自己在实现字符串转整数时，要注意一下几个要点：
- 正负号的处理
- 非数字字符的处理
- 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 <compiler.h>
#include <ctype.h>
#include <errno.h>
#include <limits.h>
#include <stdlib.h>
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);
}
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