Open

• Recall:

• #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  
  int open(const char *pathname, int flags);
  int open(const char *pathname, int flags, mode_t mode);
     

• The pathname needs to be either absolute or relative.
  • tilda substitution is done by the shell.
  • Multiple errors from a bad path name, some are:
    • EACCES : bad access, file or search path permission.
    • EEXIST : creating an existing file with special flags
    • EFAULT : pathname outside of address space?
    • EISDIR : attempt to write to a directory
    • EISDIR : attempt to manipulate a directory incorrectly
    • ELOOP : too many symbolic links in the path name.
    • ELOOP : attempt to mess with a symbolic link incorrectly
    • ENAMETOOLONG: File name too long.
    • ENODEV: the block/special device does not exist.
    • ENOENT: a component of the path name does not exist.
    • ENOSPC: No room left on device.
    • There are more,
• Or in C/C++
  • What or is this : || or
  • What or is this : |
  • What the heck, what is the difference between & and &&
  • This is a really simple example program: orexamp.C
• flags
  • This is an integer
  • It is formed by oring system constants to produce the desired action
  • First, specify what you will be doing:
    • O_RDONLY - open for read
    • O_WRONLY - open for write
    • O_RDWR - open for write
    • Use only one of these.
  • Second: special flags you may need.
    • This is just an overview.
    • He classifies these into two categories
      • Creation flags
      • Open Status flags.
    • O_APPEND : open the file and append to the end.
    • O_CLOEXEC : Close the file description on exec
    • O_CREAT : If the file does not exist, create it
    • O_EXCL : used with O_CREAT, if the file exists, fail.
    • O_TRUNC: truncate normal files to size 0.
    • O_NOFOLLOW: Don't follow symbolic links in the path.
• Finally the mode, if supplied, tells us what permission to apply to the file.
  • One or more :
    • S_IRWXU, S_IRUSR, S_IWUSR, S_IXUSR
    • S_IRWXG, S_IRGRP, S_IWGRP, S_IXGRP
    • S_IRWXO, S_IROTH, S_IWOTH, S_IXOTH
  • Used with O_CREAT
• There is much more, but this will do for now.
• creat
  • In earlier versions of unix, the mode was not part of open.
  • So you had to call this first.
  • But it is now deprecated.
  • If you see it, open now covers that operation.
• read/ write/ close
  • We mostly have these.
  • Any questions?
• The file offset pointer or read write offset pointer
  • This is a byte level pointer into the file.
  • When a file is opened, this pointer points at the beginning of the file
  • This is byte 0
  • Each time you read or write, the pointer is advanced by the amount read/written.
• lseek

  • #include <sys/types.h>
    #include <unistd.h>
    
    off_t lseek(int fd, off_t offset, int whence);
    	

  • fd is a valid file descriptor
  • Offset is measured in bytes.
  • whence:
    • SEEK_SET: set the pointer to offset (Absolute move)
    • SEEK_CUR: relative to the current location
    • SEEK_END: The pointer is set to the end of the file plus offset bytes.
  • For the last two, offset can be negative (but not beyond the beginning of the file)
  • Surprisingly the offset of the seek can be beyond the end of the file.
  • But we will discuss that in the next set of notes.
  • We can not seek in pipes, sockets, FIFOs, or terminals.
  • We can not seek in some special files, but can in others
  • The return value is
    • -1 on fail
    • The position in the file on success (0 and up)
  • And there are error numbers set.
  • A silly seek example : seekTest.C.