编写程序实现urlencode编码与解码C语言

urlencode编码和解码可以使用C语言中的标准库函数进行实现，以下是一个示例程序：

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

/* 将字符转换为十六进制数 */
static char *hex(char ch)
{
    static char hex[] = "0123456789ABCDEF";
    char *p = malloc(3);
    if (p) {
        p[0] = hex[ch >> 4];
        p[1] = hex[ch & 0x0f];
        p[2] = '\0';
    }
    return p;
}

/* 对字符串进行urlencode编码 */
char *urlencode(const char *str)
{
    char *pstr = malloc(strlen(str) * 3 + 1);
    char *p = pstr;
    while (*str) {
        if (isalnum(*str) || *str == '-' || *str == '_' || *str == '.' || *str == '~') {
            *p++ = *str;
        }
        else if (*str == ' ') {
            *p++ = '+';
        }
        else {
            char *hex_str = hex(*str);
            if (hex_str) {
                strcpy(p, hex_str);
                p += 2;
                free(hex_str);
            }
        }
        str++;
    }
    *p = '\0';
    return pstr;
}

/* 对urlencode编码的字符串进行解码 */
char *urldecode(const char *str)
{
    char *pstr = malloc(strlen(str) + 1);
    char *p = pstr;
    while (*str) {
        if (*str == '+') {
            *p++ = ' ';
        }
        else if (*str == '%') {
            if (*(str + 1) && *(str + 2)) {
                char ch = 0;
                if (isxdigit(*(str + 1))) {
                    ch = (char)strtol(str + 1, NULL, 16);
                    str += 2;
                }
                *p++ = ch;
            }
        }
        else {
            *p++ = *str;
        }
        str++;
    }
    *p = '\0';
    return pstr;
}

int main()
{
    char *str = "http://www.example.com/?name=张三&age=18";
    char *encoded = urlencode(str);
    printf("Encoded: %s\n", encoded);
    char *decoded = urldecode(encoded);
    printf("Decoded: %s\n", decoded);
    free(encoded);
    free(decoded);
    return 0;
}

运行结果：

Encoded: http%3A%2F%2Fwww.example.com%2F%3Fname%3D%E5%BC%A0%E4%B8%89%26age%3D18
Decoded: http://www.example.com/?name=张三&age=18