C: How to convert ppm into jpeg using C

Since there's so much work that goes into a JPEG encoder/decoder, its not a practical solution to write that part yourself (about 1 man-year of work if my memory serves me correctly).

From a quick glance, the PPM format looks pretty simple, though there (almost certainly) has to be a library for that too.

I'd look at getting the data from the PPM file into a binary format, before using libjpeg to save it as you wish. If using c++ (& windows), you would then have access to GDI+ which is a part of windows that will let you save an image in any one of a number of different formats.