The Universe of Discourse

Sun, 19 Apr 2015

Another use for strace (groff)

The marvelous Julia Evans is always looking for ways to express her love of strace and now has written a zine about it. I don't use strace that often (not as often as I should, perhaps) but every once in a while a problem comes up for which it's not only just the right thing to use but the only thing to use. This was one of those times.

I sometimes use the ancient Unix drawing language pic. Pic has many good features, but is unfortunately coupled too closely to the Roff family of formatters (troff, nroff, and the GNU project version, groff). It only produces Roff output, and not anything more generally useful like SVG or even a bitmap. I need raw images to inline into my HTML pages. In the past I have produced these with a jury-rigged pipeline of groff, to produce PostScript, and then GNU Ghostscript (gs) to translate the PostScript to a PPM bitmap, some PPM utilities to crop and scale the result, and finally ppmtogif or whatever. This has some drawbacks. For example, gs requires that I set a paper size, and its largest paper size is A0. This means that large drawings go off the edge of the “paper” and gs discards the out-of-bounds portions. So yesterday I looked into eliminating gs. Specifically I wanted to see if I could get groff to produce the bitmap directly.

GNU groff has a -Tdevice option that specifies the "output" device; some choices are -Tps for postscript output and -Tpdf for PDF output. So I thought perhaps there would be a -Tppm or something like that. A search of the manual did not suggest anything so useful, but did mention -TX100, which had something to do with 100-DPI X window system graphics. But when I tried this groff only said:

    groff: can't find `DESC' file
    groff:fatal error: invalid device `X100`

The groff -h command said only -Tdev use device dev. So what devices are actually available?

strace to the rescue! I did:

    % strace -o /tmp/gr groff -Tfpuzhpx

and then a search for fpuzhpx in the output file tells me exactly where groff is searching for device definitions:

    % grep fpuzhpx /tmp/gr
    execve("/usr/bin/groff", ["groff", "-Tfpuzhpx"], [/* 80 vars */]) = 0
    open("/usr/share/groff/site-font/devfpuzhpx/DESC", O_RDONLY) = -1 ENOENT (No such file or directory)
    open("/usr/share/groff/1.22.2/font/devfpuzhpx/DESC", O_RDONLY) = -1 ENOENT (No such file or directory)
    open("/usr/lib/font/devfpuzhpx/DESC", O_RDONLY) = -1 ENOENT (No such file or directory)

I could then examine those three directories to see if they existed, and if so find out what was in them.

Without strace here, I would be reduced to groveling over the source, which in this case is likely to mean trawling through the autoconf output, and that is something that nobody wants to do.

Addendum 20150421: another article about strace. ]

[ Addendum 20150424: I did figure out how to prevent gs from cropping my output. You can use the flag -p-P48i,48i to groff to set the page size to 48 inches (48i) by 48 inches. The flag is passed to grops, and then resulting PostScript file contains

   %%DocumentMedia: Default 3456 3456 0 () ()

which instructs gs to pretend the paper size is that big. If it's not big enough, increase 48i to 120i or whatever. ]

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