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Fix broken links and the example code in signal-safe-tracing.md, and also add a note to the documentation about _dl_find_object being required for signal-safe tracing

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Jeremy 2024-07-12 17:33:35 -05:00
parent 15b6d9a2cd
commit e8fce3fad1
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2 changed files with 12 additions and 4 deletions
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4
README.md
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@ -507,6 +507,10 @@ namespace cpptrace {
> information can't be resolved in a signal-safe way then `get_safe_object_frame` will not populate fields beyond the
> `raw_address`.
> [!IMPORTANT]
> `_dl_find_object` is required for signal-safe stack tracing. This is a relatively recent addition to glibc, added in
> glibc 2.35.
> [!CAUTION]
> Calls to shared objects can be lazy-loaded where the first call to the shared object invokes non-signal-safe functions
> such as `malloc()`. To avoid this, call these routines in `main()` ahead of a signal handler to "warm up" the library.

12
docs/signal-safe-tracing.md
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@ -35,6 +35,10 @@ memory corruption.
> [!IMPORTANT]
> Currently signal-safe stack unwinding is only possible with `libunwind`, more details later.
> [!IMPORTANT]
> `_dl_find_object` is required for signal-safe stack tracing. This is a relatively recent addition to glibc, added in
> glibc 2.35.
> [!CAUTION]
> Calls to shared objects can be lazy-loaded where the first call to the shared object invokes non-signal-safe functions
> such as `malloc()`. Because of this, the signal safe api must be "warmed up" ahead of a signal handler.
@ -102,8 +106,8 @@ knows ways to do these safely on other platforms, I'd be much appreciative.
Of the three strategies, `fork()` + `exec()`, is the most technically involved and the only way to resolve while the
signal handler is running. I think it's worthwhile to do a deep-dive into how to do this.
In the source code, [`signal_demo.cpp`](signal_demo.cpp) and [`signal_tracer.cpp`](signal_tracer.cpp) provide a working
example for what is described here.
In the source code, [`signal_demo.cpp`](../test/signal_demo.cpp) and [`signal_tracer.cpp`](../test/signal_tracer.cpp)
provide a working example for what is described here.
## In the main program
@ -136,7 +140,7 @@ struct pipe_t {
};
};
void do_signal_safe_trace(cpptrace::frame_ptr* buffer, std::size_t size) {
void do_signal_safe_trace(cpptrace::frame_ptr* buffer, std::size_t count) {
// Setup pipe and spawn child
pipe_t input_pipe;
pipe(input_pipe.data);
@ -172,7 +176,7 @@ void handler(int signo, siginfo_t* info, void* context) {
constexpr std::size_t N = 100;
cpptrace::frame_ptr buffer[N];
std::size_t count = cpptrace::safe_generate_raw_trace(buffer, N);
do_signal_safe_trace(buffer, N);
do_signal_safe_trace(buffer, count);
// Up to you if you want to exit or continue or whatever
_exit(1);
}