First, let's take a look at the following two codes ：

test.c in ：

test1.c in ：

At this point, we compile and run ：

Compile and pass , And printed out 10, Why is that ？ This involves the linker resolving multiple defined global symbols .

Linux Rules for linkers to handle multiple defined symbols

The above result is that we are VS2019 Under test , Linux The rule for the linker to handle multiple defined symbols is ：

1. Multiple with the same name are not allowed Strong sign
2. If there is a Strong sign And multiple Weak sign The same name , choice Strong sign
3. If there are more than one Weak sign The same name , From these Weak sign Choose one randomly

First , What is a strong symbol , What is a weak sign ？

Strong sign ： Functions and initialized global variables are strong symbols

Weak sign ： Uninitialized global variables are weak symbols

In the above code ,test.c Medium count It's a strong sign ,test1.c Medium count Is a weak sign , When printing, strong symbols are printed count, so , stay Windows This rule is also followed under

stay Linux and Windows Next test

We can do it in Windows and Linux Let's test these three rules ：

When multiple source files have strong symbols with the same name

When multiple source files have strong symbols with the same name ：

Linux：

Compile link results ：

Windows：

thus it can be seen , stay Linux and Windows Multiple strong symbols with the same name are not supported under

When multiple source files have a strong symbol with the same name as multiple weak symbols

When there is a strong symbol with the same name as multiple weak symbols in multiple source files ：

Linux：

Compile and run ：

Windows：

Like the example shown at the beginning , Compile and print 10

so , When there is a strong symbol with the same name as multiple weak symbols in multiple source files ,Linux and Windows The strong symbol will be selected under

When there are multiple weak symbols with the same name

When there are multiple weak symbols with the same name ：

Linux：

Running results ：

Windows：

Running results ：

so , Allow multiple weak symbols to duplicate names , And will randomly choose a weak symbol to use .

Why print 0？ Because uninitialized global variables are assigned when linking 0 value