Using snprintf in a cross-platform application

Question

I am writing a C program that is expected to be compiled with all major compilers. Currently I am developing on GCC on a linux machine and will compile on MSVC before committing the code. To make the cross-compiling easy, I am compiling with -ansi and -pedantic flags. This worked well until I started using snprintf which is not available in C89 standard. GCC can compile this without the -ansi switch but MSVC will fail always as it doesn't have C99 support.

So I did something like,

#ifdef WIN32 
#define snprintf sprintf_s
#endif

This works well because snprintf and sprintf_s has same signatures. I am wondering is this the correct approach?

Answer 1

I found this on using _snprintf() as an alternative, and the gotchas involved if the buffer overrun protection actually triggers. From what I could see at a quick glance, similar caveats apply to sprintf_s.

Can you see the problem? In the Linux version, the output is always null-terminated. In MSVC, it's not.

Even more subtle is the difference between the size parameter in Linux and count parameter in MSVC. The former is the size of the output buffer including the terminating null and the latter is the maximum count of characters to store, which excludes the terminating null.

Oh, and don't forget to send a mail to Microsoft demanding they support current language standards. (I know they already announced they have no plan to support C99, but bugger them anyway. They deserve it.)

Bottom line, if you want to play it really safe, you'll have to provide your own snprintf() (a wrapper around _snprintf() or sprintf_s() catching their non-standard behaviour) for MSVC.