atoi.c

#include <stdio.h>
#include <limits.h>     /* for LONG_MAX, INT_MAX */
#include <stdlib.h>
#include <stdbool.h>

#define IS_ASCII_DIGIT(c) (((c >= 48) && (c <= 57)))

long __my_atoi(char* buffer)
{
        if(buffer == NULL) {

                return 0;

        }

        long ret = 0;

        bool neg = false;

        if(*buffer == '-') {
                neg = true;
                buffer++; /* advance to next position to pass ascii check */
        }

        while(*buffer) {

                if(IS_ASCII_DIGIT(*buffer)) {

                        ret = ret * 10 + (*buffer - '0');

                } else {

                        fprintf(stderr, "Fatal Error: unexpected '%c' passed to %s\n", *buffer, __func__);

                        exit(EXIT_FAILURE);
                }

                buffer++;

        }

        return neg ? -ret : ret;
}

int main()
{
        printf("INT_MAX=%d\n", INT_MAX);
        printf("LONG_MAX=%ld\n", LONG_MAX);
        printf("sizeof(long long)=%zu\n", sizeof(long long));
        printf("%ld\n", __my_atoi("-10004"));

        return EXIT_SUCCESS;
}