检查给定字符串是否可以是有效 IP 地址的建议/帮助/更好的解决方案

你有很多松散的 47、48 等值'0'。最好使用后一种语法。

有许多if范围检查。使用一些额外的状态变量可以降低复杂性。

inputString[i]到处使用很麻烦。更好的做法（例如int chr = inputString[i];，改用chr它——它更简单、更易于阅读）。

原始程序错误识别为：

192.168.0.1
280.100.92.101

我重构了代码并构建了一个诊断测试程序。注释为：

#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>

#define ERR(_expr) \
    if (err != NULL) \
        break; \
    if (_expr) { \
        err = #_expr; \
        break; \
    }

const char *bigerr;
int opt_i;

bool
isfix1(const char *inputString)
{
    int i = 0;
    int ndig = -1;
    int ndot = 0;
    int val = 0;
    int leading_zero = 0;
    unsigned char ipv[4];
    const char *err = NULL;

    while (1) {
        int chr = inputString[i++];
        int eos = (chr == 0);

        // check the last number we got
        if ((chr == '.') || eos) {
            ERR(ndig == 0);

            // we don't allow (because it might be decoded as octal):
            //   x.00.y.z x.01.y.z
            // but this must be ok:
            //   x.0.y.z
            ERR(leading_zero && (ndig > 1));

            // we can only have 3 dots in the string
            ndot += (! eos);
            ERR(ndot > 3);

            ndig = 0;
            val = 0;

            if (eos) {
                // we must have _exactly 3 dots in the string
                ERR(ndot != 3);
                break;
            }
            continue;
        }

        // collect digits
        if ((chr >= '0') && (chr <= '9')) {
            val *= 10;
            chr -= '0';
            val += chr;

            if (ndig < 0)
                ndig = 0;

            // remember whether substring started with "0"
            if (ndig == 0)
                leading_zero = (chr == 0);

            // we can only have up to 3 digits per byte
            ++ndig;
            ERR(ndig > 3);

            // max value for byte
            ERR(val > 255);

            // just for fun -- the binary IP address
            ipv[ndot] = val;
            continue;
        }

        // any invalid character
        ERR(chr != 0);
    }

    if (0)
        printf("IPV: %d.%d.%d.%d\n",ipv[0],ipv[1],ipv[2],ipv[3]);

    bigerr = err;

    return (err == NULL);
}

bool
isorig(const char *inputString)
{
    int count = 0,
        period = 0;
    int k = 0,
        i = 0;
    int digit = 0;

    while (inputString[i] != '\0') {
        count = 0;
        digit = 0;
        i = k;

        if (inputString[i] == '0' || inputString[i] == '.') {
            if (inputString[i + 1] > 47 && inputString[i + 1] < 58) {
                return false;
            }
        }

        while (inputString[i] != '.' && inputString[i] != '\0') {
            if (inputString[i] < 48 || inputString[i] > 57) {
                return false;
            }
            count += (int) inputString[i];
            i++;
            digit++;
        }

        if (digit == 3) {
            if (inputString[i - 3] > '2') {
                return false;
            }
        }

        if (inputString[i] == '.') {
            period++;
        }

        k = i + 1;

        if (count < 48 || count > 156) {
            return false;
        }

        if (inputString[i] == '\0') {
            break;
        }
    }

    if (period == 3) {
        return true;
    }
    else {
        return false;
    }
}

int
dofnc(bool (*fnc)(const char *),const char *ipaddr,const char *reason)
{
    int valid;

    bigerr = NULL;

    valid = fnc(ipaddr);
    printf("%s",valid ? "valid" : "INVALID");

    if (bigerr != NULL)
        printf(" (%s)",bigerr);

    printf(" (%s)\n",(valid != (reason == NULL)) ? "FAIL!" : "pass");

    return valid;
}

void
dotest(const char *ipaddr,const char *reason)
{
    static int sep = 0;

    if (sep)
        printf("\n");
    sep = 1;

    printf("IPADDR: %s",ipaddr);
    if (reason != NULL)
        printf(" -- %s",reason);
    printf("\n");

    int valid[2];
    printf("isorig: ");
    valid[0] = dofnc(isorig,ipaddr,reason);
    printf("isfix1: ");
    valid[1] = dofnc(isfix1,ipaddr,reason);

    do {
        if (opt_i)
            break;

        for (int idx = 0;  idx < 2;  ++idx) {
            if (valid[idx] != (reason == NULL))
                exit(1);
        }
    } while (0);
}

int
main(int argc,char **argv)
{

    --argc;
    ++argv;

    for (;  argc > 0;  --argc, ++argv) {
        char *cp = *argv;
        if (*cp != '-')
            break;

        cp += 2;
        switch (cp[-1]) {
        case 'i':
            opt_i = ! opt_i;
            break;
        }
    }

    dotest("192.168.0.1",NULL);
    dotest("255.255.255.255",NULL);
    dotest("280.100.92.101","280 is too large to be an 8-bit integer");
    dotest("255.100.81.160.172","contains 5 integers instead of 4");
    dotest("1..0.1","not properly formatted");
    dotest("1.0.1.","not properly formatted");
    dotest("17.233.00.131","contain leading zeros");
    dotest("17.233.01.131","contain leading zeros");

    return 0;
}

这是程序输出：

IPADDR: 192.168.0.1
isorig: INVALID (FAIL!)
isfix1: valid (pass)

IPADDR: 255.255.255.255
isorig: valid (pass)
isfix1: valid (pass)

IPADDR: 280.100.92.101 -- 280 is too large to be an 8-bit integer
isorig: valid (FAIL!)
isfix1: INVALID (val > 255) (pass)

IPADDR: 255.100.81.160.172 -- contains 5 integers instead of 4
isorig: INVALID (pass)
isfix1: INVALID (ndot > 3) (pass)

IPADDR: 1..0.1 -- not properly formatted
isorig: INVALID (pass)
isfix1: INVALID (ndig == 0) (pass)

IPADDR: 1.0.1. -- not properly formatted
isorig: INVALID (pass)
isfix1: INVALID (ndig == 0) (pass)

IPADDR: 17.233.00.131 -- contain leading zeros
isorig: INVALID (pass)
isfix1: INVALID (leading_zero && (ndig > 1)) (pass)

IPADDR: 17.233.01.131 -- contain leading zeros
isorig: INVALID (pass)
isfix1: INVALID (leading_zero && (ndig > 1)) (pass)