GDB中如何在$r1等于指定字符串时无条件触发断点？

Great question! Let's break this down so you can set that global conditional breakpoint in GDB exactly how you want.

First, let's clarify why your initial break [addr] if (...) approach has limits: that command only checks the condition when the program hits that specific address. So picking a single addr won't catch cases where $r1 holds your target string elsewhere in the code. To make it truly global—triggering no matter where the program is executing—we need a different strategy.

Option 1: Conditional Watchpoint (Best for Most Scenarios) #

Watchpoints are designed to monitor changes to registers or memory. We can set one on $r1 that only triggers when it points to your target string:

watch $r1 if (strcmp((char *)$r1, "hello") == 0)

• How this works: GDB will pause execution whenever $r1's value changes and the new value is a pointer to the string "hello".
• A quick note: This won't trigger repeatedly if $r1 stays pointing to "hello" (it only checks when the register's value updates). If you need to catch every single instruction where $r1 holds the string—even if it hasn't changed—use the next option.

Option 2: Instruction-Level Global Check (Full Coverage, But Slow) #

If you need to inspect $r1 at every single instruction (regardless of whether its value changed), you can set a breakpoint at the current program counter and use GDB's command system to auto-continue unless your condition is met:

1. Start the program at its first instruction to set the initial breakpoint:

   starti
   

2. Set a breakpoint at the current $pc and attach a command sequence:

   break *$pc
   commands
       if (strcmp((char *)$r1, "hello") == 0)
           echo Hit the global condition!\n
           # Leave the program paused here
       else
           # Auto-continue if the condition isn't met
           continue
       end
   end
   

• Heads up: This will slow down execution a lot, since GDB pauses at every single instruction to check the condition. Only use this for small code snippets or very specific debugging tasks.

Troubleshooting strcmp in GDB #

If you get an error about strcmp being undefined, make sure your program's debug symbols are loaded. If that's not an option, you can use a manual string comparison (it's less clean, but works):

watch $r1 if ((char *)$r1)[0] == 'h' && ((char *)$r1)[1] == 'e' && ((char *)$r1)[2] == 'l' && ((char *)$r1)[3] == 'l' && ((char *)$r1)[4] == 'o' && ((char *)$r1)[5] == '\0'

内容的提问来源于stack exchange，提问作者Neolex