RAND_MAX 是否保证 <= UINT_MAX？

Question

I'm trying to determine if RAND_MAX can fit inside an unsigned int variable. After looking through the C99 standard, I have only found that RAND_MAX is guaranteed to have a value of at least 32767. While I know that RAND_MAX expands to int value, I'm not sure if it's a long int. Right now I'm working with unsigned long int variables, but I would like to simplify my source code to use unsigned int variables.

To summarize, is this assumption statement true for the C99 standard:

RAND_MAX <= UINT_MAX <= ULONG_MAX

Also, is RAND_MAX is explicitly less than (ie not equal to) UINT_MAX or ULONG_MAX. I realize that the value of RAND_MAX is often a Mersenne prime, but I'm not sure if this is a standard.

Answer 1

RAND_MAX is the maximum value that can be returned by rand() . Since rand() is defined as returning int , RAND_MAX will be no more than INT_MAX , and therefore also no more than UINT_MAX .

Answer 2

rand has signature int rand(void); , so any non-negative value it returns must fit inside an unsigned int , since the non-negative range of int is contained in the range of unsigned int .

This doesn't explicitly state that RAND_MAX is <= INT_MAX , but the requirement that rand return values in the range 0 to RAND_MAX along with the lower limit of 32767 for RAND_MAX indicates that rand is expected to cover the range 0 to RAND_MAX , which would not be the case if it were greater than INT_MAX .

Answer 3

Try this .

I agree with the answerer to this question that the Mersenne Twister will give you all the control and speed you need. It's also a very easy addition to any C++ program.

But like many others have said, rand()'s return type of int virtually guarantees RAND_MAX to be less than UINT_MAX.

Answer 4

Is RAND_MAX guaranteed to be <= UINT_MAX ?

Yes.

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32767 <= RAND_MAX <= INT_MAX <= UINT_MAX
65535 <= UINT_MAX

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... is RAND_MAX is explicitly less than (ie not equal to) UINT_MAX or ULONG_MAX (?)

No. RAND_MAX may equal UINT_MAX and ULONG_MAX in uncommon implementations.

Commonly RAND_MAX <= UINT_MAX/2 , yet that is not specified by the C Library spec.

To assert this common case:

#include <limits.h>
#include <stdlib.h>
_Static_assert(RAND_MAX <= UINT_MAX/2, "Large RAND_MAX");

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I realize that the value of RAND_MAX is often a Mersenne prime, but I'm not sure if this is a standard.

Not quite. RAND_MAX is commonly a Mersenne number - a power of 2 minus 1. It is not part of the C library standard, yet commonly a very desirable property.

To assert this desirable property:

#include <stdlib.h>
_Static_assert((RAND_MAX & 1) && ((RAND_MAX/2 + 1) & (RAND_MAX/2)) == 0, 
    "RAND_MAX is not a Mersenne number");

Sometimes RAND_MAX is a Mersenne prime .