Can I shorten a function name I use repeatedly?

if I use define, does it affect runtime?

define works by doing text-based substitution at compile time. If you #define s(x) sin(x) then the C pre-processor will rewrite all the s(x) into sin(x) before the compiler gets a chance to look at it.

BTW, this kind of low-level text-munging is exactly why define can be dangerous to use for more complex expressions. For example, one classic pitfall is that if you do something like #define times(x, y) x*y then times(1+1,2) rewrites to 1+1*2, which evaluates to 3 instead of the expected 4. For more complex expressions like it is often a good idea to use inlineable functions instead.

There are at least three options for using s for sin:

Use a preprocessor macro:

#define s(x) (sin(x))
#define c(x) (cos(x))
float a = s(b)*c(c)+s(c+d)*c(d)....
#undef c
#undef s

Note that the macros definitions are immediately removed with #undef to prevent them from affecting subsequent code. Also, you should be aware of the basics of preprocessor macro substitution, noting the fact that the first c in c(c) will be expanded but the second c will not since the function-like macro c(x) is expanded only where c is followed by (.

This solution will have no effect on run time.

Use an inline function:

static inline double s(double x) { return sin(x); }
static inline double c(double x) { return cos(x); }

With a good compiler, this will have no effect on run time, since the compiler should replace a call to s or c with a direct call to sin or cos, having the same result as the original code. Unfortunately, in this case, the c function will conflict with the c object you show in your sample code. You will need to change one of the names.

Use function pointers:

static double (* const s)(double) = sin;
static double (* const c)(double) = cos;

With a good compiler, this also will have no effect on run time, although I suspect a few more compilers might fail to optimize code using this solution than than previous solution. Again, you will have the name conflict with c. Note that using function pointers creates a direct call to the sin and cos functions, bypassing any macros that the C implementation might have defined for them. (C implementations are allowed to implement library function using macros as well as functions, and they might do so to support optimizations or certain features. With a good quality compiler, this is usually a minor concern; optimization of a direct call still should be good.)