Random Access Files, Why isn't this returning the position of the last character?

[voidflux]

Hello everyone, i'm trying to print a file in reverse, to do this, I need to use random acccess file functions. My idea was to go to the last character and just print until I get to the first character of the file. To find the last character, which is really the start of the file, i thought i would tell the file market to go to the last character in the file, ios::end, and then travel to the beginning of the character, so the file marker should stop at the first character, put it all together and u get seekg(0,ios::end); here's my code

void reverseFile(ifstream& theCopy)
{
	char ch;
	int last;
	theCopy.seekg(0,ios::end);		//returns the size of the file
	last = theCopy.tellg();
	cout <<"Last: " << last << endl;

	for(int offset = 1; offset <= last; offset++)
	{
		theCopy.seekg(-offset,ios::end);
		ch = theCopy.get();
		cout <<ch;
	}
}

When i ouptut last, it says -1, which isn't right!
any idea's where I went wrong?
Thanks

[visualAd]

-1 - Probably means EOF (End of File)

[jim mcnamara]

Try some simple C first -

/* rev.c */
#include <stdio.h>
#include <sys/stat.h>
#include <stdlib.h>
off_t filesize(char *);
int main(int argc,char *argv[]){
        char *buf;
        off_t i=0;        
        int j=0;
        FILE *in;
        i=filesize(argv[1]);
        buf=malloc(i+1);
        memset(buf,0x00,(size_t)i+1);
        in=fopen(argv[1],"rb");
        if(fread(buf,i,1,in))
             fclose(in);
        else{
             perror("Error reading file");
             exit(EXIT_FAILURE);
        }               
        while(i>(-1)){
            if(*(buf+i) ) printf("%c",*(buf+i));
            i--;
        }
        printf("\n");
	return 0;
}

off_t filesize(char *fname){
     struct stat st;
     if(stat(fname,&st)){
          printf("Cannot stat file: %s\n",fname);
          exit(1);
     }
     return st.st_size;     
}

[voidflux]

Thanks for the replies, if -1 signal's EOF, its still not correct, because the way the seekg works, it should have returned the current file marker, which should have been the start of the file, have to use random access files, so i can't just use c to do it unfortunatley.

[visualAd]

EOF doesn't always mean EOF - it can mean there are was error opening the files maybe due to permissions or something.

I havn't used C++ File IO functions before but C can handle random access no problem using fread and fwrite.

[CornedBee]

From tellg:

If fail is false, the member function returns rdbuf -> pubseekoff(0, cur, in). Otherwise, it returns pos_type(-1).

Might this be your problem?

[voidflux]

THis was a very weird problem, the reason it wasn't working correct was I had to create 2 ifstreams, one ifstream to copy the file into an output file, such as:

                ifstream inFile;
	ifstream reverse;
	ofstream outFile;

	inFile.open("fileOne.txt",ios::in);
	outFile.open("fileCopy.txt",ios::out);
	copy(outFile,inFile);
	inFile.close();
	reverseFile(reverse);

If i tried the following code:

    ifstream inFile;
	//ifstream reverse;
	ofstream outFile;

	inFile.open("fileOne.txt",ios::in);
	outFile.open("fileCopy.txt",ios::out);
	copy(outFile,inFile);
	inFile.close();
	reverseFile(inFile);

It wouldn't open the file from the inFile stream, I created a test, such as

if(inFile.fail())
{
cout<<"Cannot open file, inside reverseFile function\n";
}

Any idea's why I had to make another ifstream and why I couldn't just close the orginal ifstream after I read the file and copied its contents into the ostream, then re-opened the file inside the reverseFile function?

[CornedBee]

Maybe the open/close pair never gets the ifstream out of the EOF state it undoubtly is in after copy? Try calling reset after closing the first file.

[voidflux]

When you said, try calling reset, do you mean set the file marker back to the start of the file? if thats what you ment, It still wont' read the file, I did this to set the filemarker to the start of the file:

void reverseFile(ifstream& theCopy)
{
	theCopy.open("fileCopy.txt",ios::in);
	theCopy.seekg(0,ios::beg);
	if(theCopy.fail())
	{
		cout <<"Cannot open file inside reverse File\n";
	}

........
........
........

[CornedBee]

No, I mean the reset method of ifstream.