srand(0) set random seed to 0 instead random

[adic3x]

Description

The following code:

<?php
function foo(): int
{
	srand(0);
	$bar = rand(0, 1023);
	var_dump($bar);
	return $bar;
}

var_dump(foo() === foo());

Resulted in this output:

int(684)
int(684)
bool(true)

But I expected this output instead:

int(23)
int(756)
bool(false)

But this code:

<?php
function foo(): int
{
	srand();
	$bar = rand(0, 1023);
	var_dump($bar);
	return $bar;
}

var_dump(foo() === foo());

Works fine:

int(722)
int(132)
bool(false)

https://www.php.net/manual/en/function.srand.php

Seeds the random number generator with seed or with a random value if seed is 0.

It's seems like srand(0) process 0 as new seed value instead 0 must reset seed to new random value.

PHP Version

PHP 8.2.0

Operating System

Win 11 build 25272 (insider)

[hormus]

<?php

function foo()
{
	srand(0);
	$bar = rand(0, 1023);
	return $bar;
}

var_dump(foo() === foo());
?>

From php >= 4.3.0 expected result bool true
mt_srand(0) From php >= 7.1.0 expected result bool true
https://3v4l.org/JNHF8

[damianwadley]

The srand page is a bit weird because it seems to try to document a function that doesn't exist anymore: it's mt_srand now, and that does not do the "with a random value if seed is 0" behavior. And given that 3v4l, it seems the original rand didn't actually do this?

Either way this seems like a documentation issue.

[cmb69]

The srand page is a bit weird because it seems to try to document a function that doesn't exist anymore: it's mt_srand now

The docs mention that srand() is an alias of mt_srand() as of PHP 7.1.0, and since the docs are currently supposed to document PHP 7.0 - 8.2, that looks okay.

Anyhow, the problem is that the stubs are not matching the implementation; the stubs claim the default of $seed would be zero:

php-src/ext/random/random.stub.php

Line 19 in 4427b2e

function mt_srand(int $seed = 0, int $mode = MT_RAND_MT19937): void {}

However, actually the implementation just checks whether an argument has been passed, or not:

php-src/ext/random/random.c

Line 691 in 4427b2e

if (ZEND_NUM_ARGS() == 0) {

So there is no default value.

So back to php-src. Documentation should be fixed afterwards.

[cmb69]

The big question is how to solve this: fix the stubs (resulting in a painful signature), or fix the implementation (resulting in a BC break). @kocsismate, thoughts?

[cmb69]

Ah, and since this is now part of ext/random, maybe @zeriyoshi and @TimWolla have thoughts on this as well.

[TimWolla]

I do have thoughts on this: The only good reason to use mt_rand() and mt_srand() in the first place since PHP 7.0 is getting a reproducible sequence, thus changing the long-standing behavior of 0 is a no-go in my book, especially since this would either also affect the OOP Mt19937 engine or result in differences between Mt19937 and mt_rand, which directly goes against the intended behavior.

I suggest to change the actual default value to null, like Mt19937's default is null as well and make null equivalent to a random uint32_t.

[adic3x]

https://www.php.net/manual/en/function.mt-srand.php

Seeds the random number generator with seed or with a random value if no seed is given.

Ok, but it factually contradicts to signature of this function:

mt_srand(int $seed = 0, int $mode = MT_RAND_MT19937): void

Where default value of $seed is 0. And I expect mt_srand(); and mt_srand(0); must works identically. For example, if I declare some function foo(int $ i = 0) then foo(); and foo(0); works identically.

As for me

srand(?int $seed = null, int $mode = MT_RAND_MT19937): void

looks more intuitive. For example substr takes int or null:

substr(string $string, int $offset, ?int $length = null): string