This section discussed SNOBOL4.2 input / output considerations. FILE NAMES, MODIFIERS and other I/O considerations:
File names can be associated with input/output unit numbers as mentioned in the section on invoking SNOBOL4.2 above. File names (more correctly, path names with optional modifiers) are specified in the command line or as the optional fourth operand of the INPUT or OUTPUT functions. If no fourth parameter is given for the INPUT or OUTPUT functions, the previous file name and modifiers assigned to the I/O unit (either on the command line or prior INPUT/OUTPUT function call) are used.
Path name syntax is described in the DOS manual. Paths names are used to gain access to files not in the current directory. In SNOBOL4.2, "file modifiers" may be optionally appended to the file name. Each modifier begins with the slash character "/" followed by the modifier name. All of the modifiers follow the path name with no intervening blanks. The modifier names and their functions are described below.
1. The /A modifier specifies that the file is to be considered an ASCII file with records terminated by the carriage control character (0Dh, which is discarded on input). The end of the file is indicated by the actual end of the file or the end-of-file character (1Ah), whichever comes first. The /B modifier is the opposite of /A and indicates a binary file. Characters are read until the buffer is full, except for possibly the last record read in the file, which may be shortened if the file size is not a multiple of the buffer size. The buffer size is given by the third parameter of the input function. Certain other modifiers only apply to a file read or written in one of the modes (ASCII /A or binary /B). /A is the default.
2. /NBUF (no buffering) disables SNOBOL4.2 buffering of output. This is particularly needed when writing to the console or perhaps a communications port. /BUF enables buffering and is the default, except for devices.
3. /TABX is the default and causes tab characters to be expanded into blanks on input of ASCII (/A) files. The tabs expand into blanks such that the next character after the tab has an offset in the line which is the next multiple of 8. This eliminates the need to preprocess lines when they were written by some editor or other program/utility to eliminate the tab characters. On output, /TABX compresses the output lines (in ASCII /A mode) so that they use tabs where possible. Quotation marks terminate the tab expansion. /NTABX disables this tab expansion/compression. Since /TABX is the default, SNOBOL4.2 is incompatible with SNOBOL4 versions 1.xx on this item. If you have any problems, just append /NTABX to the file name.
4. The /AP (append) modifier indicates that a file opened for write should be positioned to the end of the file, so that the next write appends to the file. /R is the opposite (replace) and is the default. It causes writing to start at the beginning of the file. For /AP, if the file is ASCII (/A and not /B), then a check is made for the end of file character 1Ah at the end of the file. If it appears there, the next write will replace the end of file character. /AP is ignored for devices, such as the console.
5. /NPR turns "?" prompt off when input is from the console. /PR turns prompting on and is the default. These only apply to ASCII files (/A) connected to the console.
6. The /VL (variable length) option provides the ability to read variable length records for ASCII files. The maximum record length is determined by the third parameter to the INPUT() function. The record ends with but does not include the carriage-return character in the file. Use of the /VL option is sometimes preferable to using &TRIM = 1 because less processing is involved and the reads reflect more precisely the content of the file. /VL applies to input on files read in ASCII mode (/A). Note, on binary files (/B), the last record may be less than the buffer size regardless of the /VL setting, when the number of bytes in the file is not an exact multiple of the record length specified in the third parameter of the INPUT function.
7. /LF enables (/NLF) disables) line feed character processing. If enabled, on input, line feeds at the start of a record are ignored. On output, a line feed character is inserted after the carriage control character. /LF is ignored if /CC is specified on output. Also, /LF only applies to files processed in ASCII mode (/A). /LF is the default.
8. /CC enables (/NCC disables) carriage control on output. /CC interprets the first character of the output record as a carriage control character if it is a legal carriage control character, otherwise it assumes a blank was given. Instead of ending the output records with carriage return and line feed characters, /CC causes these to be written at the front of the line instead. The codes are:
9. "/STD" DOS Standard I/O handles can be named by using:
10. SUMMARY and RULES
11. If I/O unit is not assigned to a file when starting to read or write, it is then assigned to the standard device IN/STD or OUT/STD.
12: Device names such as CON, LPT1, and AUX must NOT have a colon (:) after them as was the practice for DOS 1.X.
13. INPUT() and OUTPUT() functions fail if the file name is not legal or if the modifiers are incorrect, in the fourth parameter.
14. The third parameter of the OUTPUT function, which is the FORTRAN format, now defaults to the null string, (no format). In the green book, it defaults to a specific format. This is an incompatibility between SNOBOL4.2 and SNOBOL4 as described in the green book. This was done to keep SNOBOL4 version 1.xx programs from truncating records and putting blanks in front of them on output, when interpreted by SNOBOL4.2.
Minnesota SNOBOL4.2 now supports FORTRAN output formatting as used in the green book. People have noted that various examples depend on this formatting and it makes SNOBOL4.2 even more compatible with the mainframe versions of SNOBOL. Formatting is often used in conjunction with the /CC (carriage control) file modifier to achieve double spacing, page skips etc.
The details of FORTRAN formatting are covered in many books and FORTRAN manuals. However, only a subset of the full FORTRAN formatting facilities apply to SNOBOL. These are described below.
FORTRAN formatting is used only on output and only if the format (operand 3 of the OUTPUT function) was not null. A format is a string directs the characters to be written to particular columns and spacing. As the characters to be written are processed left to right, corresponding format elements are processed until the entire record to be written is processed. If there is an error in the syntax of the format string, then a write error will occur.
For example, if the format is
If an entire format string is processed before the source characters are all consumed, then a new output record is started and the format is repeated from the beginning, unless there is a list of codes specified that has no repetition factor in front of it. Then a new record is not started (unless the output reaches 255 bytes) and format codes are used starting with the last indefinite code list in the format.