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 the 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.