Two handy GDB breakpoint tricks

Over the past couple months I’ve discovered a couple of handy tricks for working with GDB breakpoints. I figured these out on my own, and I’ve not seen either discussed elsewhere, so I really ought to share them.

Continuable assertions

The assert macro in typical C implementations leaves a lot to be desired, as does raise and abort, so I’ve suggested alternative definitions that behave better under debuggers:

#define assert(c)  while (!(c)) __builtin_trap()
#define assert(c)  while (!(c)) __builtin_unreachable()
#define assert(c)  while (!(c)) *(volatile int *)0 = 0

Each serves a slightly different purpose but still has the most important property: Immediately halt the program directly on the defect. None have an occasionally useful secondary property: Optionally allow the program to continue through the defect. If the program reaches the body of any of these macros then there is no reliable continuation. Even manually nudging the instruction pointer over the assertion isn’t enough. Compilers assume that the program cannot continue through the condition and generate code accordingly.

The MSVC ecosystem has a solution for this on x86: int3. The portable name is __debugbreak, a name I’ve borrowed elsewhere.

#define assert(c)  do if (!(c)) __debugbreak(); while (0)

On x86 it inserts an int3 instruction, which fires an interrupt, trapping in the attached debugger, or otherwise abnormally terminating the program. Because it’s an interrupt, it’s expected that the program might continue. It even leaves the instruction pointer on the next instruction. As of this writing, GCC has no matching intrinsic, but Clang recently added __builtin_debugtrap. In GCC you need some less portable inline assembly: asm ("int3").

However, regardless of how you get an int3 in your program, GDB does not currently understand it. The problem is that feature I mentioned: The instruction pointer does not point at the int3 but the next instruction. This confuses GDB, causing it to break in the wrong places, possibly even in the wrong scope. For example:

for (int i = 0; i < n; i++) {
    // ...

With int3 at the very end of the loop, GDB will break at the top of the next loop iteration, because that’s where the instruction pointer lands by the time GDB is involved. It’s a similar story when placed at the end of a function, leaving GDB to break in the caller. To resolve this, we need the instruction pointer to still be “inside” the breakpoint after the interrupt fires. Easy! Add a nop:

#define breakpoint()  asm ("int3; nop")

This behaves beautifully, eliminating all the problems GDB has with a plain int3. Not only is this a solid basis for a continuable assertion, it’s also useful as a fast conditional breakpoint, where conventional conditional breakpoints are far too slow.

for (int i = 0; i < 1000000000; i++) {
    if (/* rare condition */) breakpoint();
    // ...

Could GDB handle int3 better? Yes! Visual Studio, for instance, does not require the nop instruction. As far as I know there is no ARM equivalent compatible with GDB (or even LLDB). The closest instruction, brk #0x1, does not behave as needed.

Named positions

GDB’s built-in user interface understands three classes of breakpoint positions: symbols, context-free line numbers, and absolute addresses. When you set some breakpoints and (re)start a program under GDB, each kind of breakpoint is handled differently:

The first is the best case because it adapts to program changes. Modify the code, recompile, and the breakpoint generally remains where you want it.

The third is the least useful. These breakpoints rarely survive across rebuilds, and sometimes not even across reruns.

The second is in the middle between useful and useless. If you edit the source file which has the breakpoint — likely, because you placed the breakpoint there for a reason — chances are high that the line number is no longer correct. Instead it drifts, requiring manual replacement. This is tedious and GDB ought to do better. Think that’s unreasonable? The Visual Studio debugger does exactly that quite effectively through external code edits! GDB front ends tend to handle it better, especially when they’re also the code editor and so directly observe all edits.

As a workaround we can get the first kind by temporarily naming a line number. This requires editing the source, but remember, the very reason we need it is because the source in question is actively changing. How to name a line? C and C++ labels give a name to program position:

void example(double *nums, int n, ...)
    for (int i = 0; i < n; i++) {
        loop:  // named position at the start of the loop
        // ...

The name loop is local to example, but the qualified example:loop is a global name, as suitable as any other symbol. I could, say, reliably trace the progress of this loop despite changes to its position in the source.

(gdb) dprintf example:loop,"nums[%d] = %g\n",i,nums[i]

One downside is dealing with -Wunused-label (enabled by -Wall), and so I’ve considered disabling the warning in my defaults. Update: Matthew Fernandez pointed out that the unused label attribute eliminates the warning, solving my problem:

    for (int i = 0; i < n; i++) {
        loop: __attribute((unused))
        // ...

More often I use an assembly label, usually named b for convenience:

    for (int i = 0; i < n; i++) {
        asm ("b:");
        // ...

Like int3, sometimes it’s necessary to give it a nop so that GDB has something on which to break. “Enabling” it at any time is quick:

(gdb) b b

Because it’s not .globl, it’s a weak symbol, and I can place up to one per translation unit, all covered by the same GDB breakpoint item (less useful than it sounds). I haven’t actually checked, but I probably more often use dprintf with such named lines than actual breakpoints.

If you have similar tips and tricks of your own, I’d like to learn about them!

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null program

Chris Wellons (PGP)
~skeeto/ (view)