is this the correct/best way to set file size?

George2 · 07-17-2006, 05:00 AM

Hello everyone,

I have verified that the following approach works to set the size of a file (newly created file) to be 100 bytes, but I am not sure whether it is the correct/best way to have a maximum portability (I need to write code on both Windows and Linux).

Could anyone give me any comments?

Code:

#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <io.h>
#include <stdio.h>

int main()
{
	FILE* file = fopen ("foo123", "w+");
	
	fseek (file, 99, SEEK_SET);

	fprintf (file, "x");

	fclose (file);

	return 0;
}

thanks in advance,
George

ugenn · 07-17-2006, 06:00 AM

There's no portable (as far as the ISO C specs are concerned) way to determine file size.

You should have a look at stat() (non ISO C, but part of POSIX/SUS and supported as an extension in VC++).

And besides, your code will produce different results on windows and unix when text files are used since windows differentiates between the 2, so it's not too portable either.

jim mcnamara · 07-17-2006, 09:06 AM

George -

Don't do that. You create a file with holes in it. It screws up backups.
If there is a sysadmin he/she will throw rocks at you

There is no way to effectively pre-allocate a file in either Windows or Linux.

Windows (MS) does NOT follow any ANSI or ISO standards for it's compilers, so if your code ports it's pure luck. There are other free compilers that do a better job...

You have also included several extra header files for the code you show us. It does not need those header files.

Try this:

Code:

#include <stdio.h>
#include <stdlib.h>
int main()
{
     FILE *out=fopen("filename","w");
     char ch=0xff;
      
     if(out==NULL){
     {
         perror("Error on file open");
         exit(EXIT_FAILURE);
     }
     if( !fwrite(&ch,sizeof(char),100,out) || fclose(out)==EOF)
     {
         perror("File I/O failure);
         exit(EXIT_FAILURE);
     }
     return 0;
}

If this is meant to be production code you have to check return codes to assure success, especially with disk operations.

This also uses code that will compile/run on Windows.

File holes: http://www.scit.wlv.ac.uk/~jphb/cp3025/fcons.html

When "hoely" files are backed up, they don't always restore back to disk correctly, depneidng on the file system. Therefore they essentially report incorrect file sizes to the file system. If you want to look at it that way. This drives sysadmins insane when they think they are restoring files of one size and get different results.

George2 · 07-18-2006, 02:41 AM

Thank you ugenn,

Quote:

Originally Posted by ugenn

There's no portable (as far as the ISO C specs are concerned) way to determine file size.

You should have a look at stat() (non ISO C, but part of POSIX/SUS and supported as an extension in VC++).

You mean using stat to set file size? As far as I know, stat can only get the information of the file, other than set its size.

Quote:

Originally Posted by ugenn

And besides, your code will produce different results on windows and unix when text files are used since windows differentiates between the 2, so it's not too portable either.

Do you mean on Windows, end of line is \r\n and on Linux end of line is \n?

regards,
George

George2 · 07-18-2006, 02:46 AM

Thank you jim,

Quote:

Originally Posted by jim mcnamara

George -

Don't do that. You create a file with holes in it. It screws up backups.
If there is a sysadmin he/she will throw rocks at you

There is no way to effectively pre-allocate a file in either Windows or Linux.

Windows (MS) does NOT follow any ANSI or ISO standards for it's compilers, so if your code ports it's pure luck. There are other free compilers that do a better job...

You have also included several extra header files for the code you show us. It does not need those header files.

Try this:

Code:

#include <stdio.h>
#include <stdlib.h>
int main()
{
     FILE *out=fopen("filename","w");
     char ch=0xff;
      
     if(out==NULL){
     {
         perror("Error on file open");
         exit(EXIT_FAILURE);
     }
     if( !fwrite(&ch,sizeof(char),100,out) || fclose(out)==EOF)
     {
         perror("File I/O failure);
         exit(EXIT_FAILURE);
     }
     return 0;
}

If this is meant to be production code you have to check return codes to assure success, especially with disk operations.

This also uses code that will compile/run on Windows.

File holes: http://www.scit.wlv.ac.uk/~jphb/cp3025/fcons.html

When "hoely" files are backed up, they don't always restore back to disk correctly, depneidng on the file system. Therefore they essentially report incorrect file sizes to the file system. If you want to look at it that way. This drives sysadmins insane when they think they are restoring files of one size and get different results.

Your reply makes senses. But from the link you provided, I can not find more information about the file hole problem and the confusing file size issue you mentioned above. Could you provide more information please?

regards,
George

ugenn · 07-18-2006, 06:44 AM

Quote:

Originally Posted by George2

Thank you ugenn,
You mean using stat to set file size? As far as I know, stat can only get the information of the file, other than set its size.

Sorry, my bad. I misread you original post.

Quote:

Do you mean on Windows, end of line is \r\n and on Linux end of line is \n?

On windows, when opened in "text" mode, '\n' is expanded into ascii 0xD and 0xA.

ugenn · 07-18-2006, 06:46 AM

George,
to find out more about "holey" files. Go research on sparce files.

AnanthaP · 07-18-2006, 07:32 AM

Maximum portability?

I mean, declare a straight character literal of hundred bytes, fill it with non-white space (presumably handles 0d, 0a and ctrl+z in doze) and write it with fopen, fclose etc - the works.

End

chrism01 · 07-18-2006, 11:19 PM

I think you meant 'sparse' files ;-)