C is case sensitive—case matters.
Fortran ignores case by default.
There are two usual solutions to the uppercase/lowercase problem:
In the C subprogram, make the name of the C function all lowercase.
Compile the Fortran program with the -U option, which tells the compiler to preserve existing uppercase/lowercase distinctions on function/subprogram names.
Use one of these two solutions, but not both.
Most examples in this chapter use all lowercase letters for the name in the C function, and do not use the f95 -U compiler option.