C pointer format displaying useless characters-CodePudding

When trying to print a pointer, for example

#include <stdio.h>

main() {
    int a = 3;
    printf("The address : %p\n", &a);
    printf("As an hex   : %x\n", &a);
    return 0;
}

I get a 000000ac4f5ffd58 and the nice 4f5ffd58 (the first two characters mismatch is from unknown origin). It's just for comfort and beauty, the %x "works" to display it. Is there any way to have the correct (0x4f5ffd58) way to print my pointer (its format) ?

I'm using CLion (I resetted the settings), and here is the CMakeLists.txt associated with it :

cmake_minimum_required(VERSION 3.22)
project(untitled C)
set(CMAKE_C_STANDARD 99)
add_executable(untitled main.c)

CodePudding user response：

The "correct" way I was thinking of it the 0x4f5ffd58.

What is the "correct" part of it? The length? If you're on a 64bit system, a 32bit value is by no means "correct".

If you're looking for the 0x intro, you might want to try the "alternative" format (printf("The address : %#p\n", (void *)&a);). What exactly the standard and the alternative format might be is implementation-defined, but usually the alternative form provides the 0x intro.

CodePudding user response：

Seems the problem was in my expectation. In every C tutorial, the pointers is printed with the format 0x4f5ffd58, but of course, as I redid every calculation now, 8*4 only makes a 32-bit address, where my system is a 64-bit, thus the longer displayed address (which confused me because I was first printing with the 0x prefix, but no more, maybe due to an IDE update).

CodePudding user response：

You are using the conversion specifier x designed to output objects of the type unsigned int. Using it with a pointer invokes undefined behavior. Instead you could write for example

#include <stdio.h>
#include <inttypes.h>

int main( void ) 
{
    int a = 3;

    printf( "The address : %p\n", ( void * )&a );
    printf( "As an hex   : %#" PRIxPTR "\n", ( uintptr_t )&a );

    return 0;
}

The program output might look like

The address : 0x7ffe9bf5f9b8
As an hex   : 0x7ffe9bf5f9b8

Also bear in mind that according to the C Standard (7.21.6.1 The fprintf function)

p The argument shall be a pointer to void. The value of the pointer is converted to a sequence of printing characters, in an implementation-defined manner.

And pay attention to that according to the C Standard the function main without parameters shall be declared like

int main( void )