MACHAR is a C library which dynamically computes constants that characterize the floating point arithmetic system on a computer, by William Cody.
This includes the value of the "machine epsilon", the smallest number that can be added to 1 and make a difference. However, it includes many other quantities of interest, including the arithmetic base, the largest and smallest magnitudes, and so on.
Many compilers now do certain kinds of optimization that may cause MACHAR to fail. The most noticeable symptom is that MACHAR may fall into an infinite loop. If you notice this, recompile MACHAR with compiler optimization turned off, or set to the lowest level.
The FORTRAN77 version of these routines was supplied as part of ACM TOMS algorithm 665.
A C version of these routines was supplied as part of ACM TOMS algorithm 722.
The computer code and data files described and made available on this web page are distributed under the GNU LGPL license.
MACHAR is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
MACHINE, a C library which stores the appropriate values of machine constants for a given machine.
PARANOIA, a C program which checks the accuracy of floating point arithmetic on a given computer, by William Kahan.
Original FORTRAN77 version by William Cody. C version by John Burkardt.
You can go up one level to the C source codes.