strcpy... want to replace with strcpy_mine which will strncpy and null terminate

Question

The clue is in the title but basically I've inherited some code which has 800+ instances of strcpy. I want to write a new function and then to replace strcpy with strcpy_mine.

So I'm trying to work out what parameter list strcpy_mine will have.

I tried:

void strcpy_mine( char* pTarget, const char* const pCopyMe )
{
  const unsigned int lenAlwaysFour = sizeof(pCopyMe ); //:(
  strncpy( pTarget, pCopyMe, lenAlwaysFour );

  //add extra terminator in case of overrun
  pTarget[lenAlwaysFour] = 0;
}

but the sizeof is always 4 pCopyMe is a pointer

what I don't want to do is replace

strcpy (buf, pCopyMe);

with

strncpy (buf, pCopyMe, sizeof(pCopyMe)); buf[sizeof(pCopyMe)] = 0;

Any ideas? (strcpy_l isn't available)

Answer 1

sizeof() returns the size of the type - in this case const char* const which will be 4 on 32-bit machines.

I think you think you want strlen(). But that isn't the right way to use strncpy functions. You need the size of the output buffer for strncpy.

To fix this you need to examine the code at each call site, and work out the size of the output buffer, and pass that as an argument to strcpy_mine. If the call-site for strcpy (or strcpy_mine) doesn't know the size of the output buffer, you need to search backwards in the code for the location that allocates the buffer, and pass the size all the way down to the strcpy site.

Basically you can't write a drop in replacement for strcpy that takes the same arguments and hope to avoid the problems that produced strncpy in the first place (and better replacements beyond that). You can create a function that takes the same arguments as strncpy, but ensures the result is null-terminated - look at the implementation of OpenBSD's strlcpy() function. But the first step has to be to change the calling sites to pass on knowledge of the output buffer size.

Answer 2

Depending on how the call-sites look like, often majority of cases can be handled by a simple template:

#include <string.h>

template <int bufferSize>
void strcpy_mine( char (&pTarget)[bufferSize], const char* const pCopyMe )
{
  strncpy( pTarget, pCopyMe, bufferSize-1 );

  //add extra terminator in case of overrun
  pTarget[bufferSize-1] = 0;
}

int main()
{
  char buf[128];
  strcpy_mine(buf,"Testing");
  return 0;
}

If you are using Microsoft Visual Studio 2005 or newer, see Secure Template Overloads for a Microsoft implementation.