프로그램이 실행 도중 변경된 환경변수의 값은 반영할수 없는건가요??

$ ps eww p 4380
  PID TTY      STAT   TIME COMMAND
 4380 pts/1    Ss     0:00 -bash CVS_RSH=ssh DISPLAY=localhost:10.0 EDITOR=vim G_BROKEN_FILENAMES=1 HISTSIZE=1000 HOME=/home/test01 HOSTNAME=testpc INPUTRC=/etc/inputrc LANG=ko_KR.UTF-8 LESS=-iMSx4 -FX LESSOPEN=|/usr/bin/lesspipe.sh %s LOGNAME=test01 MAIL=/var/spool/mail/test01 PAGER=less PATH=/usr/local/sbin:/sbin:/usr/sbin:/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin ....

$ strings /proc/4380/environ
CVS_RSH=ssh
DISPLAY=localhost:10.0
EDITOR=vim
G_BROKEN_FILENAMES=1
HISTSIZE=1000
HOME=/home/test01
HOSTNAME=testpc
INPUTRC=/etc/inputrc
LANG=ko_KR.UTF-8
LESS=-iMSx4 -FX
LESSOPEN=|/usr/bin/lesspipe.sh %s
LOGNAME=test01
MAIL=/var/spool/mail/test01
PAGER=less
PATH=/usr/local/sbin:/sbin:/usr/sbin:/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/home/test01/bin:/usr/hs/bin
...

$ strings /proc/4380/environ
$ cat /proc/4380/environ | xargs -0 -L 1 echo
$ cat /proc/4380/environ | tr "\0" "\n"

#include <unistd.h>
#include <stdio.h>
#include <string.h>
 
// <string.h>의 문자열함수를 이용해 /proc/?/environ 내용물을 직접 다뤄도 되지만
// 번거로와서 그냥 UNIX 도구를 사용.
void read_proc_env(pid_t pid, char *key)
{
    char cmd[256];
    sprintf(cmd, "strings /proc/%d/environ | grep '^%s='", pid, key);
    FILE *f = popen(cmd, "r");
    char buf[256];
    while (fgets(buf, sizeof(buf), f) != 0) {
        printf("READ: %s", buf);
    }
    pclose(f);
}
 
int main(int argc, char **argv, char **env)
{
    pid_t pid = fork();
    if(pid) {
        // parent process
        // search for the position of the parent variable
        while(*env) {
            //puts(*env);
            if(!strncmp(*env, "PARENT_VAR=", 11))
                break;
            env++;
        }
 
        if(!*env)   return 1;
        char *var = *env;
        int size = strlen(var) + 1;
 
        int i = -1;
        while(i-- > -6) {
            sleep(3);
            snprintf(var, size, "PARENT_VAR=%d", i);    // modify parent variable
            read_proc_env(pid, "CHILD_VAR");            // read child env
        }
    }
    else {
        // child process
        // search for the position of the child variable
        while(*env) {
            //puts(*env);
            if(!strncmp(*env, "CHILD_VAR=", 10))
                break;
            env++;
        }
 
        if(!*env)   return 1;
        char *var = *env;
        int size = strlen(var) + 1;
 
        int i = 5;
        while(i++ < 10) {
            sleep(3);
            snprintf(var, size, "CHILD_VAR=%d", i);     // modify child variable
            read_proc_env(getppid(), "PARENT_VAR");     // read parent env
        }
    }
 
    sleep(1);
 
    return 0;
}

$ gcc -o print_env print_env.c
$ PARENT_VAR=000 CHILD_VAR=000 ./print_env
READ: CHILD_VAR=6
READ: PARENT_VAR=-2
READ: CHILD_VAR=7
READ: PARENT_VAR=-3
READ: CHILD_VAR=8
READ: PARENT_VAR=-4
READ: CHILD_VAR=9
READ: PARENT_VAR=-5
READ: CHILD_VAR=10
READ: PARENT_VAR=-6