Objectives

1. Understand the basics of the open call.

Notes

• int open(const char *pathname, int flags, ... /* mode_t mode */ );
  • Needs fcntl.h
  • This takes a path to the file we wish to open
  • It then takes at least one, but as many flags as you need to provide3
    • Normally these are bitwise ored together (|), and only one is given
    • See demo1.cpp
    • But we can give as many as we want.
    • See demo2.cpp
    • If the file is opened with the flag O_CREAT, or O_TMPFILE
      • The file will be created.
      • The file is owned by the process owner
      • The group is either the process group id, or the file group owner if the group setgid bit is set.
      • The mode argument must be supplied
      • If the mode argument is not supplied, the next values on the stack will be used. (IE THE ARGUMENT IS EXPECTED)
      • More on this when we discuss O_CREAT
    • But this is done is demo1.cpp
    • The return value is
      • A non negative integer on success
      • -1 on error, and errno is set.
  • The path argument
    • This can be absolute, relative or even ~
    • See demo3.cpp.
      • This is a primitive version of cat.
• Open flags
  • Must include exactly one of the following
    1. O_RDONLY
    2. O_WRONLY
    3. O_RDWR
  • Read and write are obvious, let's delay RW for a bit.
  • If you add O_CREAT as a flag
    • If the file does not exist, it will be created.
  • When creating a file, the last argument, as we said is the mode.
    • These are permissions
    • S_I*
      • RWXU, RWXG, RWXO
      • RUSR, WUSR, XUSR and for GRP and OTH
      • SUID, SGID, SVTX for setuid user, group and sticky
    • Again, demo1
    • But setting the setuid bit might be a problem , or even ignored
    • See demo5