warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘int*’ [-Wformat=]

Question

While I'm compiling this simple program in GCC compiler I'm getting this error:- warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘int’ [-Wformat=]*

#include <stdio.h>
int main()
{
    printf("Pointer\n");
    printf("*******\n\n");
    int i=3;
    printf("Address of Variable i : %u",&i);
    printf("Value stored in Variable i : %d\n",i);
    printf("Value stored in Variable i : %d\n",*(&i));
    return 0;
}

I'm getting this kind of Address :- Address of Variable i : 0x7ffc836a8534 — Souvik Pal, Mar 26 '21 at 06:50
Well, hex is conventional for a pointer. It's a bit of a hack, but you might cast to unsigned long and print with %lu — T W Bennet, Mar 26 '21 at 06:55
Use `%z` https://stackoverflow.com/questions/2524611/how-can-one-print-a-size-t-variable-portably-using-the-printf-family — Alan Birtles, Mar 26 '21 at 07:42
@AlanBirtles No, wouldn't help here, `%z` is a modifier for int type `size_t` — Olaf Dietsche, Mar 26 '21 at 07:45
@OlafDietsche a pointer and `size_t` are the same size on the vast majority of platforms — Alan Birtles, Mar 26 '21 at 08:06

score 2 · Answer 1 · answered Mar 26 '21 at 07:43

Even without any extra option, gcc (9.3.0) shows a detailed warning message

a.cpp: In function 'int main()':
a.cpp:7:38: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'int*' [-Wformat=]
    7 |     printf("Address of Variable i : %u",&i);
      |                                     ~^  ~~
      |                                      |  |
      |                                      |  int*
      |                                      unsigned int
      |                                     %n

So there's a mismatch between the format %u (unsigned int) and the argument &i (pointer).

Looking at printf Conversion specifier

u converts an unsigned integer into decimal representation dddd.
...
p writes an implementation defined character sequence defining a pointer.

The proper format in this case would be %p for a pointer argument &i

printf("Address of Variable i : %p", &i);

warning: format ‘%u’ expects argument of type ‘unsigned int’, but argument 2 has type ‘int*’ [-Wformat=]

1 Answers1