Tuesday, September 11, 2012

URL Decode in C ( http query string )

/*

* to decode http query_string ie to convert special characters in
* Query String to their original form like %20 to space %22 to  
* double-quote
*
* if 'str' contains  type=it%20is%20simple%20%22text%22
* ie when called as
* http://localhost/cgi-bin/a.out?type=it is simple "text"

* when used as cgi-bin with apache

* NOTE:
* for GET method there is environment variable set by apache QUERY_STRING
* for POST method, need to read from STDIN 
*
*/ 

int urlDecode(char *str);

int main(int argc, char *argv[])
{
    char *query_string;
    printf("Content-type: text/html\r\n\r\n"); /* required */
    query_string = getenv("QUERY_STRING");
    if (!query_string)
    {
        printf("QUERY_STRING not found\n"); return 1;
    }
    urlDecode(query_string);
    printf("text received: %s \n", query_string);
}

int urlDecode(char *str)
{
    unsigned int i;
    char tmp[BUFSIZ];
    char *ptr = tmp;
    memset(tmp, 0, sizeof(tmp));

    for (i=0; i < strlen(str); i++)
    {
        if (str[i] != '%')
        {
            *ptr++ = str[i];
            continue;
        }

        if (!isdigit(str[i+1]) || !isdigit(str[i+2]))
        {
            *ptr++ = str[i];
            continue;
        }

        *ptr++ = ((str[i+1] - '0') << 4) | (str[i+2] - '0');
        i += 2;
    }
    *ptr = '\0';
    strcpy(str, tmp);
    return 0;
}

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