Undefined reference to 'pow' (newbie mistake)

/*
Exercise 2-3. Write a function htoi(s), which converts a string of hexadecimal digits
(including an optional 0x or 0X) into its equivalent integer value. The allowable digits are 0
through 9, a through f, and A through F.
*/
#include <stdio.h>
#include <string.h>
#include <math.h>

#define MAXI 1000

double htoi(char s[], int len);

int main()
{
	char hex[MAXI];
	/* 
	Take a string of characters input by user, verify it's in the
	format of a hex number, then use htoi() to turn it from a hex
	string to a double number. Use strlen to pass to htoi.
	*/
	return 0;
}

double htoi(char s[], int len)
{
	double n = 0;
	int j = 0;
	int i;
	if(len >= 2 && s[0] == 0 && (s[1] == 'x' || s[1] == 'X'))
	{
		s[1] = 0;
		s[0] = 0;
	}
	for(i = len; i >= 0; i--, j++)
	{
		if(s[i] >= '0' && s[i] <= '9')	
			n += pow(16.0, (double) j ) * (s[i] - '0');
		else if(s[i] == 'A' || s[i] == 'a')
			n += pow(16.0, (double) j) * 10;
		else if(s[i] == 'B' || s[i] == 'b')
			n += pow(16.0, (double) j) * 11;
		else if(s[i] == 'C' || s[i] == 'c')
			n += pow(16.0, (double) j) * 12;
		else if(s[i] == 'D' ||  s[i] == 'd')
			n += pow(16.0, (double) j) * 13;
		else if(s[i] == 'E' || s[i] == 'e')
			n += pow(16.0, (double) j) * 14;
		else if(s[i] == 'F' || s[i] == 'f')
			n += pow(16.0, (double) j) * 15;
	}
	return n;
}

/*

Exercise 2-3. Write a function htoi(s), which converts a string of hexadecimal digits
(including an optional 0x or 0X) into its equivalent integer value. The allowable digits are 0
through 9, a through f, and A through F.

* user inputs a string of characters -> getLine
* verify string of characters fits the hex format
	-> have in a nested loop that runs through testing to see if it fits against an array like hexDigits[] = "0123456789ABCDEF" after toupper()ing string. once it matches, continue through, else if hexDigits[i] = '\0', isHex = 0;
* convert hex characters to decimal by multiplying through on a base 16 system
-	multiply digit by 16, assigning values to the letters as if > '9', (s[i] - 'A' + 1) * pow(16, pow) else (s[i] - '0') * pow(16, pow)
* display number

*/

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

#define MAXI 30

long htoi(char array[], int len);
int isHex(char array[], int len);
int getLine(char array[], int lim);

main()
{
	char hexDigits[MAXI];
	long int deci;
	int hexLen;
	printf("input digits");
	hexLen = getLine(hexDigits, MAXI);
	while(isHex(hexDigits, hexLen) != 1)
	{
		printf("input digits again");
		getLine(hexDigits, MAXI);
	}
	deci = htoi(hexDigits, hexLen);
	printf("number is: %d", deci);
}

long htoi(char array[], int len)
{
	long num = 0;
	int i, power;

	for(i = 0, power = len - 1; i < len;i++, power--)
		if(array[i] > '9')
			num += (array[i] - 'A' + 11) * pow(16, power);
		else
			num += (array[i] - '0') * pow(16, power);
	return num;
}

int isHex(char array[], int len)
{
	char hexDigits[16] = "0123456789ABCDEF";
	toupper(array);
	int i, j;
	int ishex = 0;
	
	//If leading 0X or 0x, convert x to 0.
	if(len >= 2 && array[0] == '0' && array[1] == 'X')
		array[1] = 0;

	for(i = 0; array[i] < len;i++)
	{
		ishex = 0;
		for(j = 0;j < 16;j++)
			if(array[i] == hexDigits[j])
				ishex = 1;
		if(ishex == 0) break;
	}
	return ishex;
}
//All strings of hex will end in \0, so len-1 returned to work with other functions
int getLine(char array[], int lim)
{
	int c, i;
	for(i = 0; i < lim-1 && (c = getChar()) != EOF && c != '\n'; i++)
	{
		array[i] = c;
	}
	if(c == '\n')
	{
		array[i] = c;
		i++;
	}
	array[i] = '\0';
	return i-1;
}

paul@paul-laptop:~/Desktop/C/2/2-3$ gcc -lm main.c
main.c: In function ‘isHex’:
main.c:59: warning: passing argument 1 of ‘toupper’ makes integer from pointer without a cast
/tmp/ccOSvZ0y.o: In function `getLine':
main.c:(.text+0x2a4): undefined reference to `getChar'
collect2: ld returned 1 exit status
paul@paul-laptop:~/Desktop/C/2/2-3$