7.5.5 The GNU extensions to ptx

This version of ptx contains a few features which do not exist in System V ptx. These extra features are suppressed by using the `-G' command line option, unless overridden by other command line options. Some GNU extensions cannot be recovered by overriding, so the simple rule is to avoid `-G' if you care about GNU extensions. Here are the differences between this program and System V ptx.

• This program can read many input files at once, it always writes the resulting concordance on standard output. On the other end, System V ptx reads only one file and produce the result on standard output or, if a second file parameter is given on the command, to that file.

  Having output parameters not introduced by options is a quite dangerous practice which GNU avoids as far as possible. So, for using ptx portably between GNU and System V, you should pay attention to always use it with a single input file, and always expect the result on standard output. You might also want to automatically configure in a `-G' option to ptx calls in products using ptx, if the configurator finds that the installed ptx accepts `-G'.

• The only options available in System V ptx are options `-b', `-f', `-g', `-i', `-o', `-r', `-t' and `-w'. All other options are GNU extensions and are not repeated in this enumeration. Moreover, some options have a slightly different meaning when GNU extensions are enabled, as explained below.

• By default, concordance output is not formatted for troff or nroff. It is rather formatted for a dumb terminal. troff or nroff output may still be selected through option `-O'.

• Unless `-R' option is used, the maximum reference width is subtracted from the total output line width. With GNU extensions disabled, width of references is not taken into account in the output line width computations.

• All 256 characters, even NULs, are always read and processed from input file with no adverse effect, even if GNU extensions are disabled. However, System V ptx does not accept 8-bit characters, a few control characters are rejected, and the tilde ~ is condemned.

• Input line length is only limited by available memory, even if GNU extensions are disabled. However, System V ptx processes only the first 200 characters in each line.

• The break (non-word) characters default to be every character except all letters of the underlying character set, diacriticized or not. When GNU extensions are disabled, the break characters default to space, tab and newline only.

• The program makes better use of output line width. If GNU extensions are disabled, the program rather tries to imitate System V ptx, but still, there are some slight disposition glitches this program does not completely reproduce.

• The user can specify both an Ignore file and an Only file. This is not allowed with System V ptx.