### 3.7 Reentrancy

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

- If configured with `--enable-alloca=malloc-notreentrant` (or with `--enable-alloca=notreentrant` when `alloca` is not available), then naturally GMP is not reentrant.
- `mpf_set_default_prec` and `mpf_init` use a global variable for the selected precision. `mpf_init2` can be used instead, and in the C++ interface an explicit precision to the `mpf_class` constructor.
- `mpz_random` and the other old random number functions use a global random state and are hence not reentrant. The newer random number functions that accept a `gmp_randstate_t` parameter can be used instead.
- `gmp_randinit` (obsolete) returns an error indication through a global variable, which is not thread safe. Applications are advised to use `gmp_randinit_default` or `gmp_randinit_lc_2exp` instead.
- `mp_set_memory_functions` uses global variables to store 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, then GMP will not be reentrant either.
- If the standard I/O functions such as `fwrite` are not reentrant then the GMP I/O functions using them will not be reentrant either.
- It’s safe for two threads to read from the same GMP variable simultaneously, but it’s not safe for one to read while another might be writing, nor for two threads to write simultaneously. It’s not safe for two threads to generate a random number from the same `gmp_randstate_t` simultaneously, since this involves an update of that variable.