When did a C compiler first become available for the IBM PC? I have fond memories of Turbo C, but that didn't become available until the mid to late eighties; it definitely wasn't the first one. Languages available at launch seem to have been Basic, assembly and a Pascal byte code system.
The first compiler for PC-DOS was Lattice C, released in June, 1982. This compiler was also distributed under license as Microsoft C v2.0. The Lattice C compiler was very popular and widely ported, available on several 1980s retrocomputers including Amiga, Atari ST, and Sinclair QL. The Lattice compiler was acquired by SAS Institute in 1987 and renamed SAS/C.
Watcom C was also popular and widely ported, but was not available for the PC until the late-1980s, around the same time as Borland's Turbo C.
I worked for SAS when it acquired Lattice C in 1983. SAS needed a C compiler so it could port the SAS System to the IBM PC. I was part of a group whose job it was to create a "cross-compiler" that ran on IBM's CMS operating system but generated code for the IBM PC. I was experienced in CMS program development so my job was to write enough of the C standard library (memory management, I/O, etc.) to be able to run the compiler on CMS. Other members of the team included ex-Lattice employees who were familiar with code generation, etc.
SAS/C was a compiler that ran on IBM operating systems that could be used to create executables that ran on CMS and IBM's MVS (now Z/OS) O/S. We leased the compiler very successfully but eventually IBM created its own C compiler that eventually took over.
I worked on that product for 6 or 7 years. It was a lot of fun and I wrote some of the best code in my 30+ year career.