In the traditional standard C library, there are many functions which, in order to be invoked with less parameters, use some static variables.

However this approach works only in single-threaded programs. In multi-threaded programs, the static variables are clobbered when invocations of the function from different threads are interleaved.

The correct method to make the standard C library reentrant is to replace all the functions that use static variables with reentrant functions having one extra parameter, which typically use the old name of the function with the suffix "_r".

This has been done, but this means that if you have an old single-threaded program, when you want to make it multi-threaded you must search the program source for all bad standard library functions and replace them with good functions.

Newlib offers an optional facility to avoid the editing of the source files when making a conversion to multi-threading, by replacing all the static variables with dynamically-allocated variables, which will be distinct in different threads.

This feature should better be avoided, because the newer reentrant standard functions also correct other problems, in many cases.

An exception is precisely the "rand_r" function, which is supposed to replace "rand", but which has been wrongly defined, with an inappropriate size of the state parameter.

None of the ancient functions "rand", "rand_r", "srand", "random" or "srandom" should be used in any remotely serious application of random numbers.

There are a huge number of good PRNGs for which the source code is freely available.