C and Fortran take opposite perspectives on case sensitivity:
C is case sensitive—case matters.
Fortran ignores case by default.
The f95 default is to ignore case by converting subprogram names to lowercase. It converts all uppercase letters to lowercase letters, except within character-string constants.
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.