Linux fork/exec to application in same directory

Question

Is there an exec variant that will use the current application directory to locate the target program?

I am using C++ and Qt to implement a "last ditch" error reporting system. Using Google Breakpad, I can create a minidump and direct execution to a handler. Because my application is in an unstable state, I just want to fork and start a separate error handling process using minimal dependencies. The error reporting application will be deployed in the same directory as the application executable.

I am quite unfamiliar with the fork and exec options, and am not finding an exec option that includes the current application directory in the search path. Here is what I have so far:

static bool dumpCallback(const char* /*dump_path*/,
                         const char* /*minidump_id*/,
                         void* /*context*/,
                         bool succeeded)
{
  pid_t pid = fork();
  if (pid == 0)
  {
    // This is what I would *like* to work.
    const char* error_reporter_path = "error_reporter";

    // This works, but requires hard-coding the entire path, which seems lame,
    // and really isn't an option, given our deployment model.
    //
    // const char* error_reporter_path = "/path/to/app/error_reporter";

    // This also works, but I don't like the dependency on QApplication at this
    // point, since the application is unstable.
    //
    // const char* error_reporter_path =
    //     QString("%1/%2")
    //    .arg(QApplication::applicationDirPath())
    //    .arg("error_reporter").toLatin1().constData();

    execlp(error_reporter_path,
           error_reporter_path,
           (char *) 0);
  }
  return succeeded;
}

Any other suggestions on best practices for using fork and exec would be appreciated as well; this is my first introduction to using them. I'm only concerned about Linux (Ubuntu, Fedora) at this point; I will work on handlers for other operating systems later.

Answer 1

What you asked for is actually quite easy:

{
  pid_t pid = fork();
  if (pid == 0)
  {
    const char* error_reporter_path = "./error_reporter";
    execl(error_reporter_path,
          error_reporter_path,
          (char *) 0);
    _exit(127);
  }
  else
    return pid != -1;
}

but it doesn't do what you want. The current working directory is not necessarily the same thing as the directory containing the current executable -- in fact, under almost all circumstances, it won't be.

What I would recommend you do is make error_reporter_path a global variable, and initialize it at the very beginning of main, using your "option 2" code

     QString("%1/%2")
    .arg(QApplication::applicationDirPath())
    .arg("error_reporter").toLatin1().constData();

The QString object (not just its constData) then has to live for the lifetime of the program, but that shouldn't be a problem. Note that you should be converting to UTF-8, not Latin1 (I guess QString uses wide characters?)

Answer 2

I think you have 2 choices:

1. Add '.' to $PATH.
2. Prepend the result of getcwd() to the executable name.

Answer 3

You should build the path to your helper executable at your program's startup, and save it somewhere (in a global or static variable). If you only need to run on Linux, you can do this by reading /proc/self/exe to get the location of your executable. Something like this:

// Locate helper binary next to the current binary.
char self_path[PATH_MAX];
if (readlink("/proc/self/exe", self_path, sizeof(self_path) - 1) == -1) {
  exit(1);
}
string helper_path(self_path);
size_t pos = helper_path.rfind('/');
if (pos == string::npos) {
  exit(1);
}
helper_path.erase(pos + 1);
helper_path += "helper";

Excerpted from a full working example here: http://code.google.com/p/google-breakpad/source/browse/trunk/src/client/linux/minidump_writer/linux_dumper_unittest.cc#92