C language to show the hex symbol from unsigned char pointer

Question

//How to conversion the number 12345 change to the unsigned char*(why is '90') and how to modify code to show the hex value (eg.0x80...)?

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

typedef unsigned char *byte_pointer;

void show_bytes(byte_pointer, size_t);
void show_bits(int);
void show_int(int);
void show_float(float);
void show_pointer(void *) ;
int  mask_LSbits(int);



void show_bytes(byte_pointer start, size_t len)
{
    size_t i;
    printf("address=%p",start);
    printf("\n");
    for (i = 0; I <len; i++)
    printf(" %x.2x", start[i]);  //modify here, how to change char to hex value
    printf("\n");
}
void show_int(int x) {
    printf("\nival = %d\n", x);
    show_bytes((byte_pointer) &x, sizeof(int));
} 
int main()
{
    int ival = 12345;
    float fval = (float) ival;
    int *pval = &ival;

    show_int(ival);

    return 0;

}

Answer 1

To display each of the four component bytes of the int value passed (after casting) to your show_bytes function (assuming you want these in the format 0x01), change your printf format string as follows:

printf(" 0x%02x", (unsigned int)start[i]);

The 0x is printed as is; the %02x formats the actual value of start[i] in hexadecimal, always using a field width of 2 and adding leading zeros, if required.

Note that the %x format expects an unsigned int as the corresponding argument, which is why I've added the cast.

The output I get when I run your code (with my suggested modification) is:

ival = 12345
address = 000000CE690FFA30
0x39 0x30 0x00 0x00

Which corresponds to an integer with the following (full) hex value: 0x00003039 - which is 12345, in decimal.

Note, also, that if you add the following line:

printf("address=%p", pval);

in your main function, you will see that the value of the pval address is not the same as the value printed in your show_bytes function! This is because your are passing a copy of ival to show_int, which is placed (most probably) on the stack, and thus is, effectively, a different variable from the original (which pval points to).