Question

This a code that would reverse the data of a document and save it in the same document itself.
However I am getting a Segmentation Fault.Please Help,I don’t know why it gives a SegFault.

#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>


int main (int argc,char* argv[])
{

    int fd,n,i,j;

    char* buf;

    if(argc<2)
    printf("USAGE: %s file-to-reverse.\n",argv[0]);

    fd=open(argv[1], O_RDWR);
    if(fd==-1)
    printf("ERROR: Cannot reverse %s,file does not exist.\n",argv[1]);

    i = 0;
    j = n-1;

    while(i < j)
    {   
        read(fd,buf,n);

        char ib = buf[i];
        char jb = buf[j];

        jb = i++;
        ib = j--;

        write(fd,buf,n);
    }

    free(buf);
    close(fd);
}

EDIT1
I tried adding :

#include <sys/stat.h>
struct stat fs;


  fstat(fd, &fs);
  n= fs.st_size;
  buf = malloc(n * sizeof (char));

but now it just duplicates the characters inside the document again and again instead of
reversing them.

Answer 1

Regarding your second EDIT – your loop is wrong.

(1) Take the read & write out of the loop – that’s why it keeps writing again & again.

(2) You need to seek back to the beginning of the file, otherwise you will just be appending the new data to the end of the file.

(3) You actually have to reverse the chars in the buffer before writing them out.

read(fd, buf, n);

while (i < j)
{

    char t = buf[i];

    buf[i] = buf[j];

    buf[j] = t;

    i++;
    j--;
}

lseek(fd, 0, SEEK_SET);

write(fd, buf, n);