c++ GMP mpz_init() causes Segmentation Fault 11 -

i have program receives segmentation fault 11 notice simplicity of main. here entire working script:

#include <iostream> #include "gmp.h"  void  makeprime () {     // *********************** variable declaration *********************** //     // initilize variables gmp class instances     mpz_t l, rand;     unsigned long seed;     // perform inits create variable pointers 0 value     mpz_inits(l, rand);     //mpz_init(rand);      // calculate random number floor     mpz_ui_pow_ui(l, 2, 199);      // initilze state object random generator functions     gmp_randstate_t rstate;     // initialize state mersenne twister algorithm. algorithm fast , has randomness properties.     gmp_randinit_mt(rstate);      // create generator seed random engine reference      gmp_randseed_ui(rstate, seed);      /*     function:     int mpz_probab_prime_p (const mpz_t n, int reps)      determine whether n prime. return 2 if n prime, return 1 if n prime (without being certain),      or return 0 if n composite.     */     {         // return uniformly distributed random number in range 0 n-1, inclusive.         mpz_urandomb(rand, rstate, 310);          // add random number low number, make sure random number between low , high ranges         mpz_add(rand, rand, l);          gmp_printf("randomly generated number: %zd\n", rand);      } while ( !(mpz_probab_prime_p(rand, 25)) );              // *********************** garbage collection *********************** //     // empty memory location random generator state     gmp_randclear(rstate);     // clear memory locations variables used avoid leaks     mpz_clear(l);     mpz_clear(rand); }  int main (void) {     makeprime();     return 0; } 

ok, i'll add following 2 lines @ beginning of main (not changing else script):

int main (void) {     mpz_t r;      //added line     mpz_init (r); //and added line      makeprime();     return 0;  } 

now program doesn't execute nor print or execute makeprime(), instead segmentation fault 11 notice.

what gives? doing wrong here?

i trying debug code using gdb , gave me segfault @ line mpz_inits(l, rand). modified line , not give segfault. see if can find reason segfault.

#include <iostream> #include "gmp.h"  void  makeprime () {     // *********************** variable declaration *********************** //     // initilize variables gmp class instances     mpz_t l, rand;     unsigned long seed;     // perform inits create variable pointers 0 value      //mpz_inits(l, rand);     mpz_init(rand);     mpz_init(l);      // calculate random number floor     mpz_ui_pow_ui(l, 2, 199);      // initilze state object random generator functions     gmp_randstate_t rstate;     // initialize state mersenne twister algorithm. algorithm fast , has randomness properties.     gmp_randinit_mt(rstate);      // create generator seed random engine reference      gmp_randseed_ui(rstate, seed);      /*     function:     int mpz_probab_prime_p (const mpz_t n, int reps)      determine whether n prime. return 2 if n prime, return 1 if n prime (without being certain),      or return 0 if n composite.     */     {         // return uniformly distributed random number in range 0 n-1, inclusive.         mpz_urandomb(rand, rstate, 310);          // add random number low number, make sure random number between low , high ranges         mpz_add(rand, rand, l);          gmp_printf("randomly generated number: %zd\n", rand);      } while ( !(mpz_probab_prime_p(rand, 25)) );              // *********************** garbage collection *********************** //     // empty memory location random generator state     gmp_randclear(rstate);     // clear memory locations variables used avoid leaks     mpz_clear(l);     mpz_clear(rand); }  int main (void) {     makeprime();     return 0; } 

edit:

i looked @ gmp documentation , says following mpz_inits.

— function: void mpz_init (mpz_t x)

initialize x, , set value 0.

— function: void mpz_inits (mpz_t x, ...)

initialize null-terminated list of mpz_t variables, , set values 0.

you must end list of variables want initialize null. need replace mpz_inits(l, rand) mpz_inits(l, rand, null) , works fine.

edit:

you forgot initialize seed. in function makeprime declare unsigned long seed; not assign value seed hence same numbers every time because value of seed same every execution.

typically random seeds initialized using system time. can use

seed = (int) time(null); 

to initialize seed after declaration. works fine (don't forget include ctime or time.h).