Copy the last 'n' lines of a file to another file in reverse order

I am working on system calls in Ubuntu and implementing it using C language. I wrote this code to do the aforesaid but in the destination file say, "b.txt", I am only getting 'n' number of blank lines instead of the desired texts in those lines.
I can't figure out if there is some logical error here or do I need to change the permission conditions/values (mode) for open() corresponding to fd2?

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main()
{
int n, fd1, fd2, choice=0;
char sPathName[100], dPathName[100], c;
printf("Enter name of the source file: \n");
scanf("%s", sPathName);
while(access(sPathName, F_OK) != 0)	//Given file doesn't exist (in the this directory)
{
printf("The file \"%s\" does not exist in the given directory!", sPathName);
printf("\n\nEnter name of a valid source file: \n");
scanf("%s", sPathName);
}
fd1 = open(sPathName, O_RDONLY);
printf("\nEnter the integer value of number of lines, 'n': \n");
scanf("%d", &n);
printf("\nEnter name of the destination file: \n");
scanf("%s", dPathName);
if(access(dPathName, F_OK) != 0)	//Given file doesn't exist (in this directory)
{
fd2 = open(dPathName, O_CREAT|O_WRONLY, 0666);
}
else
{
printf("\nA file by the name \"%s\" already exists in this directory! \nDo you want to: \n1) Overwrite it \n2) Append to it? \nChoice: ", dPathName);
scanf("%d", &choice);
if(choice == 1)
{
fd2 = open(dPathName, O_WRONLY);
}
else if(choice == 2)
{
fd2 = open(dPathName, O_WRONLY|O_APPEND);
}
else
{
printf("\nInvalid choice! Exiting program...\n");
return 0;
}
}
while(n != 0)
{
while(1)
{
lseek(fd1, -1, SEEK_END);
read(fd1, &c, 1);
write(fd2, &c, 1);
if(c == '\n')
{
break;
}
}
n--;
}
}

The problem is the following loop:

while(n != 0)
{
while(1)
{
lseek(fd1, -1, SEEK_END);
read(fd1, &c, 1);
write(fd2, &c, 1);
if(c == '\n')
{
break;
}
}
n--;
}

Before reading every character, you seek to the last character of the file. This means that you are always only reading and writing the last character of the file. If this character is a newline character, then you will write lots of blank lines.

In order for your approach to work, you should first read and write the last character, then the second-to-last character, then the third-to-last character, etc., until you have found the required number of newline characters.

In order to find the newline character of the last line in the file that should not be written, you should first seek to the last character of the file using

lseek(fd1, -1, SEEK_END);

which you are already doing, but after reading that character, you should not use that function call again, but should instead use

lseek(fd1, -2, SEEK_CUR);

to jump back one character and to also jump over the character that you just read (so 2 characters in total). Afterwards, you should always use

lseek(fd1, -2, SEEK_CUR);

instead of

lseek(fd1, -1, SEEK_END);

after reading every character, until you find the newline character of the last line in the file that should not be written.

Also, it is generally advisable to check the return values of the system calls. Otherwise, your program will likely misbehave if, for example, the entire text file consists of less than n lines.

Therefore, I recommend that you change your code to the following:

//attempt to seek to the last character of the file
if ( lseek(fd1, -1, SEEK_END) == -1 )
{
perror( "lseek" );
exit( EXIT_FAILURE );
}
while ( 1 )
{
ssize_t ret;
//attempt to read a character
ret = read(fd1, &c, 1);
if ( ret != 1 )
{
if ( ret == -1 )
{
perror( "read" );
}
else
{
fprintf( stderr, "Encountered unexpected return value from \"read\"!\n" );
}
exit( EXIT_FAILURE );
}
//attempt to write a character
ret = write( fd2, &c, 1 );
if ( ret != 1 )
{
if ( ret == -1 )
{
perror( "write" );
}
else
{
fprintf( stderr, "Encountered unexpected return value from \"write\"!\n" );
}
exit( EXIT_FAILURE );
}
//count the number of newline characters and break out
//of infinite loop if we have reached the limit
if(c == '\n')
{
if ( n-- == 0 )
{
break;
}
}
//jump back two characters in the file
if ( lseek(fd1, -2, SEEK_CUR) == -1 )
{
perror( "lseek" );
exit( EXIT_FAILURE );
}
}

Note that this code additionally requires #include <stdlib.h> at the top of the file.

For the input

This is line1.
This is line2.
This is line3.
This is line4.
This is line5.
This is line6.
This is line7.
This is line8.

and with n equal 5, this code has the following output:

.8enil si sihT
.7enil si sihT
.6enil si sihT
.5enil si sihT
.4enil si sihT

Note that all characters are written in reverse order, including the newline characters. As a consequence, the first line is empty, because the first character written is a newline character.

It is also worth noting that performing three system calls for every character is highly inefficient. A more efficient approach would read several kilobytes at once.