Thread local storage understanding

Thread Local Storage understand

Learn with problems

1. What is? thread local storage？

A method of computer programming , Use thread local static or global memory .

2. What's the main function ？

1. Avoid resource competition ; When multiple threads access the same resource , There will be competition . When a resource is declared as thread local storage when , There will be no competition .
2. Use the reentrancy of global object methods ; For example, a function uses global variables to set an error code （ such as c In the library errno）, If errno It's a global variable , A system method will override the value that has just been modified by another thread , However, the code of another thread is right again errno For verification （ At this point, the results do not meet expectations ）; Use thread local storage Can solve ： send errno It looks like a global variable , But each thread has one .

3. What segment is stored after compilation ？

After compilation, it is stored in TLS section, If there is no initial value , It's in .tbss; If there is an initial value , It's in .tdata;

#include <stdio.h>
#include <thread>

thread_local int hello = 3;
int main(int argc, char** argv){
    printf("hello:%d\n",hello);
    return 0;
}

1. Use readelf -s see hello The position of the symbol ( First compile the above code into main Binary system )

   readelf -s main | grep -E ‘hello|Num’

   Output :

      Num:    Value          Size Type    Bind   Vis      Ndx Name
      Num:    Value          Size Type    Bind   Vis      Ndx Name
       58: 0000000000000000     4 TLS     GLOBAL DEFAULT   21 hello
   

2. Ndx Indicates the segment to which the symbol belongs , Use readelf -S see 21 What paragraph is it

   readelf -S main | grep -A 1 -E ‘[21|Name’

   Output

     [Nr] Name              Type             Address           Offset
          Size              EntSize          Flags  Link  Info  Align
   --
     [21] .tdata            PROGBITS         0000000000003db4  00002db4
          0000000000000004  0000000000000000 WAT       0     0     4
   

   therefore thread_local Variable hello The symbol type of is TLS, Global scope , be located .tdata In the paragraph .

ps: When thread_local When a variable is not initialized , be located .tbss paragraph （ Similar to global variables , The initialized global variables are located in .data paragraph , Uninitialized global variables are located in .bss paragraph ）, Those who are interested can make thread_local int hello = 3; Change it to thread_local int hello; Then follow the above steps to view .

What segment is stored after compilation ？

answer ： The initialization of the thread_local After the variable is compiled, it is located in .tdata paragraph , Uninitialized at .tbss paragraph .

4. How the runtime initializes , What area of memory is it stored in ？

When a thread uses a variable , Copy data from the corresponding data segment , Then store it in thread local storage Area .