reset the file location using lseek() system call - c

Question

I tried to use the system call lseek() to get back the beginning of a file or reach the end of the file.
The exact code I used is:
int location = lseek(fd, 0, SEEK_SET) //get back to the beginning
int location = lseek(fd, 0, SEEK_END) //reach to the end
However, after the file location has been reset, whenever I tried to use read(), the return value of read() is always set to -1, which means something was wrong. Furthermore, the errno message I got was Bad file descriptor. Does anyone know what should I do?
PS: I tried to close and reopen the file to help me get back to the beginning of the file and it worked. But I have no ideas on how should I get to the end of the file and read the entire file in the reverse order without using lseek().
Plus: a reproducible example would be:
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
int main(void)
{
int fd;
char buffer[1000];
fd = creat("newFile", 0777);
memset(buffer, 'a', 500);
write(fd, buffer, 500); // fill up
int location = lseek(fd, 0, SEEK_SET); //get back to the beginning
int read_bytes = read(fd, buffer, 500);
// this should return the bytes it reads but it actually returns -1
printf("%d\n", read_bytes);
return 0;
}

Answer 1

The creat function does not allow you to read from the file. It only allows you to write to it.
From creat(2):
creat()
A call to creat() is equivalent to calling open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
The important part here is O_WRONLY. That means "write only".
If you want to open the file (and create it) for reading and writing, then you can use open like so:
int fd = open("newFile", O_CREAT|O_RDWR|O_TRUNC, 0777);
The important part here is O_RDWR. That means "read and write".
If you want to have open give an error if the file already exists, add the O_EXCL flag; this causes -1 to be returned and errno to be set to EEXIST if the file already exists when you try to create it.

Answer 2

The following proposed code:
cleanly compiles
properly declares the variable types
properly creates the file
properly includes the needed header files
properly checks for error indications from C library functions (and properly handles any error)
And now, the proposed code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>
int main(void)
{
int fd;
char buffer[1000];
// fd = creat("newFile", 0777);
fd = open("newFile", O_CREAT|O_RDWR|O_TRUNC, 0777);
if( fd < 0 )
{
perror( "open failed" );
exit( EXIT_FAILURE );
}
memset(buffer, 'a', 500);
write(fd, buffer, 500); // fill up
off_t location = lseek(fd, 0, SEEK_SET); //get back to the beginning
printf( "%ld\n", location );
ssize_t read_bytes = read(fd, buffer, 500);
if( read_bytes < 0 )
{
perror( "read failed" );
exit( EXIT_FAILURE );
}
// this should return the bytes it reads but it actually returns -1
printf("%ld\n", read_bytes);
return 0;
}
A run of the program results in:
0
500
Suggest reading/understanding the MAN pages for any C library functions your code uses