Is this a good way to check wheter a file already exists?

out = fopen(out_name, "r")
if (out != NULL) {
fclose(out);
printf("File %s already exists. ", out_name);
if (ask("Do you want to overwrite it?") == 1)
overwrite++;

No, that's not a good idea. There could be lots of other reasons
why the fopen() failed, starting from missing permission to open
it for reading up to e.g. having run out of file descriptors. So
you shouldn't blindly assume that the file does not already exist
just because fopen() in read mode failed (and, by the way, it is
also possible on a multitasking system that the file gets created
by another process between the time you did the test and the time
you open the it). If you want to avoid overwriting data in the
file if it already exists fopen it in append mode and check if
with ftell() if you're at the very start of the file (but that
still will not allow to distinguish the cases of the file not
existing at all and the file existing but being empty).

Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de

[Daniel Rudy]

At about the time of 4/7/2007 6:51 AM, Army1987 stated the following:

Is this a good way to check wheter a file already exists?

#include <stdio.h>
#include <stdlib.h>
int ask(const char *prompt);
typedef char filename[FILENAME_MAX];

int main(int argc, char *argv[])
{
FILE *in, *out;
filename in_name, out_name;
int overwrite = 0;
/* get filenames */
out = fopen(out_name, "r")
if (out != NULL) {
fclose(out);
printf("File %s already exists. ", out_name);
if (ask("Do you want to overwrite it?") == 1)
overwrite++;
else
exit(EXIT_FAILU RE);
}
/* etc... */
}

I think that Microsoft implements a file exists call. On Unix systems,
you can stat(2) the file and check if errno = ENOENT. Granted, this is
not portable, but it works.
--
Daniel Rudy

Email address has been base64 encoded to reduce spam
Decode email address using b64decode or uudecode -m

Why geeks like computers: look chat date touch grep make unzip
strip view finger mount fcsk more fcsk yes spray umount sleep

[Keith Thompson]

"Army1987" <pl********@for .itwrites:

Is this a good way to check wheter a file already exists?

[snip]

Testing whether a file exists often (not always, but often) means
you're asking the wrong question.

Why do you want to know whether the file exists? If it's so you can
decide whether to attempt some operation (e.g., reading from the file
if it does exist, or creating it if it doesn't), it's often better to
just go ahead and attempt the operation, and handle the error if it
fails. (This can be difficult in standard C if you're trying to
create a file; if I recall correctly, it's implementation-defined
whether attempting to create an existing file will clobber the file or
fail, but there are often system-specific ways to control this
behavior.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Eric Sosman]

Keith Thompson wrote:

"Army1987" <pl********@for .itwrites:

>Is this a good way to check wheter a file already exists?

[snip]

Testing whether a file exists often (not always, but often) means
you're asking the wrong question.

Why do you want to know whether the file exists? If it's so you can
decide whether to attempt some operation (e.g., reading from the file
if it does exist, or creating it if it doesn't), it's often better to
just go ahead and attempt the operation, and handle the error if it
fails. (This can be difficult in standard C if you're trying to
create a file; if I recall correctly, it's implementation-defined
whether attempting to create an existing file will clobber the file or
fail, but there are often system-specific ways to control this
behavior.)

I would prefer to see "File IRREPLACABLE.DA T exists.
Overwrite?" than to regret it at leisure ...

This is what you allude to in "system-specific ways," but
unfortunately Standard C's I/O is too diluted to do the job
unaided.

--
Eric Sosman
es*****@acm-dot-org.invalid

[Army1987]

"Richard Heathfield" <rj*@see.sig.in validha scritto nel messaggio
news:tM******** *************** *******@bt.com. ..

Army1987 said:

>Is this a good way to check wheter a file already exists?

#include <stdio.h>
#include <stdlib.h>
int ask(const char *prompt);
typedef char filename[FILENAME_MAX];

int main(int argc, char *argv[])
{
FILE *in, *out;
filename in_name, out_name;
int overwrite = 0;
/* get filenames */
out = fopen(out_name, "r")
if (out != NULL) {
fclose(out);
printf("File %s already exists. ", out_name);
if (ask("Do you want to overwrite it?") == 1)
overwrite++;
else
exit(EXIT_FAILU RE);
}
/* etc... */
}

And there is no
guarantee that the file will continue to exist after the fclose.

That's the problem. If the file couldn't be opened for some other reason
other than not existing, I would expect the fopen(out_name, "w") later in
the program to fail (unless the user has permission -w- on the file, in
which case I think he (or its owner) *deserves* to lose it, or some other
problem which I can't imagine).

Now if it is possible that out = fopen(out_name, "r"); fclose(out); deletes
the file, I'd better find another way...

[Army1987]

"Keith Thompson" <ks***@mib.orgh a scritto nel messaggio
news:ln******** ****@nuthaus.mi b.org...

"Army1987" <pl********@for .itwrites:

>Is this a good way to check wheter a file already exists?

[snip]

Testing whether a file exists often (not always, but often) means
you're asking the wrong question.

Why do you want to know whether the file exists? If it's so you can
decide whether to attempt some operation (e.g., reading from the file
if it does exist, or creating it if it doesn't), it's often better to
just go ahead and attempt the operation, and handle the error if it
fails.

7.19.5.3.3 says that fopen(name, "w") "truncate[s] to zero length or
create[s] text file for writing".
Simply opening the file for writing loses its contents if it already exists.

[Richard Heathfield]

Army1987 said:

<snip>

Now if it is possible that out = fopen(out_name, "r"); fclose(out);
deletes the file, I'd better find another way...

Well, I wouldn't expect that to happen! But I would not be surprised by
the following sequence of events:

1) you open the file read-only
2) you close the file
3) another process deletes the file
4) you act on your belief that the file exists

Multi-user / multi-process systems can be tricky, can't they?

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.

[Jens Thoms Toerring]

Army1987 <pl********@for .itwrote:

"Keith Thompson" <ks***@mib.orgh a scritto nel messaggio
news:ln******** ****@nuthaus.mi b.org...

"Army1987" <pl********@for .itwrites:

Is this a good way to check wheter a file already exists?

[snip]

Testing whether a file exists often (not always, but often) means
you're asking the wrong question.

Why do you want to know whether the file exists? If it's so you can
decide whether to attempt some operation (e.g., reading from the file
if it does exist, or creating it if it doesn't), it's often better to
just go ahead and attempt the operation, and handle the error if it
fails.

7.19.5.3.3 says that fopen(name, "w") "truncate[s] to zero length or
create[s] text file for writing".
Simply opening the file for writing loses its contents if it already exists.

In order to avoid just that there's the "a" or "a+" mode you can
pass to fopen() instead of "w" to open it in append mode. Then an
already existing file won't get truncated and you're positioned
at the end of the file while, if the file doesn't exist, it be-
haves as if you had used "w".
Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de