c - fcntl not working -

i have small program tires change files access mode after has been opened.

 int main(int argc, char* argv[]) {     int fd;     char *filename = argv[1];     char data[1];     int curval;                         //current flag value     int newval;                         //new flag value
  
fd = open(filename, o_rdonly);  while(read(fd, data, 1)>0) {     write(stdout_fileno, data, 1); }  lseek(fd, 0, seek_set);  if((curval = fcntl(fd, f_getfl, 0))<0)               {     perror("file flag failed"); } 
  
printf("%d\n", curval);     newval = curval | o_wronly | o_append; printf("%d\n", newval);
  
    if(fcntl(fd, f_setfl, newval)<0) {     perror("file flag set failed"); }  if(write(fd, argv[2], strlen(argv[2]))<0)   //appending more data file {     perror("write failed");     }  lseek(fd, 0, seek_set);  while(read(fd, data, 1)>0) {     write(stdout_fileno, data, 1); } close (fd); return 0; 
  
} 

here output when run program text file input.

$ cat input inital data $ ./a.out input newdata inital data 0 1025 write failed: bad file descriptor inital data 

why write in program failing? i'm not able find file status flag constants defined. checked in usr/include/

the behavior trying perform not allowed. fcntl(2) man page:

   f_setfl (long)          set   file status flags value specified arg.  file          access mode (o_rdonly, o_wronly, o_rdwr) , file creation flags          (i.e.,  o_creat,  o_excl, o_noctty, o_trunc) in arg ignored.          on linux command can  change   o_append,  o_async,          o_direct, o_noatime, , o_nonblock flags.