Convert String to Integer

Code

#include <stdio.h>
#include <stdint.h>
int custom_atoi(const char *str) 
{
    int i,sum=0;
    if(str[0]=='-')
    {
        for(i=1;str[i];i++)
        {
            if(str[i]>='0' && str[i]<='9')
            {
                sum=(sum*10)+(str[i]-'0');
            }
            else
            {
                return -sum;
            }
        }
        return -sum;
    }
    else if((str[0]>='0' && str[0]<='9') || (str[0]=='+'))
    {
        if(str[0]=='+')
        {
            for(i=1;str[i];i++)
            {
                if(str[i]>='0' && str[i]<='9')
                {
                    sum=(sum*10)+(str[i]-'0');
                }
                else
                {
                    return sum;
                }
            }
            return sum;
        }
        else if(str[0]>='0' && str[0]<='9')
        {
            for(i=0;str[i];i++)
            {
                if(str[i]>='0' && str[i]<='9')
                {
                    sum=(sum*10)+(str[i]-'0');
                }
                else
                {
                    return sum;
                }
            }
            return sum;
        }
    }
    else
        return 0;
}

int main() 
{
    char str[101];
    fgets(str, sizeof(str), stdin);
    uint8_t i = 0;
    while (str[i]) 
    {
        if (str[i] == '\n') {
            str[i] = '\0';
            break;
        }
        i++;
    }
    printf("%d", custom_atoi(str));
    return 0;
}

Solving Approach