File: groff.info,  Node: Other Differences,  Prev: Compatibility Mode,  Up: Implementation Differences

5.38.3 Other Differences
------------------------

'groff' request names unrecognized by other 'troff' implementations will
likely be ignored by them; escape sequences that are 'groff' extensions
are liable to be interpreted as if the escape character were not
present.  For example, the adjustable, non-breaking escape sequence '\~'
is also supported by Heirloom Doctools 'troff' 050915 (September 2005),
'mandoc' 1.9.5 (2009-09-21), 'neatroff' (commit 1c6ab0f6e, 2016-09-13),
and Plan 9 from User Space 'troff' (commit 93f8143600, 2022-08-12), but
not by Solaris or Documenter's Workbench 'troff's.  *Note Manipulating
Filling and Adjustment::.

   GNU 'troff' does not allow the use of the escape sequences '\|',
'\^', '\&', '\{', '\}', '\', '\'', '\`', '\-', '\_', '\!', '\%', and
'\c' in identifiers; AT&T 'troff' does.  The '\A' escape sequence (*note
Identifiers::) may be helpful in avoiding use of these escape sequences
in names.

   When adjusting to both margins, AT&T 'troff' at first adjusts spaces
starting from the right; GNU 'troff' begins from the left.  Both
implementations adjust spaces from opposite ends on alternating output
lines in this adjustment mode to prevent "rivers" in the text.

   GNU 'troff' does not always hyphenate words as AT&T 'troff' does.
The AT&T implementation uses a set of hard-coded rules specific to
English, while GNU 'troff' uses language-specific hyphenation pattern
files derived from TeX.  Furthermore, in old versions of 'troff' there
was a limited amount of space to store hyphenation exceptions (arguments
to the 'hw' request); GNU 'troff' has no such restriction.

   GNU 'troff' predefines a string '.T' containing the argument given to
the '-T' command-line option, namely the current output device (for
example, 'pdf' or 'utf8').  The existence of this string is a common
feature of post-CSTR #54 'troff's(1) (*note Other
Differences-Footnote-1::) but valid values are specific to each
implementation.

   AT&T 'troff' ignored attempts to remove read-only registers; GNU
'troff' honors such requests.  *Note Built-in Registers::.

   The (read-only) register '.T' interpolates 1 if GNU 'troff' is called
with the '-T' command-line option, and 0 otherwise.  This behavior
differs from AT&T 'troff', which interpolated 1 only if 'nroff' was the
formatter and was called with '-T'.

   AT&T 'troff' and other implementations handle the 'lf' request
differently.  For them, its LINE argument changes the line number of the
_current_ line.

   AT&T 'troff' had only environments named '0', '1', and '2'.  In GNU
'troff', any number of environments may exist, using any valid
identifiers for their names (*note Identifiers::.)

   Fractional type sizes cause one noteworthy incompatibility.  In AT&T
'troff' the 'ps' request ignores scaling units and thus '.ps 10u' sets
the type size to 10 points, whereas in GNU 'troff' it sets the type size
to 10 _scaled_ points.  *Note Using Fractional Type Sizes::.

   The 'ab' request differs from AT&T 'troff': GNU 'troff' writes no
message to the standard error stream if no arguments are given, and it
exits with a failure status instead of a successful one.

   The 'bp' request differs from AT&T 'troff': GNU 'troff' does not
accept a scaling unit on the argument, a page number; the former
(somewhat uselessly) does.

   The 'pm' request differs from AT&T 'troff': GNU 'troff' reports the
sizes of macros, strings, and diversions in bytes and ignores an
argument to report only the sum of the sizes.

   Unlike AT&T 'troff', GNU 'troff' does not ignore the 'ss' request if
the output is a terminal device; instead, the values of minimal
inter-word and additional inter-sentence space are each rounded down to
the nearest multiple of 12.

   In GNU 'troff' there is a fundamental difference between
(unformatted) characters and (formatted) glyphs.  Everything that
affects how a glyph is output is stored with the glyph node; once a
glyph node has been constructed, it is unaffected by any subsequent
requests that are executed, including 'bd', 'cs', 'tkf', 'tr', or 'fp'
requests.  Normally, glyphs are constructed from characters immediately
before the glyph is added to an output line.  Macros, diversions, and
strings are all, in fact, the same type of object; they contain a
sequence of intermixed character and glyph nodes.  Special characters
transform from one to the other: before being added to the output, they
behave as characters; afterward, they are glyphs.  A glyph node does not
behave like a character node when it is processed by a macro: it does
not inherit any of the special properties that the character from which
it was constructed might have had.  For example, the input

     .di x
     \\\\
     .br
     .di
     .x

produces '\\' in GNU 'troff'.  Each pair of backslashes becomes one
backslash _glyph_; the resulting backslashes are thus not interpreted as
escape _characters_ when they are reread as the diversion is output.
AT&T 'troff' _would_ interpret them as escape characters when rereading
them and end up printing one '\'.

   One correct way to obtain a printable backslash in most documents is
to use the '\e' escape sequence; this always prints a single instance of
the current escape character,(2) (*note Other Differences-Footnote-2::)
regardless of whether or not it is used in a diversion; it also works in
both GNU 'troff' and AT&T 'troff'.

   The other correct way, appropriate in contexts independent of the
backslash's common use as a 'troff' escape character--perhaps in
discussion of character sets or other programming languages--is the
character escape '\(rs' or '\[rs]', for "reverse solidus", from its name
in the ECMA-6 (ISO/IEC 646) standard.(3)  (*note Other
Differences-Footnote-3::)

   To store an escape sequence in a diversion that is interpreted when
the diversion is reread, either use the traditional '\!' transparent
output facility, or, if this is unsuitable, the new '\?' escape
sequence.  *Note Diversions:: and *note Gtroff Internals::.

   In the somewhat pathological case where a diversion exists containing
a partially collected line and a partially collected line at the
top-level diversion has never existed, AT&T 'troff' will output the
partially collected line at the end of input; GNU 'troff' will not.