osdev/libc/ctype.c

#include <ctype.h>

int atoi(const char *s) {
    int n = 0, neg = 0;
    while (isspace(*s)) s++;
    switch (*s) {
        case '-':
            neg = 1;
        case '+':
            s++;
    }
    /* Compute n as a negative number to avoid overflow on INT_MIN */
    while (isdigit(*s))
        n = 10 * n - (*s++ - '0');
    return neg ? n : -n;
}