Why is it so hard to find TeX sources in C? I know the full source code is included in the
pdftex.web files, but isn't there any way to convert them to plain C code?
It's hard to find them because they don't exist. TeX is written in Pascal (one of the many flavors of it), but the source is interspersed with description of the code in the Web system devised by Knuth and others as part of a "literate programming" project.
The commented source is in
and then compiling the file with
Modern distributions translate the Pascal code into C with a helper program, but I don't think that the C code is particularly readable. If you have available a TeX Live distribution, you can say
which will open a PDF file with the commented Pascal code.
The C source code to Y&Y TeX, a once commercial product (Windows), has been released as free software. You can download it from http://code.google.com/p/yytex/