校验IP地址的合法性是一个常见需求，今天用C语言来实现一下，欢迎指正。
#include <string.h> #include <stdio.h> #include <stddef.h> char *is_valid_dotted_decimal(char *p, int is_first, int is_last, int *pvalid) { //printf("input dotted decimal: %s\n", p); if (!is_first) { if (p[0] != '.') { printf("first char for dotted decimal should be '.'!\n"); goto RET_INVALID; } p++; } char tmp[4] = {0, 0, 0, 0}; int j = 0; while (p[j] && p[j] != '.' && j < 3) { tmp[j] = p[j]; j++; } //printf("parsed dotted decimal block: %s\n", tmp); if (j == 0) { printf("dotted decimal length should not be zero!\n"); goto RET_INVALID; } if (is_last && p[j]) { printf("last dotted decimal length should be less 4!\n"); goto RET_INVALID; } int decimal = atoi(tmp); if (tmp[0] == '0' && decimal != 0) { printf("dotted decimal should not have redundant zero!\n"); goto RET_INVALID; } if (!(decimal >= 0 && decimal <= 255)) { printf("dotted decimal should between 0 and 255!\n"); goto RET_INVALID; } *pvalid = 1; return p + j; RET_INVALID: *pvalid = 0; return NULL; } int is_valid_ipv4(char *ip) { if (ip == NULL) { return 0; } int len = strlen(ip); if (len < 7 || len > 15) { printf("invalid length for ipv4!\n"); return 0; } int i = 0, j = 0; for (i = 0; i < len; ++i) { if (!((ip[i] >= '0' && ip[i] <= '9') || ip[i] == '.')) { printf("invalid char for ipv4!\n"); return 0; } } int is_valid = 0; char *p = ip; for (i = 0; i < 4; ++i) { p = is_valid_dotted_decimal(p, (i == 0), (i == 3), &is_valid); if (!is_valid) { printf("invalid dotted decimal for ipv4 in %d-th block!\n", (i + 1)); return 0; } } return 1; }
