4.6.7 Differences from AT&T ms

The groff ms macros are an independent reimplementation, using no AT&T code. Since they take advantage of the extended features of GNU troff, they cannot be used with AT&T troff. groff ms supports features described above as Berkeley and Research Tenth Edition Unix extensions, and adds several of its own.

• The internals of groff ms differ from those of AT&T ms. Documents that depend upon implementation details of AT&T ms may not format properly with groff ms. Such details include macros whose function was not documented in the AT&T ms manual.¹⁷
• The error-handling policy of groff ms is to detect and report errors, rather than to ignore them silently.
• Research Tenth Edition Unix supported P1/P2 macros to bracket code examples; groff ms does not.
• groff ms does not work in GNU troff’s AT&T compatibility mode. If loaded when that mode is enabled, it aborts processing with a diagnostic message.
• Multiple line spacing is not supported. Use a larger vertical spacing instead.
• groff ms uses the same header and footer defaults in both nroff and troff modes as AT&T ms does in troff mode; AT&T’s default in nroff mode is to put the date, in U.S. traditional format (e.g., “January 1, 2021”), in the center footer (the CF string).
• Many groff ms macros, including those for paragraphs, headings, and displays, cause a reset of paragraph rendering parameters, and may change the indentation; they do so not by incrementing or decrementing it, but by setting it absolutely. This can cause problems for documents that define additional macros of their own that manipulate indentation. Use the ms RS and RE macros instead of the in request.
• AT&T ms interpreted the values of the registers PS and VS in points, and did not support the use of scaling units with them. groff ms interprets values of the registers PS, VS, FPS, and FVS equal to or larger than 1,000 (one thousand) as decimal fractions multiplied by 1,000.¹⁸ This threshold makes use of a scaling unit with these parameters practical for high-resolution devices while preserving backward compatibility. It also permits expression of non-integral type sizes. For example, ‘groff -rPS=10.5p’ at the shell prompt is equivalent to placing ‘.nr PS 10.5p’ at the beginning of the document.
• AT&T ms’s AU macro supported arguments whose values were used with some non-RP document types; that of groff ms does not.
• Right-aligned displays are available. The AT&T ms manual observes that “it is tempting to assume that ‘.DS R’ will right adjust lines, but it doesn’t work”. In groff ms, it does.
• To make groff ms use the default page offset (which also specifies the left margin), the PO register must stay undefined until the first ms macro is called.

  This implies that ‘\n[PO]’ should not be used early in the document, unless it is changed also: accessing an undefined register automatically defines it.

• groff ms supports the PN register, but it is not necessary; you can access the page number via the usual % register and invoke the af request to assign a different format to it if desired.¹⁹
• The AT&T ms manual documents registers CW and GW as setting the default column width and “intercolumn gap”, respectively, and which applied when MC was called with fewer than two arguments. groff ms instead treats MC without arguments as synonymous with 2C; there is thus no occasion for a default column width register. Further, the MINGW register and the second argument to MC specify a minimum space between columns, not the fixed gutter width of AT&T ms.
• The AT&T ms manual did not document the QI register; Berkeley and groff ms do.

Register: \n[GS]

groff ms sets the register GS to 1; AT&T ms does not use it. A document can test its value to determine whether it is being formatted with groff ms or another implementation.