Your application can contain both C and Fortran source files. If your main program is a Fortran source file (myprog.for) that calls a routine written in C (cfunc.c), you can use the following sequence of commands to build your application:
icc -c cfunc.c ifort -o myprog myprog.for cfunc.o
The icc (for Intel C++) command compiles cfunc.c. The -c option specifies that the linker is not called. This command creates cfunc.o. The ifort command compiles myprog.for and links cfunc.o with the object file created from myprog.for to create myprog.