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Asked: 2026-05-11T07:13:35+00:00

Compare these two largely identical functions. In the first, the memory for buff is

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Compare these two largely identical functions. In the first, the memory for buff is allocated using _alloca. This works fine. In the second, calloc and free are used instead of _alloca. This crashes.

The weird thing is that I use the calloc/free technique in almost every other GMP wrapping function I have and they all work. Here they don’t. Any ideas?

1:

#define Z(x) mpz_t (x); mpz_init( (x) ); #define BUFF_SIZE (1024 * 32)  BSTR __stdcall IBIGDIV(BSTR p1, BSTR p2 ) {      USES_CONVERSION;      Z(n1);     Z(n2);     Z(res);      char * buff =  (char *) _alloca( mpz_sizeinbase( res, 10 ) + 2 );      LPSTR sNum1 = W2A( p1 );     LPSTR sNum2 = W2A( p2 );      mpz_set_str( n1, sNum1, 10 );     mpz_set_str( n2, sNum2, 10 );      if ( mpz_sgn( n2 ) != 0 ) {          mpz_div( res, n1, n2 );         mpz_get_str(buff, 10, res);     } else {         strcpy( buff, '-0' );     }      BSTR bResult = _com_util::ConvertStringToBSTR( buff );     return bResult; }

2:

#define Z(x) mpz_t (x); mpz_init( (x) ); #define BUFF_SIZE (1024 * 32)  BSTR __stdcall IBIGDIV(BSTR p1, BSTR p2 ) {      USES_CONVERSION;      Z(n1);     Z(n2);     Z(res);      char * buff =  (char *) calloc( mpz_sizeinbase( res, 10 ) + 2, sizeof( char ) );      LPSTR sNum1 = W2A( p1 );     LPSTR sNum2 = W2A( p2 );      mpz_set_str( n1, sNum1, 10 );     mpz_set_str( n2, sNum2, 10 );      if ( mpz_sgn( n2 ) != 0 ) {          mpz_div( res, n1, n2 );         mpz_get_str(buff, 10, res);     } else {         strcpy( buff, '-0' );     }      BSTR bResult = _com_util::ConvertStringToBSTR( buff );     free( buff );     return bResult; }
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1 Answer

  1. Add logging and dump everything along the way to find what goes wrong. This is usually more efficient than trying to guess.

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