Memory leak concern

[chigley]

I am writing a function to wrap the winsock2.h function gethostname. I am concerned I have introduced a memory leak by using malloc, as if I put a free() after the return statement it will not fire. Can someone guide me?

char * getLocalHostName(){
     char * hostName = (char *)malloc(256);
     int nameLen = 256; 
     if (gethostname(hostName,nameLen)>0){
        return "Error retrieving local host name";
     }    
     else{
          return hostName;
          free(hostName);
     }
}

Charlie Benger-Stevenson
Hart Hill IT Ltd

http://www.harthill-it.co.uk

 

[cpjust]

As long as the function that calls getLocalHostName() remembers to free() the returned pointer, there is no leak. However, since you return malloc'ed memory in one place and a constant literal string in another, the caller cannot safely free() the pointer because freeing a constant literal string is undefined behavior and will most likely blow up.

A better implementation would be:

int getLocalHostName( char*  buf, int bufSize )
{
   return gethostname( buf, bufSize );
}

But of course in this case, what's the point in just wrapping gethostname() in an extra function call?

So the question comes down to: what exactly are you trying to do with your custom function that can't be done with gethostname() by itself?

 

[chigley]

Well, partly I am trying to resurrect my C programming skills so I can write a Kernel mode driver that does some low level network monitoring using the Windows Filter Driver.

I need to be able to do stuff like

enumerate NICs
Get network information about the host system
Get network information about wireless interfaces

I notice that some of this stuff lives in winsock2.h, some in iputils etc etc so I thought I could write some wrapper functions so everything could be in one logical unit.

In this case I have simplified the call to gethostname, but this is only one part of the function I was writing, when I got spooked about memory leaks. The rest of the function should return ip address, subnet mask, default gateway, primary and secondary dns, mac address etc etc



Charlie Benger-Stevenson
Hart Hill IT Ltd

http://www.harthill-it.co.uk

 

[cpjust]

Actually, if you don't mind your function not being multi-thread safe, you can use a static variable:

const char* getLocalHostName()
{
   static char buf[1024];
   if ( gethostname( buf, 1024 ) == 0 )
   {
      return buf;
   }

   return NULL;
}