allocate and free BN_CTX structures
BN_CTX is a structure that holds BIGNUM temporary variables used by library functions. Since dynamic memory allocation to create BIGNUMs is rather expensive when used in conjunction with repeated subroutine calls, the BN_CTX structure is used.BN_CTX_new() allocates and initializes a BN_CTX structure.BN_CTX_free() frees the components of the BN_CTX, and if it was created by BN_CTX_new(), also the structure itself. If BN_CTX_start(3) has been used on the BN_CTX, BN_CTX_end(3) must be called before the BN_CTX may be freed by BN_CTX_free().BN_CTX_init() (deprecated) initializes an existing uninitialized BN_CTX. This should not be used for new programs. Use BN_CTX_new() instead.
BN_CTX. If the allocation fails, it returns NULL and sets an error code that can be obtained by ERR_get_error(3).BN_CTX_init() and BN_CTX_free() return no value.