Open

• You need
  • sys/types.h
  • And also sys/stat.h
  • And probably fcntl.h
• Open: Open a file.
  • man 2 open
  • This is more complex than it sounds.
  • int open(const char *pathname, int flags);
  • int open(const char *pathname, int flags, mode_t mode);
    • Path is the file name, absolute or relative.
    • Flags: A description of the requested file.
    • Perms: The permissions used to open the file.
  • Return:
    • On error: -1
    • Otherwise: The lowest available file descriptor, starting at 0.
      • This turns out to be important.
      • Look at cool1.C
• Mandatory Flag:
  • You must use one of the following flags
  • On traditional *nix:
    • O_RDONLY : read only
    • O_WRONLY : write only
    • O_RDRW : both reading and writing
    • In the past O_RDONLY was 0,
      • so O_RDONLY | O_WRONLY = O_WRONLY
      • So O_RDRW ≠ O_RDONLY | OWRONLY in older systems.
      • This has been deprecated, but ...
• Optional Flags (man 2 open)
  • Far too many to discuss here
  • Some will can not be used with others.
  • Some common flags
    • O_CREAT : if the file does not exist, create it.
      • This uses the mode argument given later.
    • O_TRUNC: if the file exists, truncate it to size 0
    • O_APPEND: perform all output at the end of the file.
    • O_EXCL: used with O_CREAT, fail if the file exists.
    • O_CLOEXEC: close on exec
• Modes
  • S_IPUUU
  • P = {R,W,X}
  • UUU = {USR, GRP, OTH}
  • S_IRGRP, S_IWUSR, ...
  • Also I_SRWXW
  • W = {U,G,O}
  • S_IRWXU, S_IRWXG, S_IRWXO
  • Only applys with O_CREAT, O_TMPFILE
  • Gives future access permission.
  • Other elements influence this as well.
• Errors
  • There are MANY
  • Some depend on the type of file you are trying to open
  • Others are related to system limits.
  • Or permissions
  • Look em over.