hitting my head against gcc

people

39 posts

#8477

7 years, 7 months ago

For the code added in day 328, I've been trying to understand why gcc won't create usable code, it doesn't seem to like "va_list" pointers like ArgList.

The gcc error:

src/handmade_shared.h: In function ‘umm FormatStringList(umm, char*, char*, __va_list_tag*)’:
src/handmade_shared.h:388:84: error: cannot convert ‘__va_list_tag**’ to ‘__va_list_tag (*)[1]’ 
for argument ‘2’ to ‘s64 ReadVarArgSignedInteger(u32, __va_list_tag (*)[1])’
                         s64 Value = ReadVarArgSignedInteger(IntegerLength, &ArgList);

First it changes the (va_list *) type to (__va_list_tag *), I guess with a macro or something, but "__va_list_tag" isn't a real type, (it is built into gcc), so I can't cast to it. I can cast to (va_list *) and it will compile, but it will crash with a segmentation fault when I try to open "Profile" in the debug menu.

I'm not sure how to cast to a type with a [1] in it, does it mean an array of length 1? Does it need the length information or can you replace the [1] with an * in the cast? I couldn't get a cast to work with any form of [1] in it.

By not passing "ArgList" by reference, but by value, I can get it to compile and run normally, so I can only guess that gcc is doing something strange that prevents passing a "va_list" by reference.

I've included a link to the compiling/running mod in case it's unclear.

Thanks in advance to anyone who's more familiar with this stuff.

mmozeiko

Mārtiņš Možeiko

2562 posts / 2 projects

#8478

hitting my head against gcc

7 years, 7 months ago Edited by Mārtiņš Možeiko on September 7, 2016, 4:37pm

Yeah, this is an issue on 64-bit Intel architecture.
We already have this filed in github issues: https://github.com/HandmadeHero/cpp/issues/26
Here's an explanation why passing va_list by address is a problem: http://stackoverflow.com/a/8048892/675078

Correct way to do this is either to cast arglist to (va_list*) directly (because it is an array, and address of array is same as array) or to use va_copy to make a copy of va_list:

va_list args_copy;
va_copy(args_copy, args);
OtherFunction(..., &args_copy);
va_end(args_copy);

people

39 posts

#8479

hitting my head against gcc

7 years, 7 months ago Edited by people on September 7, 2016, 3:31pm

Man, that was a quick reply :)

I guess I should get around to activating a github account :P