First byte is not printed when using lseek

Question

I am trying to output the content of a file using system calls. The code is working nice until we have a file that contains only one line. in that case, the output is the line without the first char. Where does this char get lost?

ssize_t rres,wres;
char buff[1];
off_t offset;
int fd = open("tester.txt",O_CREAT | O_RDWR , 0664 ); 
int line_count = 5;
for(count = 0;line_count != 0;count--){
    offset = lseek(fd, count, SEEK_END);
    rres = read(fd,buff,1);
    if(rres < 0){
        return -1;
    }
    if(*buff =='\n') line_count--;

}

count *= -1; // get abs value of bytes
count++;
while(count--){
    wres = write(STDOUT_FILENO,buff,read(fd, buff,1)); // output
}

I am trying to output the last 5 lines of a file. But if I have less than 5 lines, the first char is not outputed.

The for loop is so that I can break after i reach the 5th line from down to up — Karina Kozarova, Nov 16 '17 at 18:59
If the file has less than 5 lines, the loop will never end because you can't read 5 newlines. — Barmar, Nov 16 '17 at 18:59
You need to stop the loop if you get to the beginning of the file. — Barmar, Nov 16 '17 at 19:01
Ok, step back, forget your program for the moment and tell us what you're trying to achieve. This looks like an [XY Problem](http://xyproblem.info/) — Jabberwocky, Nov 16 '17 at 19:01
How do you test less than 5 lines? That is - when you only have one line in the file? — Grantly, Nov 16 '17 at 19:06
Possible duplicate of [read file backwards (last line first)](https://stackoverflow.com/questions/6922829/read-file-backwards-last-line-first) [Reading a text file backwards in C](https://stackoverflow.com/questions/14834267/reading-a-text-file-backwards-in-c) [ading a text file backwards and put in C](https://stackoverflow.com/questions/34180537/reading-a-text-file-backwards-and-put-in-c) Many others — Alexandre Alencar, Nov 16 '17 at 19:21

score 2 · Accepted Answer · answered Nov 16 '17 at 19:09

The for loop is reading a character from the file before it ends. If there are at least 5 lines, it reads a newline the last time, and then the rest of the code starts reading from after that newline.

But if it gets to the beginning of the file without finding 5 newlines, the last read() reads the first character of the file, and the rest of the code starts reading from after that.

When *buff != '\n' you need to print that character first.

if (*buff != '\n') {
    write(STDOUT_FILENO, buff, 1);
}
count *= -1; // get abs value of bytes
count++;
while(count--){
    wres = write(STDOUT_FILENO,buff,read(fd, buff,1)); // output
}

First byte is not printed when using lseek

1 Answers1