Segfault on ofstream::open() if defined outside of function

Question

I'm writing a little shared library to test some things via LD_PRELOAD and I want to write logs to a file.

The following code works:

void ctor() __attribute__((constructor));
void dtor() __attribute__((destructor));

void ctor() {
  std::ofstream log_file;
  log_file.open("/home/tristan/Test.log");
  log_file << "Log Stuff..." << std::endl;
  log_file.close();
}

This causes a Segfault:

void ctor() __attribute__((constructor));
void dtor() __attribute__((destructor));

std::ofstream log_file;

void ctor() {
  log_file.open("/home/tristan/Test.log");
  log_file << "Log Stuff..." << std::endl;
  log_file.close();
}

Why is that? Something to do with the constructor attribute perhaps?

My GCC flags are as follows:

gcc -fPIC -m64 -shared -lstdc++ -o Test.so *.cpp

Is there a reason that `log_file` shouldn't belong to the function `ctor`? — Sailanarmo, Mar 05 '19 at 14:54
I was planning on using the log_file globally. Opening it in ctor, outputting in my other functions, and closing it dtor. — Tristan, Mar 05 '19 at 23:59

score 2 · Accepted Answer · answered Mar 05 '19 at 14:37

2

This is because of the __attribute__((constructor)). The ctor function is called before global variable std::ofstream log_file is initialized and so causes the segfault.

answered Mar 05 '19 at 14:37

bartop

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Segfault on ofstream::open() if defined outside of function

1 Answers1