How to use mkdir() from sys/stat.h


You want to use the mkdir() function from the sys/stat.h POSIX header, but you don’t know what the mode_t argument should look like.


For a detailed reference, see the Opengroup page on mkdir

The first argument should be obvious – just enter the path name of the directory you intend to create. If you use a std::string (in C++); use it’s c_str() member function to get a C string.

The second argument defines the permissions the newly created directory shall have. This How-to assumes you’re already familiar with Unix file permissions. If you are not, please read the corresponding Wikipedia Page.

First, decide which rights the directory shall have. This boils down to these 9 questions:

  • Shall the owner be able to read/write/execute?
  • Shall the group be able to read/write/execute?
  • Shall everyone else (= others) be able to read/write/execute? The second argument has the type mode_t, but this is basically just an alias for any type of integer.

sys/stat.h provides you with several integers you can bytewise-OR (|) together to create your mode_t:

  • User: S_IRUSR (read), S_IWUSR (write), S_IXUSR (execute)
  • Group: S_IRGRP (read), S_IWGRP (write), S_IXGRP (execute)
  • Others: S_IROTH (read), S_IWOTH (write), S_IXOTH (execute)

Additionally, some shortcuts are provided (basically a bitwise-OR combination of the above

  • Read + Write + Execute: S_IRWXU (User), S_IRWXG (Group), S_IRWXO (Others)
  • DEFFILEMODE: Equivalent of 0666 = rw-rw-rw-
  • ACCESSPERMS: Equivalent of 0777 = rwxrwxrwx Therefore, to give only the user rwx (read+write+execute) rights whereas group members and others may not do anything, you can use any of the following mkdir() calls equivalently:
mkdir("mydir", S_IRUSR | S_IWUSR | S_IXUSR);
mkdir("mydir", S_IRWXU);

In order to grant all rights to everyone (mode 0777 = rwxrwxrwx), you can use any of the following calls equivalently:

mkdir("mydir", S_IRWXU | S_IRWXG | S_IRWXO);
mkdir("mydir", ACCESSPERMS);