What's the difference between strtok_r and strtok_s in C?

strtok_s is simply the Windows version of strtok_r which is standard everywhere else.

One (common I would think) way to make a program portable when it comes to functions like strtok_s/strtok_r is to use the preprocessor:

#if defined(_WIN32) || defined(_WIN64)
/* We are on Windows */
# define strtok_r strtok_s
#endif

As the prototypes and functionality is the same, you can now use only strtok_r.

I don't have enough reputation to comment on other answers, so I'll have to provide my own.

1) To address this statement:

"strtok_s is a buffer overrun safe version of strtok on Windows. The standard strtok on windows is thread safe..."

This is not true. strtok_s is the thread safe version for the MSVC compiler. strtok is not thread safe!

2) To address this statement:

"This would probably break if compiling on Cygwin which reports itself as windows but has POSIX interfaces like strtok_r already defined."

Again, not true. The difference is which compiler you use. When using Microsoft's Visual C++ compiler, MSVC, the function is strtok_s. Another compiler, such as the GNU Compiler Collection, GCC, may use a different standard library implementation such as strtok_r. Think compiler, not target platform, when identifying which function to use.

In my opinion, Joachim Pileborg's answer is the best one on this page. However, it needs a small edit:

#if defined(_WIN32) /* || defined(_WIN64) */
#define strtok_r strtok_s
#endif

Both _WIN32 and _WIN64 are predefined macros provided by the MSVC compiler. _WIN64 is defined when compiling a 64 bit target. _WIN32 is defined for both 32 and 64 bit targets. This is a compromise that Microsoft made for backwards compatibility. _WIN32 was created to specify the Win32 API. Now you should consider _WIN32 to specify Windows API -- it is not specific to a 32 bit target.

Both of these functions are really ugly, unintuitive idioms for parsing strings, and usually fail to meet your particular application's requirements in subtle ways. Even moreso for the plain strtok in standard C. Just throw them out and write your own code to iterate over the char array and break it up as needed. strchr, strspn, and strcspn can be helpful in doing this or you can just work from scratch on the array.