Crazy idea? Program itself tells GDB to watch variable for changes

Why would I need this? 开发者_如何学GoThe location of a datum keeps changing because input data varies too much, so other than printing it, sleeping for 30 seconds so I can manually enter it into GDB, then continuing the program, it might be useful to just let the program tell GDB where to watch. But is such a thing possible?

You can get close; assuming for simplicity C/C++ language

Define a function that returns a reference to your datum to track:

 // debug.h
 extern "C" mydatastruct* GetDatumForDebug();

 // debug.cpp
 mydatastruct* GetDatumForDebug()
 {
     if (s_initialized)
          return &some_complicated_address_lookup_perhaps_in_Cpp_or_java_orwhatever();
     return (mydatastruct*) 0;
 }

You can then subsequently just

 (gdb) display GetDatumForDebug()

or even

 (gdb) display GetDatumForDebug()->s

I assume it will be possible to use the result of GetDatumForDebug() in your debug watches, I'm not sure what you do/how you do that :)

Here is a working example, crammed in a single source (test.cpp) for speed: compile with g++ -g test.cpp -o test:

static bool s_initialized = false;
struct mydatastruct { const char* s; };
static mydatastruct& some_complicated_address_lookup_perhaps_in_Cpp_or_java_orwhatever() 
{ 
    static mydatastruct s_instance = { "hello world" };
    s_initialized = true;
    return s_instance;
}

extern "C" mydatastruct* GetDatumForDebug();

// debug.cpp
mydatastruct* GetDatumForDebug()
{
    if (s_initialized)
        return &some_complicated_address_lookup_perhaps_in_Cpp_or_java_orwhatever();
    return (mydatastruct*) 0;
}

int main()
{
    // force initialize for demo purpose:
    some_complicated_address_lookup_perhaps_in_Cpp_or_java_orwhatever();
    return 42;
}

Automate gdb commands

Append the following to .gdbinit in your working directory:

break main
run
call some_complicated_address_lookup_perhaps_in_Cpp_or_java_orwhatever()
display GetDatumForDebug()? GetDatumForDebug()->s : ""

This will automatically execute these commands on launching gdb in that directory