Is `std::string(strerror(errno))` dangerous?

Question

At some places in my code, I print debug messages like this:

int ret = getLinkSpeed(device.getSysName(), linkSpeed);
if (ret < 0)
{
    logDebug("Failed to obtain port speed for this device. Error: " + std::string(strerror(errno)));
}

From the documentation it is not entirely clear if strerror will return 0 under certain conditions (which would cause my code to crash). Does anyone know if it's safe?

Answer 1

Why not write a function to do this:

string ErrStr() {
   char * e = strerror(errno);
   return e ? e : "";
}

This is easy to use, self-documenting, can be adapted to reformat the output and covers the possibility that strerror() might return NULL (I don't know if it can).

Answer 2

Good question (+1), the documentation seems quite vague. I'm not sure if there is a "harder" source, such as the POSIX specification for instance.

Thinking a bit pragmatically, here is GNU libc's implementation. It returns a pointer to a static string buffer, so it cannot return 0.

In response to p00ya's comment, the safe (and also very pragmatical, heh) thing to do in the face of conflicting, vague or incomplete specifications is of course to assume the worst, and not assume that the return value will always be a valid string.

Answer 3

Where you might get problems, is if you use a multi-threaded application. In this case, you need to use strerror_r