How to programmatically break into gdb from gcc C++ source

cj12345 · 09-15-2004, 06:19 AM

When I'm developing a C++ program and debugging under gdb, I want to be able to get the program to fall into gdb.

In MS Visual Studio you can do this from a C++ app using asm { int 3 }

How, if at all, can you do this with gcc/gdb ?

rjlee · 09-15-2004, 08:16 AM

There is a way to attach the debugger process explicitly (I've seen it done but have no idea how to do it).

If you're starting from the debugger, then you can signal() your own process with SIGINT to interrupt it and return control to the debugger, effectively a breakpoint. That won't work very well if you're doing signal-handling though. See the signal manpage for more.

cj12345 · 09-15-2004, 09:05 AM

Basically what I want is for the program to cause a break into the debugger. E.g. is MS Visual Studio I can have a function like this:

void call_debugger()
{
_asm{ int 3 };

}

which breaks into the debugger. Ie has the same effect as a breakpoint set in the debugger itself.

This is incredibly useful and I cant believe this is not possible in gcc/gdb, yet I havent found out how.

rjlee · 09-15-2004, 09:09 AM

Sending a signal of SIGINT to your own PID will do this. You just need to run the program from inside gdb initially, with the run command.

slackie1000 · 09-15-2004, 02:42 PM

hi there,

from the top of my head:

you start the program in gdb :

Code:

run

then you make a break at the line that you would like to break

Code:

 break $line_nr

then you run again

Code:

run

everything inside gdb

i think it works.
sorry i have no ressources to test it.

regards

slackie1000

ps. if i misunderstood your question is due the lack of english knowledge.

jim mcnamara · 09-15-2004, 03:52 PM

It's a one-line in any Unix
try:

Code:

#include <signal.h>

#define BREAK_HERE raise(SIGINT)

int main(int argc, char *argv[]){
	  int i,j;
	  for(j=0,i=0;i<20;i++)
	  {
	  	j+=i;
	  	if(j>20) 
	  	     BREAK_HERE;
	  }
	  return 0;
}

at the command line:

Code:

gdb mycode
gdb> run
Program received signal SIGINT, Interrupt.
0x705b28a0 in kill () from /usr/lib/libc.2
... you are now in the debugger right after the BREAK_HERE statement

You will have to step twice to come back out of the raise() call into your code.

cj12345 · 09-15-2004, 05:23 PM

Thanks - just what I wanted. Knew it must be possible somehow.

cj12345 · 09-16-2004, 05:15 AM

Just a note on this. I'm using ddd, and I find that after the SIGINT, I have to use 'stepi' 4 times (ie step 4 instructions), and then 'step' once (i.e step to next source line). This gets ddd synchronised with the source line.

Otherwise I get "Cannot find bounds of current function" errors in ddd.

Would be nice if there was a way to get ddd to go straight to the offending source line on the SIGINT, but I can live with this.