Dispatch (swift-corelibs-libdispatch) Debug Logging

rlovelett · May 11, 2018, 2:35am

I am trying to debug Dispatch to see what is causing a bug I've stumbled into.

I've actually got a debug build of libdispatch made and am able to step through it with lldb .

I was wondering how I go about enabling some of the logging in libdispatch.

I'm not sure what or how I would send to the Swift build-script to get that part of the compilation turned on.

rlovelett · May 11, 2018, 1:26pm

I've added libdispatch-cmake-options=-DDISPATCH_ENABLE_ASSERTS=1 -DDISPATCH_DEBUG=1 -DDISPATCH_IO_DEBUG=1 to my .swift-build-presets and I see it showing up the arguments sent to the libdispatch compiler. Though I'm still not seeing any logs actually being written.

dgrove-oss · May 11, 2018, 2:22pm

I think you need to set LIBDISPATCH_LOG in your environment when you are running the program as well.

github.com

apple/swift-corelibs-libdispatch/blob/a618b468b94ad6abb14a19af76e33018700af6bc/src/init.c#L687-#L705


      
          static dispatch_once_t _dispatch_logv_pred;
          
          
static void
          _dispatch_logv_init(void *context DISPATCH_UNUSED)
          {
          #if DISPATCH_DEBUG
          	bool log_to_file = true;
          #else
          	bool log_to_file = false;
          #endif
          	char *e = getenv("LIBDISPATCH_LOG");
          	if (e) {
          		if (strcmp(e, "YES") == 0) {
          			// default
          		} else if (strcmp(e, "NO") == 0) {
          			dispatch_log_disabled = true;
          		} else if (strcmp(e, "syslog") == 0) {
          			log_to_file = false;
          		} else if (strcmp(e, "file") == 0) {
          			log_to_file = true;
          		} else if (strcmp(e, "stderr") == 0) {

rlovelett · May 11, 2018, 2:53pm

LIBDISPATCH_LOG=stderr ./test

Thank you @dgrove-oss