float and double don't store decimal places. They store binary places: float is (assuming IEEE 754) 24 significant bits (7.22 decimal digits) and double is 53 significant bits (15.95 significant digits).
Converting from double to float will give you the closest possible float, so rounding won't help you. Goining the other way may give you "noise" digits in the decimal representation.
#include <stdio.h>
int main(void) {
double orig = 12345.67;
float f = (float) orig;
printf("%.17g\n", f); // prints 12345.669921875
return 0;
}
To get a double approximation to the nice decimal value you intended, you can write something like:
double round_to_decimal(float f) {
char buf[42];
sprintf(buf, "%.7g", f); // round to 7 decimal digits
return atof(buf);
}