Fseek Backwards

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March 7th, 2006, 06:12 PM

ecit12

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Join Date: Jun 2003

Posts: 418

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Fseek Backwards

Basically, I am trying to go back x amount of spaces in the file and start writing from there.

Here is the code I wrote.

C Code:

Original - C Code

#include <stdio.h>
char initXMLDoc(int systemNumber, char *fileName);
int main()
{
    FILE *fp;
    printf("\nInit XML doc");
    initXMLDoc(1234,  "files/test.xml");
    if((fp = fopen("files/test.xml", "a+b"))==NULL){
        printf("\nFailed to open file");
        return -1;
    }
    fseek(fp, 9, SEEK_END);
    fwrite("<a>HELLO</a>", 1, 12,fp);
    fclose(fp);
    return (0);
}
 
char initXMLDoc(int systemNumber, char *fileName){
    FILE *fp;
    if((fp=fopen(fileName, "w"))==NULL){
        printf("Failed to create initial XML document");
        return 0;
    }
    // write encoding msg
    //fprintf(fp, "<?xml version=\"1.0\" encoding=\"ASCII\"?>");
    fprintf(fp, "<system number=\"%d\"></system>",systemNumber );
    fclose(fp);
    return 1;
}

any ideas as to what I'm doing wrong?

my output looks like this

XML Code:

Original - XML Code
<system number="1234"></system><a>HELLO</a>

<system number="1234"></system><a>HELLO</a>

but I want it to look like this

XML Code:

Original - XML Code
<system number="1234"><a>HELLO</a></system>

<system number="1234"><a>HELLO</a></system>

Last edited by ecit12 : March 7th, 2006 at 06:31 PM.

March 7th, 2006, 06:39 PM

ecit12

Contributing User

Join Date: Jun 2003

Posts: 418

Time spent in forums: 2 Days 13 h 53 m 25 sec
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hmm... a fix seems to be to open the file in "rb+" mode..
the end component gets overwriteen, but thats ok.

March 7th, 2006, 09:41 PM

Nathan87

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Location: Missouri

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if you open a file in append mode, you can not seek...found that out the hard way!

And to seek backwards you supply a negative offset.
fseek(fp, -9, SEEK_END)

March 8th, 2006, 01:31 PM

ecit12

Contributing User

Join Date: Jun 2003

Posts: 418

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yup... I kind of figured it out the hard way.. lol..

I'm just disappointed that my book
"C the complete reference manual" by Herbert Schildt failed to mention that.

March 8th, 2006, 03:56 PM

Dave Sinkula

Contributing User

Dev Shed Intermediate (1500 - 1999 posts)

Join Date: Apr 2004

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Quote:

Originally Posted by ecit12

I'm just disappointed that my book
"C the complete reference manual" by Herbert Schildt failed to mention that.

Sorry to chuckle, but get used to it.

http://www.catb.org/jargon/html/B/bullschildt.html

[BTW, it was my first C book. I've since done my penance however.]

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March 8th, 2006, 06:15 PM

ecit12

Contributing User

Join Date: Jun 2003

Posts: 418

Time spent in forums: 2 Days 13 h 53 m 25 sec
Reputation Power: 11

Actually, my bad, the book I am using is the C++ the complete reference which includes a big section on C (basically stripped out from the original C book he wrote)
well, I've learned most of my C through this book and the devShed community. Its a decent book to start with when you just want to do the basics, but beyond that its far from the COMPLETE REFERENCE

I had no idea that ppl hated it that much to make a word for it..Interesting site none the less.. lol

Last edited by ecit12 : March 8th, 2006 at 06:18 PM.

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