Größex1 = Werty1
Größex2 = Werty2
Größex3 = Werty3
...

typedef struct{
double Größex1;
double Größex3;
char[][] = [Größex1, Größex3]
}config;

speichere(Werty1, config.größex1)
speichere(Werty3, config.größex3)

#define N 3
typedef struct {
 double sizes[N];
 char[][N] names = ["Größex1", "Größex2", "Größex3"] 
} config;

void write(config c, char* name, double value) {
    for(int i = 0; i < N; ++i) {
       if (strcmp(c.names[i], name) == 0) {
            c.sizes[i] = value;
            return;
       }
    }
}

#include <stddef.h>
#include <string.h>
typedef struct {
	const char* name;
	size_t offset;
} lookup;

typedef struct {
	double x1;
	double x2;
} config;

#define N 2
lookup configtable[N] = {
	{"x1", offsetof(config, x1)},
	{"x2", offsetof(config, x2)}
};

void write(config* c, char* name, double value) {
	for(int i = 0; i < N; ++i) {
		if (strcmp(configtable[i].name, name) == 0) {
			double* val = (double*) ((char*) c + configtable[i].offset);
			(*val) = value;
		}
	}
} 

#define VARIABLES \
	X(x1) \
	X(x2) \
	X(x3)

typedef struct {
#define X(n) double n;
VARIABLES
#undef X
} config;

lookup configtable[] = {
#define X(n) {#n, offsetof(config, n)},
VARIABLES
#undef X
};
static const size_t N = sizeof(configtable)/sizeof(lookup);