Issue 0019: Are printing characters implementation defined?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Richard Wiersma, WG14
Date: 1992-12-10
Reference document: X3J11/91-014
Submitted against: C90
Status: Closed
Cross-references: 0132
Converted from: dr.htm, dr_019.html

Background:

Subclause 7.3.1.5 states that “the isgraph function tests for any printing character except space.” Subclause 7.3.1.7 states that “the isprint function tests for any printing character including space.”

The third paragraph of subclause 7.3 defines the term printing character as “a member of an implementation-defined set of characters, each of which occupies one printing position on a display device.”

Subclause 5.2.1 defines the source and execution character sets and provides a list of characters which must be contained in both sets.

Question for interpretation: Are the isprint and isgraph functions required to return a non-zero value for all of the characters defined in subclause 5.2.1?

A scenario for use of isprint/isgraph that depends on the interpretation is: A developer may wish to use these functions to determine whether a particular character can be displayed as itself (e.g., whether a square bracket is actually displayed as a square bracket). This could be useful for formatting output in a device-independent manner, since the application could substitute some other character for ones that do not print “correctly.”

If isprint and isgraph are required to return non-zero for all characters in subclause 5.2.1, developers cannot use them for this purpose.

This problem has occurred in a real implementation. The most commonly used terminals and printers for IBM System/370 computers do not support all of the characters listed in subclause 5.2.1. For example, most IBM printers and terminals do not print the square brackets.

The SAS/C implementation of isprint and isgraph assumes that subclause 7.3 controls the behavior of these functions, and returns non-zero only for those characters that print “correctly.” The Plum Hall test suite, however, assumes that isprint and isgraph return non-zero for all characters listed in subclause 5.2.1.

Comment from WG14 on 1997-09-23:

Response

Subclause 7.3, page 102, line 8 says that printing character is implementation-defined. In particular, the value (zero or non-zero) of isprint('[') is implementation-defined, even in the "C" locale.