..... Myabs() is of type int; fabs() was of type double in C89 <math.h>, it may be
system tells me abs is in stdlib.h and not just math.h. I don't know if
that's true or not. Anyway abs takes ints and my variables are all doubles.
in <tgmath.hif your compiler has some C99 capability. Most compilers
can generate single instruction in-line implementation of fabs().
It's not difficult to check your "system," the answer would be in the
compiler or library docs, and probably in the header files in the
compiler's include folder.
I don't see a connection with the source code you showed.