Let's just ignore the C dialects outside UNIX like Small-C and RatC, or that we had to wait until 1990 for proper standard, and not even K&R C was a given outside UNIX.
The 1980s C++ compilers on the PC also dominated the C++ compiler use. C++ on the PC vaulted the language from obscurity into the major language it is today.
A while back I chatted with someone who worked on both C and Pascal compilers around that time period and got the impression that the majority of their customers were people running on 68k based Unix workstations. May have just been their niche I suppose.
I didn't start programming until closer to 1990, and started with Mix software's C compiler on a 286, because that's what I could afford.
And if we go into the Amiga it was about Assembly and AMOS mostly.
On Apple, Object Pascal and Assembly, HyperCard, MPW with C++ came later into the pictures.
On thing I do agree, by the time Windows and OS/2 were taking off, C++ on the PC was everywhere and only masochistics would insist in using C instead of C++ frameworks like OWL, MFC or CSet++.
Sadly I never saw it anywhere on sale, as the graphical debugging for C++ data structures was quite cool to read about.
Then Slackware came out on CD!
For example, there was a Small-C compiler available for the Atari 800 in 1982:
http://www.atarimania.com/utility-atari-400-800-xl-xe-c-65_1...
"... based on the Small C compiler published in Dr. Dobb's Journal"
If you look in the beginning of the manual it has a list of what is and is not supported. They claim it is sufficient to compile C/65 itself but there are lots of things we take for granted missing.
https://books.google.com/books/about/A_Book_on_C.html?id=e5p...
So it is kind of ironic this revisionism how great was C "portability", when in reality it was full of dialects outside UNIX just like the competition.
Except "near" and "far" pointers, which were an absolute plague.