Can I set a breakpoint on memory access in GDB

Question

I am running an application through gdb and I want to set a breakpoint for any time a specific variable is accessed   changed  Is there a good method for doing this  I would also be interested in other ways to monitor a variable in C C   to see if when it changes

User · Answer

What you re looking for is called a watchpoint   Usage    gdb  watch foo  watch the value of variable foo   gdb  watch   int  0x12345678  watch the value pointed by an address  casted to whatever type you want   gdb  watch a b   c d  watch an arbitrarily complex expression  valid in the program s native language  Watchpoints are of three kinds    watch  gdb will break when a write occurs rwatch  gdb will break wnen a read occurs awatch  gdb will break in both cases   You may choose the more appropriate for your needs   For more information  check this out

User · Answer

I just tried the following      cat gdbtest c  int abc   43    int main         abc   10        gcc -g -o gdbtest gdbtest c    gdb gdbtest        gdb  watch abc  Hardware watchpoint 1  abc   gdb  r  Starting program   home mweerden gdbtest         Old value   43  New value   10  main    at gdbtest c 6  6           gdb  quit   So it seems possible  but you do appear to need some hardware support

User · Answer

watch only breaks on write  rwatch let you break on read  and awatch let you break on read write   You can set read watchpoints on memory locations   gdb  rwatch  0xfeedface Hardware read watchpoint 2   0xfeedface   but one limitation applies to the rwatch and awatch commands  you can t use gdb variables in expressions   gdb  rwatch  ebx 0xec1a04f Expression cannot be implemented with read access watchpoint    So you have to expand them yourself    gdb  print  ebx   13   0x135700 gdb  rwatch  0x135700 0xec1a04f Hardware read watchpoint 3   0x135700   0xec1a04f gdb  c Hardware read watchpoint 3   0x135700   0xec1a04f  Value   0xec34daf 0x9527d6e7 in objc msgSend      Edit  Oh  and by the way  You need either hardware or software support  Software is obviously much slower  To find out if your OS supports hardware watchpoints you can see the can-use-hw-watchpoints environment setting    gdb  show can-use-hw-watchpoints Debugger s willingness to use watchpoint hardware is 1

User · Answer

Use watch to see when a variable is written to  rwatch when it is read and awatch when it is read written from to  as noted above  However  please note that to use this command  you must break the program  and the variable must be in scope when you ve broken the program      Use the watch command  The argument to the watch command is an   expression that is evaluated  This implies that the variabel you want   to set a watchpoint on must be in the current scope  So  to set a   watchpoint on a non-global variable  you must have set a breakpoint   that will stop your program when the variable is in scope  You set the   watchpoint after the program breaks

User · Answer

Assuming the first answer is referring to the C-like syntax  char    0x135700  0xec1a04f  then the answer to do rwatch  0x135700 0xec1a04f is incorrect  The correct syntax is rwatch   0x135700 0xec1a04f    The lack of   s there caused me a great deal of pain trying to use watchpoints myself

[c++] Can I set a breakpoint on 'memory access' in GDB?

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