nextfloat.c

#include <stdio.h>

int main() {
	union {
		unsigned int a;
		float b;
	} theunion;
	theunion.a = (0x80 << 23) | 0x7fffff;
	printf("%x %.30f\n", theunion.a, theunion.b);
	theunion.a += 1;
	printf("%x %.30f\n", theunion.a, theunion.b);
}

output.txt

$ clang nextfloat.c 
$ ./a.out 
407fffff 3.999999761581420898437500000000
40800000 4.000000000000000000000000000000