Why I can't print out environment variables in gdb?

Question

#include <unistd.h>
#include <stdio.h>

extern char **environ;
int main(int argc, char *argv[]) { 
  int i = 0;
  while(environ[i]) {
    printf("%s\n", environ[i++]);
  }
  return 0;
}

Here's my ops:

(gdb) n
8       printf("%s\n", environ[i++]);
(gdb) p environ[i]
Cannot access memory at address 0x0
(gdb) n
LOGNAME=root
7     while(environ[i]) {

As you can see,printf can print out environ[i],but p environ[i] gives me Cannot access memory at address 0x0,why?

Answer 1

gdb resolves the wrong environ symbol. I don't know why though. See below as to why.

But you can test it. Change the program to:

#include <unistd.h>
#include <stdio.h>

extern char **environ;
int main(int argc, char *argv[]) {
  int i = 0;
  printf("%p\n", &environ);
  while(environ[i]) {
    printf("%s\n", environ[i++]);
  }
  return 0;
}

Now let's run this in the debugger.

(gdb) n
7         printf("%p\n", &environ);
(gdb) n
0x8049760
8         while(environ[i]) {
(gdb) p &environ
$1 = (char ***) 0x46328da0
(gdb)

So. The actual program has, during its linking, resolved environ to the address 0x8049760. When gdb wants to access the environ symbol, it resolves to 0x46328da0, which is different.

Edit. It seems your environ symbol is actually linked to the environ@@GLIBC_2.0 symbol. In gdb write this:

(gdb) p environ

And hit the tab key (twice), it'll autocomplete the symbols. Which yields:

(gdb) p environ
environ             environ@@GLIBC_2.0

environ@@GLIBC_2.0 is the one actually linked to the extern char **environ

Printing this yields the same address as the program sees, 0x8049760:

(gdb) p &'environ@@GLIBC_2.0'
$9 = ( *) 0x8049760
(gdb) p ((char**)'environ@@GLIBC_2.0')[i]
$10 = 0xbffff6ad "XDG_SESSION_ID=1"

So, at one point glibc deprecated the environ symbol, and added a newer version