Stream I/O treats data in the external file as a stream of ASCII, EBCDIC, or GRAPHIC characters. In a stream I/O operation, a list of program variables is associated with actual input or output fields of data.
Two forms of stream I/O statements exist:
In both list-directed and edit-directed stream I/O, the files are written by the PUT statement and are read by the GET statement. In edit-directed stream I/O, data is transmitted to or from a stream file under the control of a format-list. In list-directed stream I/O, data is transmitted to and from the stream file without a format-list. In edit-directed stream I/O, format-lists can be part of the GET or PUT statement or can be given by a FORMAT statement.
Execution of a GET or PUT statement transmits data from or to the current line of a stream file and may not consume or produce an entire line of the stream file. Several PUT statements can be used to build a given line, or a single PUT statement can be used to output several lines. Similarly, several GET statements can read values from a given line, or a single GET statement can be used to read one or more lines. Lines read from a disk file are not modified by Open PL/I in any way; however, lines read from all other devices have trailing blanks removed and have one blank appended to the end of each line that is read by a GET statement. This blank ensures that a field typed at the beginning of a line is not appended to a field typed at the end of the previous line.
Each stream file has an associated line size that is determined when the file is opened. Lines of an output file contain n characters, where n is the line size of the file. (For more information, see the discussion of the LINESIZE option in the section Stream File Attributes.) A shorter line can be created by using the SKIP option on a PUT statement, as shown in the following example of a list-directed stream I/O statement. The shorter lines are padded to the right with blanks to obtain a length of n characters. For example:
PUT FILE(F) SKIP LIST(A,B,C); PUT FILE(F) SKIP;
In this example, the current line (consisting of data previously output) is output and one or more new lines containing the values of A, B, and C are created. The second PUT statement forces the last line containing A, B, and C to be output and starts a new line. The SKIP option is always executed before any new data is transmitted.
On input, each line can be of any length up to the value of the line size. Execution of a GET statement reads from the current line until a new line is reached, the new line is ignored, and execution continues reading values and lines until the list of variables has been read. A SKIP option can be used to force a new line to be read prior to reading any values, as shown in the following example.
GET FILE(F) LIST(A,B,C); GET FILE(F) LIST(D); GET FILE(F) SKIP LIST(X,Y);
In the previous example, A, B, and C are read from the current line or from as many lines as are necessary. D is read from the same line as C, unless C happened to be the last value on its line. The SKIP option skips to a new line and consequently ignores any values remaining on the line after the value received by D. X and Y are read from the line that is current after the SKIP option, as well as any additional, necessary lines.
The PAGE option can be used in a PUT statement to begin a new page prior to transmitting any values, as shown in the following example.
PUT FILE(F) PAGE LIST(A,B,C);
For a complete discussion of GET, PUT, and FORMAT statements, see the chapter Statements.
Standard PL/I, in a stream file, does not provide a way to read a variable-length input line from the current position to a newline indicator. Open PL/I provides two nonstandard methods to read all characters from current position to new line into a character varying variable: a READ statement and an edit-directed GET statement. To read a variable-length line with a GET statement, you must use an A-format without a field width.