int atoi(const char* str)

自己在实现字符串转整数时，要注意一下几个要点：

1. 正负号的处理
2. 非数字字符的处理
3. 溢出的处理 （难点）

下面给出一个实现（转自Nut/OS: c/stdlib/strtol.c)

/*
 * Copyright (C) 2004 by egnite Software GmbH. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY EGNITE SOFTWARE GMBH AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL EGNITE
 * SOFTWARE GMBH OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * For additional information see http://www.ethernut.de/
 *
 *-
 * Copyright (c) 1990 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * $Log$
 * Revision 1.4  2008/02/15 17:13:01  haraldkipp
 * Use configurable constant attribute.
 *
 * Revision 1.3  2006/10/08 16:48:08  haraldkipp
 * Documentation fixed
 *
 * Revision 1.2  2005/08/02 17:46:47  haraldkipp
 * Major API documentation update.
 *
 * Revision 1.1  2004/09/08 10:23:35  haraldkipp
 * Generic C stdlib added
 *
 */

#include 
#include 
#include 
#include 
#include 


long strtol(CONST char *nptr, char **endptr, int base)
{
    register CONST char *s;
    register long acc, cutoff;
    register int c;
    register int neg, any, cutlim;

    /*
     * Skip white space and pick up leading +/- sign if any.
     * If base is 0, allow 0x for hex and 0 for octal, else
     * assume decimal; if base is already 16, allow 0x.
     */
    s = nptr;
    do {
        c = (unsigned char) *s++;
    } while (isspace(c));
    if (c == '-') {
        neg = 1;
        c = *s++;
    } else {
        neg = 0;
        if (c == '+')
            c = *s++;
    }
    if ((base == 0 || base == 16) && c == '0' && (*s == 'x' || *s == 'X')) {
        c = s[1];
        s += 2;
        base = 16;
    }
    if (base == 0)
        base = c == '0' ? 8 : 10;

    /*
     * Compute the cutoff value between legal numbers and illegal
     * numbers.  That is the largest legal value, divided by the
     * base.  An input number that is greater than this value, if
     * followed by a legal input character, is too big.  One that
     * is equal to this value may be valid or not; the limit
     * between valid and invalid numbers is then based on the last
     * digit.  For instance, if the range for longs is
     * [-2147483648..2147483647] and the input base is 10,
     * cutoff will be set to 214748364 and cutlim to either
     * 7 (neg==0) or 8 (neg==1), meaning that if we have accumulated
     * a value > 214748364, or equal but the next digit is > 7 (or 8),
     * the number is too big, and we will return a range error.
     *
     * Set any if any `digits' consumed; make it negative to indicate
     * overflow.
     */
    cutoff = neg ? LONG_MIN : LONG_MAX;
    cutlim = cutoff % base;
    cutoff /= base;
    if (neg) {
        if (cutlim > 0) {
            cutlim -= base;
            cutoff += 1;
        }
        cutlim = -cutlim;
    }
    for (acc = 0, any = 0;; c = (unsigned char) *s++) {
        if (isdigit(c))
            c -= '0';
        else if (isalpha(c))
            c -= isupper(c) ? 'A' - 10 : 'a' - 10;
        else
            break;
        if (c >= base)
            break;
        if (any < 0)
            continue;
        if (neg) {
            if ((acc < cutoff || acc == cutoff) && c > cutlim) {
                any = -1;
                acc = LONG_MIN;
                errno = ERANGE;
            } else {
                any = 1;
                acc *= base;
                acc -= c;
            }
        } else {
            if ((acc > cutoff || acc == cutoff) && c > cutlim) {
                any = -1;
                acc = LONG_MAX;
                errno = ERANGE;
            } else {
                any = 1;
                acc *= base;
                acc += c;
            }
        }
    }
    if (endptr != 0)
        *endptr = (char *) (any ? s - 1 : nptr);
    return (acc);
}