fcntl not working

Question

I have a small program that tires to change the files access mode after it 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 get 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 to the 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 is the output when i run this program with a text file as input.

$ cat input
this is the inital data
$ ./a.out input newdata
this is the inital data
0
1025
write failed: Bad file descriptor
this is the inital data

Why is the write in the program failing? Also I'm not able to find where the file status flag constants are defined. I checked in usr/include/

Answer 1

If you read Linux manpage, you will see that fcntl cannot change file access modes (e.g., from read-only to read-write).

Answer 2

The behavior you are trying to perform is not allowed. From the fcntl(2) man page:

   F_SETFL (long)
         Set  the  file status flags to the value specified by arg.  File
         access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags
         (i.e.,  O_CREAT,  O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored.
         On Linux this command can only  change  the  O_APPEND,  O_ASYNC,
         O_DIRECT, O_NOATIME, and O_NONBLOCK flags.