http://gynvael.coldwind.pl/?id=364

numstr-test.c

#include <stdio.h>
#include <stdlib.h>

#define NUMBER 12345678901234567890

#define _Q(n) #n
#define Q(n) _Q(n)

#define ATOL(n)   atol(Q(n))
#define SSCANF(n) ({ long val = 0; sscanf(Q(n), "%ld", &val); val; })
#define STRTOL(n) strtol(Q(n), NULL, 10)

#define PRINTL(n)   printf("%.*lx\n", 2 * (int)sizeof(long), n)
#define PRINT(m, n) do { puts(#m); PRINTL(m(n)); PRINTL(m(-n)); } while (0);

int main()
{
	puts(Q(NUMBER));
	puts(Q(-NUMBER));
	PRINT(ATOL, NUMBER);
	PRINT(SSCANF, NUMBER);
	PRINT(STRTOL, NUMBER);

	return 0;
}