指针的值不是地址？

Question

I have a CGI script (C program test.elf ) that gets called by GET method. This means that data is appended to url address that points to the same CGI script. For example:

http://.../cgi/c/test.elf?m=20&n=2000

Here we send data set m=20&n=2000 which is sepparated from the rest of the url with a question mark ? . Single variables in a data set are also sepparated with & . So actually we are sending m=20 and n=2000 .

CGI interface automatically stores sent data set m=20&n=2000 in an enviromental variable QUERY_STRING for the time of the call and I want my CGI script to first print it's (a) value and (b) address.

This is the CGI script:

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

int main(void){

    // getenv() returns a pointer to character (char*) and we need same type (char*) to store it.
    char* data;

    // We need an "array of characters" which is actualy the same as "string of characters".
    // Because strcpy() can only take pointer to the start of the array we also define another 
    // pointer and point it to array.
    char stored_enviromental_variable[1000];
    char* pointer;
    pointer = &stored_enviromental_variable[0];

    // ASCII HTML header
    printf("content-type:text/html; charset=utf-8\n\n");

    printf("<h1>Multiplication result</h1>");

    // Pointer now points to enviromental variable.
    data = getenv("QUERY_STRING");

    // We store the value of pointer
    strcpy(pointer, data);

    // We print some information.
    printf("%s: %s<br>", "Value of the \"QUERY_STRING\"", stored_enviromental_variable);
    printf("%s: %x<br>", "Address of the \"QUERY_STRING\"", data);

}

This compiles with a warning:

[ziga@localhost ~]$ gcc -Wpedantic -std=c18 -Wall -o test.elf test.c 
test.c: In function ‘main’:
test.c:42:15: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘char *’ [-Wformat=]
  printf("%s: %x<br>", "Address of the \"QUERY_STRING\"", data);
              ~^                                          ~~~~
              %s

So it looks like this source code is causing the problem:

printf("%s: %x<br>", "Address of the \"QUERY_STRING\"", data);

Which is weird to me, because I thought that "pointer always stores location" . So, why does data store a character? How can I remove the warning?

---

If I call the CGI script in the browser where I get this:

Answer 1

As the error message states, the %x format specifier to printf expects an unsigned int argument, but you are instead passing in data whose type is char * .

To print a pointer value you should use the %p format specifier. Also, the argument must be casted to void * :

printf("%s: %p<br>", "Address of the \"QUERY_STRING\"", (void *)data);