C语言实现URLDecoder功能

C语言实现URLDecoder功能可以使用以下代码：

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

char* urldecode(const char*);
char hex2char(const char*);

int main() {
    char url[] = "http%3A%2F%2Fwww.google.com%3Fq%3Dc%2B%2B";
    char* decoded = urldecode(url);
    printf("%s\n", decoded);
    free(decoded);
    return 0;
}

char* urldecode(const char* url) {
    size_t len = strlen(url);
    char* decoded = (char*)malloc(len + 1);
    int j = 0;
    for (int i = 0; i < len; i++) {
        if (url[i] == '%') {
            decoded[j++] = hex2char(&url[i+1]);
            i += 2;
        } else if (url[i] == '+') {
            decoded[j++] = ' ';
        } else {
            decoded[j++] = url[i];
        }
    }
    decoded[j] = '\0';
    return decoded;
}

char hex2char(const char* hex) {
    char ch = 0;
    for (int i = 0; i < 2; i++) {
        ch <<= 4;
        if (hex[i] >= '0' && hex[i] <= '9') {
            ch |= hex[i] - '0';
        } else if (hex[i] >= 'a' && hex[i] <= 'f') {
            ch |= hex[i] - 'a' + 10;
        } else if (hex[i] >= 'A' && hex[i] <= 'F') {
            ch |= hex[i] - 'A' + 10;
        }
    }
    return ch;
}

上述代码中，urldecode函数将编码过的URL字符串url解码成普通的字符串，并返回解码后的字符串。hex2char函数将两个十六进制字符转换成一个字符。在main函数中，我们调用urldecode函数对URL进行解码，并打印出解码后的字符串。