hello, everyone, I just want use mpn_gcd to make a program, that can calculate gcd of two big integers, the following is my code named agcd.c: #include <stdio.h> #include <gmp.h>     main (int argc, char **argv)     {       mpz_t a, b, p;       if (argc != 3)         { printf ("Usage: %s <number> <number>\n", argv[0]); exit (1); }       /* Initialize and assign a and b from base 10 strings in argv */       mpz_init_set_str (a, argv[1], 10);       mpz_init_set_str (b, argv[2], 10);       /* Initialize p */       mpz_init (p);       /* compute the gcd of a and b*/      mp_size_t mpn_gcd (mp_limb_t *p, mp_limb_t *a,           mp_size_an, mp_limb_t *b, mp_size_t bn)       /* Print p in base 10 */       mpz_out_str (stdout, 10, p);       fputc ('\n', stdout);       /* Since we're about to exit, no need to clear out variables */       exit (0);     } and when compiled, got the following error: agcd.c: In function 'main': agcd.c:9: warning: incompatible implicit declaration of built-in function 'exit' agcd.c:20: error: syntax error before 'mp_size_an' so, please, anyone can give me some advise about this problem, thanks! weifeng Jiang