In the book authored by K.N.King, there's this example:
viewmemory.c
/* Allows the user to view regions of computer memory */
#include <stdio.h>
#include <ctype.h>
typedef unsigned char BYTE;
int main(void)
{
unsigned int addr;
int i, n;
BYTE *ptr;
printf("Address of main function: %x\n", (unsigned int) main);
printf("Address of addr variable: %x\n", (unsigned int) &addr);
printf("\nEnter a (hex) address: ");
scanf("%x", &addr);
printf("Enter number of bytes to view: ");
scanf("%d", &n);
printf("\n");
printf(" Address Bytes Characters\n");
printf(" ------- ------------------------------- ----------\n");
ptr = (BYTE *) addr;
for (; n > 0; n -= 10) {
printf("%8X ", (unsigned int) ptr);
for (i = 0; i < 10 && i < n; i++)
printf("%.2X ", *(ptr + i));
for (; i < 10; i++)
printf(" ");
printf(" ");
for (i = 0; i < 10 && i < n; i++) {
BYTE ch = *(ptr + i);
if (!isprint(ch))
ch = '.';
printf("%c", ch);
}
printf("\n");
ptr += 10;
}
return 0;
}
For some reason, when I try to enter addr variable address as the parameter, it has a segmentation fault error. However, in the book's example and the screenshot from this site in Hangul, there's no such error?
When I try using gdb to check the issue, here's what I get:
gdb
$ gdb ./a.out --silent
Reading symbols from ./a.out...
(gdb) run
Starting program: /home/<username>/Desktop/c-programming-a-modern-approach/low-level-programming/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/gnu/store/zvlp3n8iwa1svxmwv4q22pv1pb1c9pjq-glibc-2.39/lib/libthread_db.so.1".
Address of main function: 401166
Address of addr variable: ffffd678
Enter a (hex) address: ffffd678
Enter number of bytes to view: 64
Address Bytes Characters
------- ------------------------------- ----------
Program received signal SIGSEGV, Segmentation fault.
0x000000000040123a in main () at viewmemory.c:31
warning: Source file is more recent than executable.
31 printf ("%.2X ", *(ptr + i));
(gdb)
What is going on? By the way, I am using Guix, if that matters in any way. Here's the output for ldd:
ldd
$ ldd ./a.out
linux-vdso.so.1 (0x00007ffecdda9000)
libgcc_s.so.1 => /gnu/store/w0i4fd8ivrpwz91a0wjwz5l0b2ralj16-gcc-11.4.0-lib/lib/libgcc_s.so.1 (0x00007fcd2627a000)
libc.so.6 => /gnu/store/zvlp3n8iwa1svxmwv4q22pv1pb1c9pjq-glibc-2.39/lib/libc.so.6 (0x00007fcd2609c000)