Setting permissions to 777, resulting file is 775

Question

I'm trying to write a string to file, and set the file's permissions to 777. The resulting file is blank and has permissions of 775. The mode mask seems to be correct, and when I check with access(), it returns 0. I can chmod the file to 777 (without using sudo). What's going on?

Running ubuntu 14.10. Hard drive is ext3/4.

if (argc == 1) {
    int fd = open ("sampleFile", O_CREAT , S_IRWXU | S_IRWXG | S_IRWXO);
    int writ = 0xDEADBEEF;

    if (fd != -1) {
        char str [] = "My permission should be set to 777.";
        writ = write (fd, &str, (int)strlen (str));
        //writ = access ("sampleFile", W_OK);
        close (fd);

        printf ("(%o) %s\n", writ, str);
        return 0;
    }

    else {
        printf ("Couldn't make sampleFile in pwd.\n");
        return 1;
    }
}

Answer 1

You have two problems:

1. The second argument to write() "must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR". Change O_CREAT to O_CREAT | O_WRONLY to allow writing to the file.

2. The mode of the created file is modified by your umask. Your umask is probably 002. (A umask of 002 prevents you from accidentally creating world-writable files.) You can use fchmod to change the permissions after creating it.:
   fchmod(fd, S_IRWXU | S_IRWXG | S_IRWXO)