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#### open

Synopsis:

      int open(const char *pathname, int flags);
      int open(const char *pathname, int flags, mode_t mode);

Request:

  ‘`Fopen,pathptr/len,flags,mode`’

  flags is the bitwise `OR` of the following values:

  `O_CREAT`

    If the file does not exist it will be created. The host rules apply as far as file ownership and time stamps are concerned.

  `O_EXCL`

    When used with `O_CREAT`, if the file already exists it is an error and open() fails.

  `O_TRUNC`

    If the file already exists and the open mode allows writing (`O_RDWR` or `O_WRONLY` is given) it will be truncated to zero length.

  `O_APPEND`

    The file is opened in append mode.

  `O_RDONLY`

    The file is opened for reading only.

  `O_WRONLY`

    The file is opened for writing only.

  `O_RDWR`

    The file is opened for reading and writing.

  Other bits are silently ignored.

  mode is the bitwise `OR` of the following values:

  `S_IRUSR`

    User has read permission.

  `S_IWUSR`

    User has write permission.

  `S_IRGRP`

    Group has read permission.

  `S_IWGRP`

    Group has write permission.

  `S_IROTH`

    Others have read permission.

  `S_IWOTH`

    Others have write permission.

  Other bits are silently ignored.

Return value:

  `open` returns the new file descriptor or -1 if an error occurred.

Errors:
  `EEXIST`

    pathname already exists and `O_CREAT` and `O_EXCL` were used.

  `EISDIR`

    pathname refers to a directory.

  `EACCES`

    The requested access is not allowed.

  `ENAMETOOLONG`

    pathname was too long.

  `ENOENT`

    A directory component in pathname does not exist.

  `ENODEV`

    pathname refers to a device, pipe, named pipe or socket.

  `EROFS`

    pathname refers to a file on a read-only filesystem and write access was requested.

  `EFAULT`

    pathname is an invalid pointer value.

  `ENOSPC`

    No space on device to create the file.

  `EMFILE`

    The process already has the maximum number of files open.

  `ENFILE`

    The limit on the total number of files open on the system has been reached.

  `EINTR`

    The call was interrupted by the user.

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