Question

I don’t understand why in the url_split function I can use a++, but in the main function I can’t use key_value++, they have the same type…

void url_split(char *src, char **host, char *a[])    
{
    char const *p1 = "?";
    char const *p2 = "&";

    *host = strtok(src, p1);

    char *str;

    while((str = strtok(NULL, p2))) 
    {
        *a = str;
        a++;   
    }

    *a = str;
}

int main(int argc, char *argv[])    
{
    char *host;
    char *key_value[100];
    char url[] = "http://www.baidu.com/s?wd=linux&cl=3";

    url_split(url, &host, key_value);

    printf("host = %s\n", host);

    while(*key_value)
    {
        printf("key-value : %s\n", *key_value);
        key_value++; 
    }

    return 0;
}

Answer 1

They actually don’t have the same type:

• key_value is of type char *[100], an array of 100 char pointers.
• a is actually of type char **, a pointer to a char*, not an array.

When you pass key_value to url_split it decays to a char**, which is why key_value is a valid argument to the function and why you use char** as the type of a function argument that is intended to be an array of char*.

The post-increment operator is incompatible with arrays, an array can’t be assigned a new value, but it works perfectly fine for pointers, since they can be assigned a new value. That’s why a++ is valid and key_value++ is not.