musl/src/stdlib/atoi.c

#include <stdlib.h>
#include <ctype.h>

int atoi(const char *s)
{
	int n=0, neg=0;
	while (isspace(*s)) s++;
	switch (*s) {
	case '-': neg=1;
	case '+': s++;
	}
	while (isdigit(*s))
		n = 10*n + *s++ - '0';
	return neg ? -n : n;
}