hex values/variables location

well ive been into the stack the last few days, hi!

here is an example i picked up from the web:

Code:

#include <stdio.h> int main(int argc, char **argv) { char *somevar; char *important; char *temp; somevar=(char *)malloc(sizeof(char)*4); important=(char *)malloc(sizeof(char)*14); strcpy(important, "command"); strcpy(somevar, argv[1]); printf("%p\n%p\n", somevar, important); printf("Starting To Print memory address:\n"); temp = somevar; while(temp < important + 14) { /* this loop will be broken when we get to the last memory address we want, last memory address of important variable */ printf("%p: %c (0x%x)\n", temp, *temp, *(unsigned int*)temp); temp++; } return 0; }

the important function i dont understand, or cant figure out, is the printf() call.

first off: we declared temp to be a pointer to type char.
so what is printf doing when called that way?

Code:

printf("%p: %c (0x%x)\n", temp, *temp, *(unsigned int*)temp);

i could imagine that %p is the pointer address.(the memory location in the stack?)
%c is used with *temp (a pointer to an array of type char?), what does that do?
and yes last but not least, there is that %x.
does %x print the hex values of the given variable? (without preceding "0x"?)

here are my questions listed: :D

1. i could imagine that %p is the pointer address.(the memory location in the stack?)

2.%c is used with *temp (a pointer to an array of type char?), what does that do?

3.does %x print the hex values of the given variable? (without preceding "0x"?)

thank you very much!!

#1. %p prints the address of the variable as you guessed

#2. %c prints a character which is currently being pointed to by temp

#3. Yes %x prints the hex value of the variable.

first off: thanks for your quick response!

Code:

#include <stdio.h> int main (void) { char *temp; printf("memory location of temp: %p\n", temp); return 0; }

when i execute that program it shows me the memory address of the temp pointer.
its 0x2804b5a9 for me.

but when i run gdb for 1 and "disass main", the memory location printed before doesnt show up anywhere.
whats the problem with that?

temp is uninitialised, so its value is unpredictable.

Don't forget to use free to uninitialize any memory that has been used from malloc or calloc.

thanks for the suggestion.

Quote:

Originally posted by Hammer
temp is uninitialised, so its value is unpredictable.

here my fixed code:

Code:

#include <stdio.h> int main (void) { char *temp; char buffer = 'A'; temp = &buffer; printf("location of temp: %p\n", temp); return 0; }

temp isnt anymore uninitialised.
i get a value like: 0xbfbffbe7.
when i run gdb on the executable again i cannot find the above value.

whats that all about?
thanks

See what results you get with this.

Code:

#include <stdio.h> int main (void) { char *temp; char buffer = 'A'; temp = &buffer; printf("location of buffer: %p\n", (void*)&buffer); printf(" value of buffer: '%c'\n", buffer); printf("location of temp: %p\n", (void*)&temp); printf(" value of temp: %p\n", (void*) temp); printf(" value of *temp: '%c'\n", *temp); return 0; } /* my output location of buffer: 0012FF87 value of buffer: 'A' location of temp: 0012FF88 value of temp: 0012FF87 value of *temp: 'A' */

my problem is still persisting. :(

well i am not quite sure that i use gdb the right way.

Code:

%gcc -o test test.c %./test location of buffer: 0xbfbffbdb value of buffer: 'A' location of temp: 0xbfbffbdc value of temp: 0xbfbffbdb value of *temp: 'A' %gdb test GNU gdb 4.18 (FreeBSD) Copyright 1998 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "i386-unknown-freebsd"...(no debugging symbols found)... (gdb) disass main Dump of assembler code for function main: 0x8048494 <main>: push %ebp 0x8048495 <main+1>: mov %esp,%ebp 0x8048497 <main+3>: sub $0x18,%esp 0x804849a <main+6>: movb $0x41,0xfffffffb(%ebp) 0x804849e <main+10>: lea 0xfffffffb(%ebp),%eax 0x80484a1 <main+13>: mov %eax,0xfffffffc(%ebp) 0x80484a4 <main+16>: add $0xfffffff8,%esp 0x80484a7 <main+19>: lea 0xfffffffb(%ebp),%eax 0x80484aa <main+22>: push %eax 0x80484ab <main+23>: push $0x804854b 0x80484b0 <main+28>: call 0x8048358 <printf> 0x80484b5 <main+33>: add $0x10,%esp 0x80484b8 <main+36>: add $0xfffffff8,%esp 0x80484bb <main+39>: movsbl 0xfffffffb(%ebp),%eax 0x80484bf <main+43>: push %eax 0x80484c0 <main+44>: push $0x8048563 0x80484c5 <main+49>: call 0x8048358 <printf> 0x80484ca <main+54>: add $0x10,%esp 0x80484cd <main+57>: add $0xfffffff8,%esp 0x80484d0 <main+60>: lea 0xfffffffc(%ebp),%eax 0x80484d3 <main+63>: push %eax 0x80484d4 <main+64>: push $0x804857d 0x80484d9 <main+69>: call 0x8048358 <printf> 0x80484de <main+74>: add $0x10,%esp 0x80484e1 <main+77>: add $0xfffffff8,%esp 0x80484e4 <main+80>: mov 0xfffffffc(%ebp),%eax 0x80484e7 <main+83>: push %eax 0x80484e8 <main+84>: push $0x8048595 0x80484ed <main+89>: call 0x8048358 <printf> 0x80484f2 <main+94>: add $0x10,%esp 0x80484f5 <main+97>: add $0xfffffff8,%esp 0x80484f8 <main+100>: mov 0xfffffffc(%ebp),%eax 0x80484fb <main+103>: movsbl (%eax),%edx 0x80484fe <main+106>: push %edx 0x80484ff <main+107>: push $0x80485ad 0x8048504 <main+112>: call 0x8048358 <printf> 0x8048509 <main+117>: add $0x10,%esp 0x804850c <main+120>: xor %eax,%eax 0x804850e <main+122>: jmp 0x8048510 <main+124> 0x8048510 <main+124>: leave 0x8048511 <main+125>: ret 0x8048512 <main+126>: mov %esi,%esi End of assembler dump.

am i looking in the right place?

I'm not sure I understand your problem. Unless it is that you are looking for run-time information at compile time (like trying to find the contents of a register in the generated assembly).

Code:

%gcc -o test test.c %./test location of buffer: 0xbfbffbdb value of buffer: 'A' location of temp: 0xbfbffbdc value of temp: 0xbfbffbdb value of *temp: 'A' %gdb test

This looks correct to me. What is the problem you see?

i wanted to print the memory location of a specific variable, is it possible to find that memory location with gdb again?

thanks