30

While tinkering for an answer to this question, I found that debug_backtrace() doesn't trace beyond the function registered to register_shutdown_function(), when called from within it.

This was mentioned in this comment for register_shutdown_function() in the PHP docs, stating:

You may get the idea to call debug_backtrace or debug_print_backtrace from inside a shutdown function, to trace where a fatal error occurred. Unfortunately, these functions will not work inside a shutdown function.

Explained with a bit more detail, comments on this answer state:

Doesn't work. The shutdown function occurs after the stack has unwinded. There is no stack information to dump.

Is there any way to circumvent this, forcing PHP to hold the stack trace until the process has terminated altogether, or should we accept it as a given due to PHP internals?

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5 Answers 5

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19

This is a very expensive solution. I never used register_tick_function() or tick and I'm not sure if it works as expected.

declare(ticks=1);

function tick_handler() {
    global $backtrace;
    $backtrace = debug_backtrace();
}
register_tick_function('tick_handler');



function shutdown() {
    global $backtrace;
    // do check if $backtrace contains a fatal error...
    var_dump($backtrace);
}
register_shutdown_function('shutdown');
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4 Comments

Thanks @powtac - Great solution, however it didn't work. I certainly see where you were going with that. I fooled with it a bit, but with no success yet.
thats clever for sure. i didnt know about tick function
Was searching for same and found this - gist.github.com/rmbl/5780995. Didn't try though. Note declare(ticks=1); at the end
You need the declare(ticks=1) statement on every file of your project for this to work
9

Is there any way to circumvent this, forcing PHP to hold the stack trace

That's rather meaningless, when the registered function is invoked, all your defined functions have returned or been cleared down from the stack.

If you need to know where your code exited, then you need to instrument your code.

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4 Comments

Thanks @symcbean - Given all other functions in a given execution have returned, and we've bubbled back up to global scope, of course; however, the part where incomplete function calls are being cleared from the stack (given exit is called at some arbitrary depth of nested calls) is what I find unfortunate, and not meaningless. I never use exit for execution control, favoring exceptions to handle my error logic; this was more of an academic inquiry, rather than real-world. Still though, it makes sense in my mind that the stack should be preserved until the process is killed completely.
Than what would happen when your registered shutdown function returned?
Touche, sir; I just thought PHP would (should) keep the stack at script termination, check for handlers, execute handlers, wash-rinse-repeat until the queue is empty, and then end execution.
You can use error_get_last to find where the error was raised, but it won't give you the stack.
7

In XDebug extension, there is a xdebug_get_function_stack() function.

This one works similar to PHP's internal debug_backtrace(), but keeps the trace even in shutdown handler.

You won't get the exact exit point though, only the last executed function before the shutdown occured (triggered by die()/exit() call or error).

This is of course suitable only for development environment.

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2 Comments

quirk: doesn't work for uncaught exceptions... unless a default exception handler has been set with set_exception_handler
quirk #2: xdebug_get_function_stack() begins with shutdown-function if there was a parse error
5

From my experience, the shutdown function starts with a clean stack, and it has no access to the "original" stack (as it no longer exists at that point).

Unfortunately, there is no way to save that original stack.

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Comments

1

Inside your registered shutdown function you can get backtrace by error_get_last function. Works for me in PHP7.

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1 Comment

This only gets the file and line where the error occurred - not the full backtrace.

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