File IO Question

[JackC]

Hi,

Im having some problems working out how I can read the last 5 bytes of
a file which i already have open. Heres what i have tried:

pFile = fopen ("/home/jc/data.txt" , "rw");

// Some IO here (preserve position pointer required)

// Read in last 5 bytes
fseek(pFile, 5, SEEK_SET);
fgets(buffer, 5, pFile);

// More IO here (resume old position pointer)

fclose(pFile);

This doesn't work because SEEK_SET adds 5 onto the end of the file, i
need a way to set the position to the current end - 5 bytes, whilst
preserving the current position once im done.

Any ideas on how to approach this? I thought about getting the file
size and using fseek with SEEK_SET from the beginning of the file, but
i think this would involve closing/reopening the file which i would
like to avoid.

Thanks alot for any help,

Jack

 

[Rolf Magnus]

Hi,

Im having some problems working out how I can read the last 5 bytes of
a file which i already have open. Heres what i have tried:

pFile = fopen ("/home/jc/data.txt" , "rw");

// Some IO here (preserve position pointer required)

// Read in last 5 bytes
fseek(pFile, 5, SEEK_SET);
fgets(buffer, 5, pFile);

// More IO here (resume old position pointer)

fclose(pFile);

This doesn't work because SEEK_SET adds 5 onto the end of the file,

No. It adds 5 to the beginning of the file. What you need to use is
SEEK_END.

i need a way to set the position to the current end - 5 bytes, whilst
preserving the current position once im done.

Use an offset of -5.

Any ideas on how to approach this? I thought about getting the file
size and using fseek with SEEK_SET from the beginning of the file, but
i think this would involve closing/reopening the file which i would
like to avoid.

Why would you need to close the file for that?

 

[JackC]

No. It adds 5 to the beginning of the file. What you need to use is
SEEK_END.


Use an offset of -5.


Why would you need to close the file for that?

Thanks, sorry that was a typo in my code i did mean SEEK_END, but i
didn't think i could use a negative offset.

If i try this:
fseek(pFile, -5, SEEK_END);
fgets(buffer, 5, pFile);

It doesn't read the last 5 characters, but instead I just get some
random chars back.

Any ideas?

 

[Jim Langston]

Thanks, sorry that was a typo in my code i did mean SEEK_END, but i
didn't think i could use a negative offset.

If i try this:
fseek(pFile, -5, SEEK_END);
fgets(buffer, 5, pFile);

It doesn't read the last 5 characters, but instead I just get some
random chars back.

Any ideas?

I tried this program to see if I could find any problems. It gives me the
first 4 characters of the file, and the last 4 characters of the file.
Rudimentary error checking since that wasn't my main concern. It seems to
work as expected for me. Remember that the fgets wants to save 1 spot for
the null terminator, so telling it to read 5 will read 4 and add a null
temrinator.

#include <cstdio>
#include <vector>
#include <iostream>

int main()
{
FILE* Input = std::fopen("C:/test.txt", "r");
if ( Input == NULL )
{
std::cout << "Error occured opening file." << "\n";
return 0;
}

std::vector<char> Data( 10 );

if ( fgets( &Data[0], 5, Input ) == NULL )
std::cout << "Error occured reading";
else
std::cout << "Data:" << &Data[0] << "\n";

std::fseek( Input, -5, SEEK_END );

if ( fgets( &Data[0], 5, Input ) == NULL )
std::cout << "Error occured reading";
else
std::cout << "Data:" << &Data[0] << "\n";

std::fclose( Input );

}