Opens a file for reading, writing, or editing. It positions the
    file at its beginning (byte 0).

    Format

      #include  <fcntl.h>

      int open  (const char *file_spec, int flags, mode_t mode);
                (ANSI C)

      int open  (const char *file_spec, int flags, . . . );
                (DEC C Extension)

1  –  Arguments

 file_spec

    A null-terminated character string containing a valid file
    specification. If you specify a directory in the file_spec and
    it is a search list that contains an error, VSI C interprets
    it as a file open error.

    If the file_spec parameter refers to a symbolic link, the open
    function opens the file pointed to by the symbolic link.

 flags

    The following values are defined in the <fcntl.h> header file:

    O_RDONLY   Open for reading only
    O_WRONLY   Open for writing only
    O_RDWR     Open for reading and writing
    O_NDELAY   Open for asynchronous input
    O_APPEND   Append on each write
    O_CREAT    Create a file if it does not exist
    O_TRUNC    Create a new version of this file
    O_EXCL     Error if attempting to create existing file

    These flags are set using the bitwise OR operator (|)  to
    separate specified flags.

    Opening a file with O_APPEND causes each write on the file to
    be appended to the end. (In contrast, with the VAX C RTL the
    behavior of files opened in append mode was to start at EOF and,
    thereafter, write at the current file position.)

    If O_TRUNC is specified and the file exists, open creates a new
    file by incrementing the version number by 1, leaving the old
    version in existence.

    If O_CREAT is set and the named file does not exist, the C RTL
    creates it with any attributes specified in the fourth and
    subsequent arguments ( . . . ). If O_EXCL is set with O_CREAT and
    the named file exists, the attempted open returns an error.

 mode

    An unsigned value that specifies the file-protection mode. The
    compiler performs a bitwise AND operation on the mode and the
    complement of the current protection mode.

    You can construct modes by using the bitwise OR operator (|)  to
    separate specified modes. The modes are:

    0400   OWNER:READ
    0200   OWNER:WRITE
    0100   OWNER:EXECUTE
    0040   GROUP:READ
    0020   GROUP:WRITE
    0010   GROUP:EXECUTE
    0004   WORLD:READ
    0002   WORLD:WRITE
    0001   WORLD:EXECUTE

    The system is given the same access privileges as the owner. A
    WRITE privilege also implies a DELETE privilege.

  . . .

    Optional file attribute arguments. The file attribute arguments
    are the same as those used in the creat function. For more
    information, see the creat function.

2  –  Description

    If a version of the file exists, a new file created with open
    inherits certain attributes from the existing file unless
    those attributes are specified in the open call. The following
    attributes are inherited: record format, maximum record size,
    carriage control, and file protection.

                                  NOTES

       o  If you intend to do random writing to a file, the file
          must be opened for update by specifying a flags value of
          O_RDWR.

       o  To create files with OpenVMS RMS default protections
          by using the UNIX system-call functions umask, mkdir,
          creat, and open, call mkdir, creat, and open with a
          file-protection mode argument of 0777 in a program that
          never specifically calls umask. These default protections
          include correctly establishing protections based on ACLs,
          previous versions of files, and so on.

          In programs that do vfork/exec calls, the new process
          image inherits whether umask has ever been called or not
          from the calling process image. The umask setting and
          whether the umask function has ever been called are both
          inherited attributes.

    See also creat, read, write, close, dup, dup2, and lseek.

3  –  Return Values

    x                  A nonnegative file descriptor number.
    -1                 Indicates that the file does not exist, that
                       it is protected against reading or writing, or
                       that it cannot be opened for another reason.

4  –  Example

        #include <unixio.h>
        #include <fcntl.h>
        #include <stdlib.h>

        main()
        {
            int file,
                stat;
            int flags;

            flags = O_
 RDWR;  /*  Open for read and write,            */
                             /*  with user default file protection,  */
                             /*  with max fixed record size of 2048, */
                             /*  and a block size of 2048 bytes.     */

        file=open("file.dat", flags, 0, "rfm=fix", "mrs=2048", "bls=2048");
            if (file == -1)
                perror("OPEN error"), exit(1);

            close(file);
        }