Re: gdb breakpoint problem

[Jim Blandy <jimb@codesourcery.com>]

kdsfinger@gmail.com writes:
> hi, all
> I am new to gdb and have trouble in setting the breakpoint. The
> problem is, the program does not stop at some of the breakpoints. I am
> using gdb6.6 in centos4.4. Here is the detail.
>
> ===================================
> /* sample.c -- Sample C program to be debugged with ddd
> */
> #include <stdio.h>
> #include <stdlib.h>
> static void shell_sort(int a[], int size){
> 	int i, j;	
> 	int h = 1;
> 	do {
> 		h = h * 3 + 1;
> 	} while (h <= size);
> 	do {
> 		h /= 3;
> 		for (i = h; i < size; i++){
> 			int v = a[i];
> 			for (j = i; j >= h && a[j - h] > v; j -= h)
> 				a[j] = a[j - h];
> 			if (i != j)
> 			a[j] = v;
> 		}
> 	} while (h != 1);
> }
>
> int main(int argc, char *argv[]){
> 	int *a;  //line 24
> 	int i; //line 25
> 	a = (int *)malloc((argc - 1) * sizeof(int)); // line 26
> 	for (i = 0; i < argc - 1; i++)
> 		a[i] = atoi(argv[i + 1]);
> 	shell_sort(a, argc);
> 	for (i = 0; i < argc - 1; i++)
> 		printf("%d ", a[i]);
> 	printf("\n");
> 	free(a);
> 	return 0;
> }
> ========================================
> gcc -g -o sample sample.c
>
> gdb sample
> GNU gdb 6.6
> Copyright (C) 2006 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public License, and you are
> welcome to change it and/or distribute copies of it under certain conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show warranty" for details.
> This GDB was configured as "i686-pc-linux-gnu"...
> Using host libthread_db library "/lib/tls/libthread_db.so.1".
> (gdb) break 24
> Breakpoint 1 at 0x804850b: file sample.c, line 24.
> (gdb) info break 1
> Num Type           Disp Enb Address    What
> 1   breakpoint     keep y   0x0804850b in main at sample.c:24
> (gdb) run 2 4 6 3 5 7
> Starting program: /home/amdx2/drive2/cpp/testddd/sample 2 4 6 3 5 7
>
> Breakpoint 1, main (argc=7, argv=0xbff515c4) at sample.c:26
> 26              a = (int *)malloc((argc - 1) * sizeof(int));
>
> -------------------------
> As you can see, the program did not stop at the breakpoint on line24.
> It stops at line26. If I add another breakpoint on line25, this 2nd
> breakpoint is skipped as well and the program stops at line26. Why is
> that? How can I correct it?

First, thanks for providing all the details in your question.  Giving
enough information to allow a reader to reproduce the problem
themselves is the right approach.

What's probably going on is that there is no machine code generated
for lines 24 and 25.  They're just declarations.  The first actual
machine instruction in main is for line 26, so the 'addresses' of lines
24, 25, and 26 are probably all the same.

You can try setting three breakpoints on lines 24, 25, and 26, and
then comparing the addresses printed for each to see if this is the
case.

> On my own program which is rather big, it got the same problem. When I
> use info break xxx, it sometimes point to a different place in "What".
> For example, I placed the breakpoint in main line 100, but the "what"
> points to other functions in different line. As the result, these
> breakpoints never work. I changed my gcc flags with combinations of
> -glevel -olevel, the problem did not solve. Thanks for your comments.

Have you tried compiling with no optimization at all (-O0)?  When
optimization is enabled, the compiler may inline functions, rearrange
code, or remove it altogether.  The debugging information often
doesn't adequately describe these rearrangements, and the debugger
gets confused.

If you have compiled your program with -O0, and setting a breakpoint
in main still results in the breakpoint's location being reported in
some other function, that sounds like a bug.  For us to help you,
you'll need to provide enough information for us to make the problem
happen on our own machines.