c_str() of an rvalue - unsafe?

Question

Could anyone explain why the following crashes with an access violation at 0xFFFFFFFF at avformat_open_input?

std::string u8(const std::wstring& str)
{
    return boost::locale::conv::utf_to_utf<char>(str);
}

AVFormatContext* open_input(const std::wstring& filename)
{
    AVFormatContext* context = nullptr;
    avformat_open_input(&context, u8(filename).c_str(), nullptr, nullptr);
    avformat_find_stream_info(context, nullptr);
    return context;
}

while the following works:

AVFormatContext* open_input(const std::wstring& filename)
{
    auto u8filename = u8(filename);
    AVFormatContext* context = nullptr;
    avformat_open_input(&context, u8filename.c_str(), nullptr, nullptr);        
    avformat_find_stream_info(context, nullptr);
    return context;
}

@Tomek: http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavformat/utils.c;h=2f926bb4e0784b031456fce7b477a7eebac8b440;hb=refs/heads/master#l583 — ronag, Apr 12 '12 at 15:59
I think you also need to provide the compiler/boost version and maybe the generated assembly for two dummy calls that exhibit the problem (if possible). — pmr, Apr 12 '12 at 16:15

bames53 · Accepted Answer · 2012-04-12T16:09:47.463

1

The result of u8(filename).c_str() should be usable until avformat_open_input returns. It's probably saving the pointer you give it and then using that during avformat_find_stream_info.

Post the docs for these avformat functions, or their implementations so we can see if it's actually doing that.

It doesn't look like avformat_open_input is doing anything wrong. Now I suspect that there's undefined behavior occurring somewhere earlier in the program. Try using a tool like valgrind or static analysis and see if that turns up anything.

edited Apr 12 '12 at 16:09

answered Apr 12 '12 at 15:58

bames53

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bames53: See second comment under question. – ronag Apr 12 '12 at 16:00
2

If this were so then the AV would not occur at `avformat_open_input`, rather in the call to `avformat_find_stream_info` – David Heffernan Apr 12 '12 at 16:02
@DavidHeffernan yeah, you're right. And the code appears to copy the string anyway. – bames53 Apr 12 '12 at 16:07
Accepted. It's undefined somewhere earlier. – ronag Apr 12 '12 at 16:18

c_str() of an rvalue - unsafe?

1 Answers1