Linking problem due to different size of std::string

Accelerated Computing CUDA CUDA Programming and Performance
1

Hello!

I’ve found out that one can’t rely on correct linking between nvcc object files and gcc object files, if std::string is used.

Here is what I’m talking about

#include <string>                                                               
#include <cstdio>                                                               
                                                                                
std::string foo;                                                                
                                                                                
int main()                                                                      
{                                                                               
    printf("%ld\n", sizeof(foo));                                               
    return 0;                                                                   
}

Compiling and running:

$ g++ test.cpp && ./a.out
32
$ nvcc test.cpp && ./a.out
8

My environment:

gcc version 6.3.0 20170516 (Debian 6.3.0-18)
Cuda compilation tools, release 8.0, V8.0.44

If some class is compiled by nvcc, and some code using this class is compiled by gcc, and the class anyhow contains std::string, then the linking will be corrupted. There will be no errors or warnings, though. Now any call to the fields after std::string will produce garbage, because in the code using the class an offset of pointers will be incorrectly calculated.

Is this a bug or something? How to avoid such problems?

2

gcc 6.3.0 is not an officially supported environment for CUDA 8.0.44
Use the same gcc version that is being used by your CUDA 8 host compiler. Make sure your nvcc is using a properly supported host compiler for the version (refer to linux install guide). Then you should see things match.

When I run your experiment on CUDA 9, Ubuntu 14.04, gcc 4.8.4 I get 8 printed out in both cases:

[url]c++ - Why is sizeof(std::string) only eight bytes? - Stack Overflow

[url]c++ - Why is sizeof(string) == 32? - Stack Overflow

3

txbob, thanks for your answer

I always use -ccbin=g++ in my projects, but now it turns out that it doesn’t work how I expect it to!

$ g++ test.cpp && ./a.out
32
$ nvcc -ccbin=g++ test.cpp && ./a.out
8

However, if I do

$ nvcc -ccbin=g++-6 test.cpp && ./a.out
32

It finally helps.

But where does this difference come from? These two commands should be identical, shouldn’t they? Because on my system g++ is the same as g+±6, take a look

$ g++ -v 2>&1 | tail -1
gcc version 6.3.0 20170516 (Debian 6.3.0-18) 
$ g++-6 -v 2>&1 | tail -1
gcc version 6.3.0 20170516 (Debian 6.3.0-18)

$ which g++
/usr/bin/g++
$ which g++-6
/usr/bin/g++-6

$ ls -la /usr/bin | grep g++-6 | head -1
lrwxrwxrwx  1 root   root           5 апр  8  2017 g++ -> g++-6

So, on my system g++ is just a symbolic link to g+±6, but not for the nvcc.

4

nvcc from CUDA 8.0.44 should refuse to use g++ 6.x

Have you made modifications to it?

I doubt I would be able to sort this out for you. You apparently have a jumble of configurations on your machine, and the problem you initially indicated in this thread is arising from that.

5

nvcc from CUDA 8.0.44 should refuse to use g++ 6.x

it does if cuda_runtime.h is included

In file included from /usr/include/cuda_runtime.h:78:0,
                 from <command-line>:0:
/usr/include/host_config.h:119:2: error: #error -- unsupported GNU version! gcc versions later than 5 are not supported!
 #error -- unsupported GNU version! gcc versions later than 5 are not supported!
  ^~~~~

In my little example test.cpp there was no CUDA headers, so compilation was successful.

But it doesn’t matter. The question is why CUDA arbitrarily downgrade gcc version, if “-ccbin=g++” was passed, where “g++” is a symlink for “g+±6” which is not supported?

6

what happens if you try -ccbin=/usr/bin/g++

7

what happens if you try -ccbin=/usr/bin/g++
If i do, the binary /usr/bin/g+±6 will be used.

It turned out that nvcc is supplied with its own compiler

$ ls /usr/lib/nvidia-cuda-toolkit/bin
cicc  crt  g++  gcc  libcuinj64.so  nvcc  nvcc.profile  nvprof

And version of that g++ is 4.9

So, when I do “-ccbin=g++”, nvcc first looks at path /usr/lib/nvidia-cuda-toolkit/bin, finds the right one, and stops with 4.9 version of g++.
If I pass anything else, nvcc doesn’t find the match in /usr/lib/nvidia-cuda-toolkit/bin, so it will search further in /usr/bin and finally takes a compiler from there.

My fault was that I wasn’t aware of this behavior and I thought that /usr/bin is always used for name resolution.

The question is closed for me. Thank you txbob