Because the Metal option produces the final code in HLASM source
code format, the build process needs to include an assembly step to
produce the object files. The build process is demonstrated in Figure 1. Note that the build process with IPA is
more elaborated. For more information, see Building Metal C programs with IPA.
Figure 1. Metal C application build process 
In summary, the C source file is sent to the C compiler, which generates the assembler source file. The assembler source file is sent to the HLASM assembler, which generates an object file and a listing.