An efficient solution to convert a signed number character string to a signed int. Was going to put it up on Stack Overflow, but the question was closed as duplicate.

my_atoi.c

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

int my_atoi(const char* snum)
{
	int idx, strIdx = 0, accum = 0, numIsNeg = 0;
	const unsigned int NUMLEN = (int)strlen(snum);
	
	/* Check if negative number and flag it. */
	if(snum[0] == 0x2d)
		numIsNeg = 1;
	
	for(idx = NUMLEN - 1; idx >= 0; idx--)
	{
		/* Only process numbers from 0 through 9. */
		if(snum[strIdx] >= 0x30 && snum[strIdx] <= 0x39)
			accum += (snum[strIdx] - 0x30) * pow(10, idx);
			
		strIdx++;
	}
	
	/* Check flag to see if originally passed -ve number and convert result if so. */
	if(!numIsNeg)
		return accum;
	else
		return accum * -1;
}

int main()
{
	/* Tests... */
	printf("Returned number is: %d\n", my_atoi("34574"));
	printf("Returned number is: %d\n", my_atoi("-23"));

    return 0;
}