Error: initializer element is not computable at load time

This is illegal in C. Initializer lists must be constant compile time expressions. Do the following instead:

int prm_arr[3];

prm_arr[0] = prm.field1;
prm_arr[1] = prm.field2;
prm_arr[2] = prm.field3;

Mike's answer is absolutely right.

However, if you're able to use the GNU C extensions, or to use the newer and better C99 standard instead (use the --std=c99 option), then initializers such as this are perfectly legal. The C99 standard has been out for, well, 9 years, and most C compilers support it quite well... especially this feature.