From man gcc:
...
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form
of an object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
...
