Trouble working with file using lseek() read() and write()

Question

i'm working on C in my university, i'm at the beggining working with buffers and this function so apolagize my lack of knowledge that i might show. I gotta do this project using lseek(), write() and read(). I wanted to read a file, and every letter 'a' that i find i would change it to '?'. My code so far is:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>

int main () {
  int fd = open("problema4file", O_RDWR);
  int fptr = lseek(fd, (off_t)(-1), SEEK_END);
  char buffer;
  while(fptr!=-1){
    read(fd, &buffer, 1);
    char changeTo = '?';
    if(buffer == 'a'){
       write(fd, &changeTo,1);
    }
    fptr=lseek(fd, (off_t)(-2), SEEK_CUR);
  }
  close(fd);
}

But this changed the first 'a' (last, because i start from the end) and no more. It stops changing. Oh and it doesn't change the 'a', changes the letter after, but that has to do with the buffer movements right? i might think about that later.. I just wanted to know why this doesn't read all the file and changes everything, it stops in the first finding.

Posting the text rather than a picture of text is more useful (cut/paste, searching) — chux - Reinstate Monica, Mar 23 '16 at 20:24
http://stackoverflow.com/questions/36185820/lseek-doesnt-do-what-its-supposed — Iłya Bursov, Mar 23 '16 at 20:24
A question is required to have the code **in** the question itself! — too honest for this site, Mar 23 '16 at 20:26
@Lashane - I remembered this one too, but he described it better here — KevinDTimm, Mar 23 '16 at 20:26
@Lashane: The question is apparently deleted, so no one with rep <10k can see it. — too honest for this site, Mar 23 '16 at 20:27
It's a different example.. I deleted the other because i actually solved it right away. Sorry for that.. I now am having trouble doing this problem now — César Pereira, Mar 23 '16 at 20:32
Don't delete a question once you have the answer unless it is rubbish! There Q&A might help other users. And if you did not get the answer, don't re-post. — too honest for this site, Mar 23 '16 at 21:00
Please reopen. This question fulfills ALL of the requirements ; desired behavior & an MVC are all present. I honestly think this was ninja closed because the OP originally had a picture rather than the code which he fixed immediately. — KevinDTimm, Mar 28 '16 at 13:13

KevinDTimm · Accepted Answer · 2016-03-23T20:44:33.237

0

You read a byte and then write a byte without repositioning the offset within the file. Then you seek backwards 2 positions.

Write down what you're doing on a piece of paper to understand what's wrong.

P.S. That last line - you should do that for everything you write from now until you don't write code anymore ;)

you seek to EOF - 1, you're before the last byte in the file.  
you read a byte, you're now at EOF.  
Then you write a byte (you've extended the file by one), you're positioned again @ EOF.  
you now seek backwards 2 bytes, which puts you back to where you began.

edited Mar 23 '16 at 20:44

answered Mar 23 '16 at 20:24

KevinDTimm

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I still don't have the skill to really understand what's going on.. I following examples and trying to do it. – César Pereira Mar 23 '16 at 20:39
read the end of my answer (the `code` part) – KevinDTimm Mar 23 '16 at 20:44
Oh ok, i'm following.. So how can i read all the file? – César Pereira Mar 23 '16 at 20:55
You have to position the cursor before each read and write. Look at what I wrote and ascertain what seeks will be necessary to be positioned correctly for what you want to do. – KevinDTimm Mar 23 '16 at 21:01

Trouble working with file using lseek() read() and write()

1 Answers1