View Source: MemoryMap - Waikato Linux Users Group

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The [Linux] VirtualMemory Map (as seen by a UserSpace program)<br>
<code>
<?plugin OldStyleTable
| __Starts at__|  __Contains__<br>
| ffffffff      |< End of the universe<br>
| ffffe000      |< vsyscall table (new in 2.5.x)<br>
| c0000000      |< Off limits, reserved for the kernel<br>
| bfffffff      |< Process stack (grows down)<br>
| bffff000      |< Process heap (grows up)<br>
| 40000000      |< Libraries<br>
| zzzzzzzz      |< Unused<br>
| yyyyyyyy      |< __.bss__, uninitialised program data<br>
| xxxxxxxx      |< __.data__ segment, initialised program data<br>
| 08048000      |< __.text__ segment, program code<br>
| 00000000      |< Unmapped to trap [NULL] pointers<br><br>
?>
</code>
"BSS" means __b__lock __s__tarted by __s__ymbol and is a segment of uninitialised that is only stored in the BinaryExecutable image as a length and offset, since it would otherwise waste space. The "text" segment on the other hand contains ''initialized'' global variables and ''is'' stored in the BinaryExecutable.

!!Practical examples
!Library-mapped memory (using ldd(1))
<verbatim>
 $ ldd /bin/ls
        librt.so.1 => /lib/librt.so.1 (0x40026000)
        libc.so.6 => /lib/libc.so.6 (0x40038000)
        libpthread.so.0 => /lib/libpthread.so.0 (0x4016a000)
        /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
</verbatim>

!Program code<br>
<verbatim>
  $ cat > x.c

  #include <stdio.h>
  int main(void) {printf("%p\n", main);return 0;}
  $ gcc -o x x.c && ./x
  0x8048344
  $
</verbatim>
(this is printing the address of the main function.)

!Process Heap
<verbatim>
  $ cat > x2.c

  #include <stdio.h>
  int main(void) {char c;printf("%p\n", &c); return 0;}
  $ gcc -o x2 x2.c && ./x2
  0xbffffab7
</verbatim>

!Process Data and bss segment

<verbatim>
 $ perl -e 'my $var; print \$var . "\n"'
  SCALAR(0x814f38c)<br>
</verbatim>
(note that this is the address in the [Perl] interpreter)

<verbatim>
  $ cat > x3.c

  #include <stdio.h>
  #include <stdlib.h>
  int main(void) {char *p=!malloc(3);printf("%p\n", p); return 0;}
  $ gcc -o x3 x3.c && ./x3
  0x8049628
</verbatim>
Also,
<verbatim>
 cat /proc/''pid''/maps
</verbatim>
gives you the memory map for a program :)
One page links to MemoryMap:
VirtualMemory
Last edited on Tuesday, March 20, 2007 12:56:45 pm by PerryLorier
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