reset the file location using lseek() system call

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;
}

Solution

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.