| open | file statement |
| Syntax | An openStatement is one of:
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| Description | The open statement connects the program to a file so the program can perform operations such as read on the file. In form (a), the open statement translates a fileName, such as "Master", to a file number such as 5. Form (b), which is less-commonly used, opens a file whose name is given by a program argument. This is described below. The read statement uses the file number, not the file name, to access the file. When the program is finished using the file, it disconnects from the file using the close statement. Each ioCapability is the name of an operation, such as read, that is to be performed on the file.
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| Example | This programs illustrates how to open, read and then close a file.
var fileName : string := "Master" % Name of file
var fileNo : int % Number of file
var inputVariable : string ( 100 )
open : fileNo, fileName, read
…
read : fileNo, inputVariable
…
close : fileNo
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| Details | The open statement always sets the fileNumber to a positive number. If the open fails (generally because the file does not exist), the fileNumber is set to a non-positive number. It is wise to check that the stream number is greater than zero before using it further. An ioCapability is one of:
A file can be accessed using only the statements corresponding to the input/output capabilities with which it was opened. Note: tell is allowed only if the open is for seek. The open statement truncates the file to length zero if the ioCapabilities includes put or write but not mod (which stands for modify). In all other cases, open leaves the existing file intact. The mod ioCapability specifies that the file is to be modified without being truncated. Each open positions to the beginning of a file. There is no mechanism to delete a file. To open for appending to the end of the file, one has to open for seek, mod and for write or put and then seek to the end of the file. See the seek statement. Mixed mode files, which combine get and read (or put and write), are supported by some operating systems, such as UNIX, but not by others, such as Microsoft Windows. On Microsoft Windows, one should note that opening files in other directories uses the backslash character. This is because the backslash is a special character in Turing (as in \t for tab and \n for a newline). To get a single backslash, use \\.
e.g. open : f, "C:\\STUDENTS\\SMITH\\ACCT.DAT", putForm (b) of the syntax allows you to open a file whose name is given as a program argument on the command line. For example, under UNIX, the command line:
prog.x infile outfilespecifies to execute prog.x with program arguments infile and outfile. Similarly, in the Turing programming environment, the run command can accept program arguments. The argNumber is the position of the argument on the command line. (The first argument is number 1.) The name of the file to be opened is the corresponding program argument. If there is no such argument, or if the file cannot be opened successfully, fileNumberVariable is set to zero. See also nargs, which gives the number of arguments, and fetcharg, which gives the n-th argument string. Program argument files referenced by argument number and used in put, get, read or write statements need not be explicitly opened, but are implicitly opened with the capability corresponding to the input/output statement in which they are first used. (The fileNumbergives the number of the argument.) The operating system standard files (error, output and input) are accessed using file numbers 0, -1, and -2, respectively (although this may be subject to change). These files are not opened explicitly, but are used simply by using form (b) with the number. Beware of the anomalous case of a failed open that gives you file number 0. A subsequent use of this number in a put will produce output that goes to the standard error stream, with no warning that the file you attempted to open is not actually being used. To append to a file, the file must be opened with the mod and seek capability and then there must be a seek to the end of file. For example:
var streamnumber : int
open : streamnumber, "myfile", put, mod, seek
seek : streamnumber, *
put : streamnumber, "This appears at the end of the file"
There is an older and still acceptable version of open that has this syntax:
open ( var fileNumber : int, fileName : string, mode : string)The mode must be "r" (for get) or "w " (for put).
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| Details | The path name specified in the open statement and elsewhere can always be in UNIX format (i.e. with forward slashes, an initial forward slash indicating an absolute directory). On the PC, absolute paths would have the form:
a:/dir1/dir2/filenameOn the Macintosh, they would have the form:
/volume name/directory1/directory2/file nameNote that in addition to the UNIX path format, on the PC, you can always use standard PC path notation and on the Macintosh, you can use standard Macintosh path notation. On the Macintosh volume, directory and file names can have spaces in them. All routines (such as the File and Dir module routines) will return files names in UNIX format, regardless of the machine the program is run on.
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| See also | close, get, put, read, write, seek and tell statements.
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