g++: crash when accessing ostringstream::str().c_str()

The code below fails on gcc 9.4.0. Is this just a bug, or have I done something stupid?

log declares an ostringstream object, writes a filename and a line number to it, and attempts to do something with the object's underlying str().c_str().

Valgrind shows this crashing at the pointer access. The output I get is:

foo.cc, line 100
cptr is at 0x55c45e8f00c0, and is

#include <iostream>
#include <sstream>
#include <cstdarg>
    
using std::cout;
using std::ostringstream;
    
void log(const char *fname, int lineno) {
   ostringstream outstr;
   outstr << fname << ", line " << lineno;
   cout << outstr.str() << '\n';  // prints Ok
    
   const char *cptr = outstr.str().c_str();
   cout << "cptr is at " << (void*) cptr << ", and is " << cptr; // crash
}
    
int main() {
   log("foo.cc", 100);
}

std::ostringstream::str() returns a temporary string which will be destructed at the end of the line, this then means cptr is a dangling pointer.

Try:

std::string str = outstr.str();
const char *cptr = str.c_str();
cout << "cptr is at " << (void*) cptr << ", and is " << cptr;