#include <zephyr.h>
#include <fs.h>
#include <stdio.h>

void main()
{
  fs_file_t filep;
  char read_buff[80];
  char test_str[] = "hello_world";
  ssize_t brw;

  //open file
  int a = fs_open(&filep, "hello.txt");
  if (a < 0){
    printf("Error opening file\n");
  }
  printf("Done opening file\n");

  //seek
  int res = fs_seek(&filep, 0, FS_SEEK_SET);
  if (res) {
        printf("Error seeking file %d\n",res);
  }

  //write
  brw = fs_write(&filep, (char *)test_str, strlen(test_str));
  if (brw<0){
    printf("Error writing file %d\n",brw);
  }

  fs_close(&filep);
  printf("Done!");
}


Output:
Done opening file