tsort: Backgroundtsort exists because very early versions of the Unix linker processed
an archive file exactly once, and in order. As ld read each object
in the archive, it decided whether it was needed in the program based on
whether it defined any symbols which were undefined at that point in
the link.
This meant that dependencies within the archive had to be handled
specially. For example, scanf probably calls read. That means
that in a single pass through an archive, it was important for scanf.o
to appear before read.o, because otherwise a program which calls
scanf but not read might end up with an unexpected unresolved
reference to read.
The way to address this problem was to first generate a set of
dependencies of one object file on another. This was done by a shell
script called lorder. The GNU tools don’t provide a version of
lorder, as far as I know, but you can still find it in BSD
distributions.
Then you ran tsort over the lorder output, and you used the
resulting sort to define the order in which you added objects to the archive.
This whole procedure has been obsolete since about 1980, because
Unix archives now contain a symbol table (traditionally built by
ranlib, now generally built by ar itself), and the Unix
linker uses the symbol table to effectively make multiple passes over
an archive file.
Anyhow, that’s where tsort came from. To solve an old problem with the way the linker handled archive files, which has since been solved in different ways.