The most useful user-made C-macros (in GCC, also C99)?

What C macro is in your opinion is the most useful? I have found the following one, which I use to do vector arithmetic in C:

#define v3_op_v3(x, op, y, z) {z[0]=x[0] op y[0]; \
                               z[1]=x[1] op y[1]; \
                               z[2]=x[2] op y[2];}

It works like that:

v3_op_v3(vectorA, +, vectorB, vectorC);
v3_op_v3(vectorE, *, vectorF, vectorJ);
...