You are subscribed to this thread how linux publish the locale data to the system

Using function setlocale (int category, const char *locale)
will set the locale in my application, and then the whole system will use this new locale.

why the whole system can get the new locale data?

They are in the different process.

why the whole system can get the new locale data?

It doesn't. If you run this bash script:

#!/bin/bash

cat > wje.c <<EOD
#include <sys/types.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(void)
{
  char *duplicated_locale_name;

  char *returned_locale_name;

  pid_t child_process;

  printf("      parent process is %d\n",getpid());

  child_process=fork();

  if(child_process<0)
  {
    perror("first fork()");

    exit(1);
  }

  if(child_process==0)
  {
    sleep(3);

    if(setlocale(LC_ALL,"de_DE")==NULL)
    {
      fprintf(stderr,"setlocale() failed in process %d\n",getpid());

      exit(1);
    }

    printf("setlocale() succeeded in process %d\n",getpid());

    sleep(3);
  }
  else
  {
    printf("first  child process is %d\n",child_process);

    child_process=fork();

    if(child_process<0)
    {
      perror("second fork()");

      exit(1);
    }

    if(child_process==0)
    {
      sleep(9);
    }
    else
    {
      printf("second child process is %d\n",child_process);

      sleep(12);
    }
  }

  returned_locale_name=setlocale(LC_ALL,NULL);

  duplicated_locale_name=strdup(returned_locale_name);

  printf("for process %d, current locale is %s\n",
         getpid(),
         duplicated_locale_name
        );

  return 0;

} /* main() */
EOD
cc -Wall wje.c -o wje
./wje

you should get output like this:

      parent process is 15900
first  child process is 15901
second child process is 15902
setlocale() succeeded in process 15901
for process 15901, current locale is de_DE
for process 15902, current locale is C
for process 15900, current locale is C

with three-second pauses before each of the final four lines.

Thanks for your reply.

There is a system wide locale,right?
why all the process can access this system wide locale?
Is this system wide locale stored in a File? Possible not, it is too slow.
Where is it? In RAM, Maybe.But If it is in a RAM, How to protect the locale data?

How is the locale initially set for each process? Where are the files that determine this? The answer varies from one Linux distribution to another.

But changing the locale in a process by using setlocale() does not change any of those files.

Originally Posted by wje_lq How is the locale initially set for each process? Where are the files that determine this? The answer varies from one Linux distribution to another. But changing the locale in a process by using setlocale() does not change any of those files.

I do not know. Can you just give an example? On how all the process can access the system wide locale?