Getting file size with lseek command in c - c

Question

I was asked to find the size of the file usjng lseek command (without using stat) and i wrote the following code
int main()
{
char buf[100], fn[10];
int fd, i;
printf("Enter file name\n");
scanf("%s", fn);
fd = open(fn, O_RDONLY);
int size = lseek(fd, 0, SEEK_END);
printf("Size is %d", size);
close(fd);
}
But i am getting -1 as file size, where am i going wrong

Answer 1

From lseek documentation available online:
RETURN VALUE        
Upon successful completion, lseek() returns the resulting offset
location as measured in bytes from the beginning of the file. On
error, the value (off_t) -1 is returned and errno is set to indicate
the error.
So you have to check the errno (print it if lseek returns -1):
The list of possible errors from the same link:
ERRORS
EBADF fd is not an open file descriptor.
EINVAL whence is not valid. Or: the resulting file offset would be
negative, or beyond the end of a seekable device.
ENXIO whence is SEEK_DATA or SEEK_HOLE, and the file offset is
beyond the end of the file.
EOVERFLOW The resulting file offset cannot be represented in an off_t.
ESPIPE fd is associated with a pipe, socket, or FIFO.
In your case, it is most likely EBADF.

Answer 2

the following proposed code:
cleanly compiles
properly checks for errors
performs the desired functionality
uses proper variable typing
and now the proposed code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
int main( void )
{
char fn[10];
int fd;
printf("Enter file name\n");
if( scanf("%9s", fn) != 1 )
{
fprintf( stderr, "scanf for file name failed\n" );
exit( EXIT_FAILURE );
}
if( (fd = open(fn, O_RDONLY) ) < 0 )
{
perror( "open failed" );
exit( EXIT_FAILURE );
}
off_t size = lseek(fd, 0, SEEK_END);
printf("Size is %ld", size);
close(fd);
}