PHP underlying analysis: About copying the plural of cow cow when writing Shengcai Forum ww7349cow Japanese cow milk stone

If you want to reach the ceiling of technology, then the learning process lies in knowing what is happening and why.

Today we will discuss the underlying copy-on-write (also called split-on-write) of PHP.

First, let’s take a look at a piece of code:

I believe everyone knows the output results of the two pieces of code, but today we will talk about what happened.

The picture below is the structure of PHP storage variables (comments have been written for convenience of explanation), zend.h is in the Zend directory.

You can see that the structure stores information about the variable value, how many variables point to the structure, variable type, whether it is a reference variable, etc.

So what happens with the first print? The variable information enters a structure, and the correlation is as follows:

$name = '8:30 in the evening';
$myName = $name;

At this time, $name and $myName share a structure, and refcount__gc is 2.

We found that $myName = $name; did not actively turn into two structures during this process (this is also considered a kind of optimization within PHP, using only one structure, saving memory).

So when the code runs to $myName = ‘gzchen’;, how does the structure change? Since the structure is shared by two variables in the first output, will changing one of the variables at this time cause the two values ​​to change together? Purely from the logic of the structure, it is possible. After all, everyone shares this structure.

Then let’s take a look at what the second printing will look like. The relevant changes are as follows:

did not change $name and $myName to 'gzchen' at the same time as we thought, but copied one more The two structures come out, and the two structures correspond to $name and $myName respectively.

This is Copy-on-write (COW) causing trouble. It does not split into two structures when $myName = $name; is assigned, but takes effect when we rewrite one of the variables. , belongs to a kind of slow replication (also called slow division).

The pseudo code is as follows:

Let’s look at another piece of code:

The output is ‘b’, what happened in the middle?

In fact, during the foreach traversal process, $arr (the original array) is not directly manipulated, but $arr is copied into an $arrcopy (actually a copy, I use $arrcopy instead here). Foreach is in the traversal process In fact, the operation is always $arrcopy. The general process is as follows:

is actually the same as the example given above. We can see that at the beginning ($arr = $arrcopy) still share the same structure. , but $arr[$k] = $v is assigned again, copy-on-write occurs, and the structure is split.

And as mentioned before, foreach operates on $arrcopy, so the structure pointer of $arr is stuck at the first position (because the structure is different, $arrcopy cannot assign a value to $arr synchronously).

In fact, this kind of technology is usually only used in interviews. After all, there are only a few people who use this writing method in daily development. Friends who don’t understand it for the time being don’t need to pay too much attention. As long as you know that there is a situation of “copy while writing”. That's it.

The above introduces the underlying analysis of PHP: Regarding copying cow when writing, including cow and php content, I hope it will be helpful to friends who are interested in PHP tutorials.