dlmalloc + CPP + strdup + Mac OS X = crash

Question

I am using the dlmalloc library on Mac OS X in a mixed C/C++ environment.

The following simple code

/// strdup-test.cpp
///
#include <iostream>
#include <string>

int main(int argc, char **argv) {   
  std::string s1("foo");

  char *c1=strdup(s1.c_str());
  std::cerr << c1 << std::endl;

  // segfault?
  free(c1); 

  return 0;
}

when compiled like this

gcc -c malloc.c
g++ strdup-test.cpp -o strdup-test malloc.o

will crash like this

$ ./strdup-test 
foo
Segmentation fault

but only on Mac OS X. If I try this same code in Ubuntu or Windows (Cygwin) it won't happen.

What's going on here? If I use malloc directly rather than strdup it won't crash. My guess is that the default malloc and dlmalloc are being mixed. Possibly because strdup is using the default malloc while the free call is using dlmalloc's free. If so, why doesn't this happen on other platforms? How do I work around this on Mac OS X?

Answer 1

I think I've figured out what's happening. It has to do with the way Mac OS X forces you to use dynamic libc. dlmalloc is compiled statically into the exe. But regular malloc is being used in the dynamic libc. When you call strdup, it uses regular malloc, but then when free is called it is using dlmalloc. Boom.