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     The open function opens the file specified by <filename> and
     prepares the file for subsequent reading or writing, as defined
     by <oflag>. The <oflag> argument is an integer expression formed
     by combining one or more of the following manifest constants
     (defined in FCNTL.H):
 
     O_APPEN      O_EXCL       O_TEXT
     O_BINARY     O_RDONLY     O_TRUNC
     O_CREAT      O_RDWR       O_WRONLY
 
     When more than one manifest constant is given, the constants are
     combined with the bitwise-OR operator (|).
 
     See ◄BINMODE.OBJ► for a discussion of binary (O_BINARY) and text
     (O_TEXT) modes.
 
     Use the O_TRUNC flag with care, as it destroys the complete
     contents of an existing file.
 
     Either O_RDONLY, O_RDWR, or O_WRONLY must be given to specify the
     access mode. There is no default value for the access mode.
 
     The <pmode> argument is required only when O_CREAT is specified.
     If the file exists, <pmode> is ignored. Otherwise, <pmode>
     specifies the file's permission settings, which are set when the
     new file is closed for the first time. The <pmode> argument is an
     integer expression containing one or both of the manifest
     constants S_IWRITE and S_IREAD (defined in SYS\STAT.H). When
     both constants are given, they are joined with the bitwise-OR
     operator (|).
 
     If write permission is not given, the file is read-only. Under
     DOS and OS/2, all files are readable; it is not possible to
     give write-only permission. Thus, the modes S_IWRITE and
     S_IREAD | S_IWRITE are equivalent.
 
     The open function applies the current file-permission mask to
     <pmode> before setting the permissions (see umask).
 
     The <filename> used in the open function is affected by the DOS
     APPEND command.
 
     Under DOS versions 3.0 and later with SHARE installed, a
     problem occurs when opening a new file with <oflag> set to
     O_CREAT | O_RDONLY or O_CREAT | O_WRONLY and <pmode> set to
     S_IREAD. In this case, the operating system prematurely closes
     the file during system calls made within open.
 
     To get around the problem, open the file with <pmode> set to
     S_IWRITE. After closing the file, call chmod and change the mode
     back to S_IREAD. Another possibility is to open the file with
     <pmode> set to S_IREAD and <oflag> set to O_CREAT | O_RDWR.
 
     Return Value
 
     The open function returns a file handle for the opened file. A
     return value of -1 indicates an error, and errno is set to EACCES,
     EEXIST, EINVAL, EMFILE, or ENOENT.
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