c++ - Using memory sanitizer with libstdc++

Question

I wish to use the -fsanitize=memory flag in clang to analyse a program like the following:

#include <string>
#include <iostream>
#include <fstream>
using namespace std;

void writeToFile(){
    ofstream o;
    o.open("dum");
    o<<"test"<<endl; //The error is here.
                     //It does not matter if the file is opened this way,
                     //or with o("dum");
    o.close();
}
int main(){
    writeToFile();
}

As far as I know, this program is correct, but when I use clang++ san.cpp -fsanitize=memory It fails (at runtime) with:

UMR in __interceptor_write at offset 0 inside [0x64800000e000, +5)  
==9685== WARNING: MemorySanitizer: use-of-uninitialized-value  
    #0 0x7f48d0899ae5 (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0x7bae5)  
    #1 0x7f48d08d1787 (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xb3787)  
    #2 0x7f48d08d21e2 (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xb41e2)  
    #3 0x7f48d08cfd1e (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xb1d1e)  
    #4 0x7f48d08b1f2d (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0x93f2d)  
    #5 0x7f48d16d60f5 in writeToFile() /home/daniel/programming/test/santest.cpp:10  
    #6 0x7f48d16d61f4 in main /home/daniel/programming/test/santest.cpp:15  
    #7 0x7f48d0261de4 (/lib/x86_64-linux-gnu/libc.so.6+0x21de4)  
    #8 0x7f48d16d5e42 in _start (/home/daniel/programming/test/a.out+0x61e42)  

SUMMARY: MemorySanitizer: use-of-uninitialized-value ??:0 ??

How can I make this work properly?

Clang version 3.5, stdlibc++ version 6

Answer 1

The code is fine, of course but many similar errors are cause by the following requirement of clang's memory sanitizer tool:

MemorySanitizer (without a dynamic component) requires that the entire program code including libraries, (except libc/libm/libpthread, to some extent), is instrumented.

from here

The cplusplus runtime you are using libstdc++ is unistrumented and causes errors. You will unfortunately have to follow a somewhat fiddly process as described at that link to rebuild an instrumented libstdc++ or switch to libc++ (easier-ish)