Precision issue when comparing matlab output with c code output

I am trying to compare my c code output with matlab output. My results are matching up to 18th decimal points. When both matlab and c compiler uses ieee754 floating point format. Then why am I observing the difference. As matlab by default uses double precision, my c code is also using double data type.

[John D'Errico]

"My results are matching up to 18th decimal points."

Then what is the problem, since that is beyond the limit of double precision?

Anyway, you say you are observing a difference someplace, on some unspecified computation. But we cannot see inside your computer, to actually know what you did. If you like, we can try using the MATLAB crystal ball, but it is always foggy these days. I think I dropped mine on the floor too many times. But the MATLAB tarot cards are an option too if that fails.

OR, you can actually show us what you did, instead of a vague description of something that does not match up. Is there a good reason why you want to make it impossible for anyone to help you? Make it easy to get help.

[dpb]

This may boil down to one of the issues in <the last thread on similar comparison>, comparing output values with inconsistent formatting/precision.

Although note @Vaishnavi that the i/o formatting routines will continue outputting characters ad infinitum, long past the actual internal precision of the data storage size limits.

[James Tursa]

@Vaishnavi Please post a small complete example that demonstrates your problem.

[Walter Roberson]

Also, it is potentially possible differences are due to the rounding mode in effect.

[dpb]

If identical past the 15-16 digits of DP, though? That's what makes this one hard to figure...OP says agree precisely to 18 decimal places?

[Walter Roberson]

"18 decimal points" is not necessary the same as 18 digits of precision. For example,

A = 0.0000000000123456789478;
fprintf('%.19f\n', A)

0.0000000000123456789

might seem to be valid to 19 decimal places, if you count starting from the period.