GMP and Reentrancy

The GMP code is reentrant and thread-safe, with some exceptions:

• The function mpf_set_default_prec saves the selected precision in a global variable.
• The function mp_set_memory_functions uses several global variables for storing the selected memory allocation functions.
• If the memory allocation functions set by a call to mp_set_memory_functions (or malloc and friends by default) are not reentrant, GMP will not be reentrant either.
• The old random number functions (mpz_random, etc) use a random number generator from the C library, usually mrand48 or random. These routines are not reentrant, since they rely on global state. (However the newer random number functions that accept a gmp_randstate_t parameter are reentrant.)
• If alloca is not available, or GMP is configured with `--disable-alloca', the library is not reentrant, due to the current implementation of `stack-alloc.c'. In the generated `config.h', USE_STACK_ALLOC set to 1 will mean not reentrant.

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