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cc 49-simplest-of-mallocs-examples.cc -lm

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You need to link to the maths library as pointed out above. You have a statement of the form include<math.h> the function log10, ceil etc are in that file and the compiler needs to know where to get it from.

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/* Transforms a number to a string
 * e.g 1234 => "1234"
 *
 * Pablo A. Torres Navarrete
 * 02-10-07
 */

# include <stdio.h>
# include <malloc.h>
# include <math.h>

int digcount(int num); 	// counts the digits of <num>
char *num2str(int num);

main() 
{
	int num;
	char *s = NULL;

	scanf("%d", &num);
	s = num2str(num);
	printf("\"%s\"\n", s + 1);	//s is pointing to the addres before the start of the string
	free(s);
}

int digcount(int num)
/* Calculates the number of digits of the parameter using the
 * decimal logarithm property:
 *
 * N = trunc(log(x), 0) + 1, where N is the number of digits of x
 *
 */
{
	double ans;

	ans = log10(num);
	ans = ceil(ans);
	return (int) ans;
}

char *num2str(int num)
{
	int digits, i, dig;

	digits = digcount(num);
	s = (char *) malloc((digits + 1) * sizeof(char));	// assign just enough space to hold the number and '\0'
	s += (digits - 1);	// go to the last address of the memory space assigned
	*s-- = '\0';		// mark the end of the string
	for (i = 0; i < digits; i++) {
		dig = num % 10;
		num /= 10;
		*s-- = '0' + dig;	// store digit by digit
	}
	return s;
}