How to get rid of "unsafe" warnings / errors in Visual Studio (strcpy, sprintf, strdup)

I'm trying to get rid of some compiler warnings that say strcpy, sprintf, etc are unsafe. I get why they're unsafe, but I can't think of a good way to fix the code, in a C++ style.

Here's a excerpt of the code:

extList->names[i]=(char *)malloc(length*sizeof(char));
strcpy(extList->names[i],extName);                     // unsafe
// strncpy(extList->names[i],extName,length);          // also unsafe

Here's the message:

C4996: 'strcpy': This function or variable may be unsafe. Consider using strcpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.

I can't think of a safe way to copy the data over in C++ without knowing the length of the stuff to copy. I know there's strlen(), but that's also unsafe since it assumes (maybe incorrectly) that the data is null-terminated.

Also:

// used to concatenate:
sprintf(extStr,"%s%s",platExtStr,glExtStr);

C4996: 'sprintf': This function or variable may be unsafe. Consider using sprintf_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.

Using std::string to concatenate is easy enough, but then I need to get the data into extStr somehow (and not using strcpy, lol). The string::c_str() function returns a pointer to un-modifiable data, so I can't just set extStr equal to it. (And I'm not even sure if the c_str() pointer needs delete called on it later? Does it allocate space using "new"?)

Any advice on this stuff? This is part of a 10,000 line file that's not mine... so I'm not exactly keen on re-writing the thing in the C++ way.