c언어 부모 자식 프로세스 변수 공유

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/shm.h>
 
#define MAX_PROCESSES 4 
#define MAX_LEN 5
 
void child();
void parent();
int sum[MAX_PROCESSES] = {0, };
 
int main(){
    pid_t pid;
    int i=0;
    int status;
    int count = MAX_PROCESSES/2;
    int value;
 
    int shm_id = shmget((key_t)1234, 1024, IPC_CREAT | 0666);
    if (shm_id == -1) perror("shmget");
    int *shm_addr = (int*) shmat( shm_id, NULL, 0);
    if (shm_addr == (int*) -1) perror("shmat");
    int *step = shm_addr;
    int *done = shm_addr + 1;
    int *result = shm_addr + 2;
 
    pid = fork();
    if (pid == -1) {
        printf("can't fork, error\n");
        exit(0);
    }
    if (pid == 0) { //child
        for (i=0; i < MAX_PROCESSES; i++) {
            child(i, step, done, result);
        }
        *done = 1;
        exit(0);
    }
    // parent
    parent(step, done, result);
    int val1 = *result++;
    int val2 = *result++;
    int res = val1 + val2;
    printf("P step: %d, %d + %d = %d\n", (*step)++, val1, val2, res);
    printf("FINAL RESULT: %d\n", res);
    shmctl(shm_id, IPC_RMID, NULL);
    return 0;
}
 
void child(int arg, int *step, int *done, int *result) {
    FILE *f_read;
    char buffer[MAX_LEN];
    int list = arg;
    int i = list*2;
    int first = 0;
    int second = 0;
    int line_count = 0;
 
    f_read = fopen("temp.txt","r");
    if (f_read == NULL)
        printf("file read error!\n");
    while (fgets(buffer, MAX_LEN, f_read) != NULL) {
        if (line_count == i) {
            first = atoi(buffer);
            second = atoi(fgets(buffer, MAX_LEN, f_read));
        }
        line_count++;
    }
    fclose(f_read);
 
    int sum_add = first + second;
    printf("C step: %d, %d + %d = %d\n", (*step)++, first, second, sum_add);
    *(result + arg) = sum_add;
}
 
void parent(int *step, int *done, int *result) {
    while (! *done) ;
    int loop = *step / 2;
    int val1, val2, sum_add;
    int *result_bak = result;
    for (int i = 0; i < loop; ++i) {
        val1 = *(result++);
        val2 = *(result++);
        sum_add = val1 + val2;
        printf("P step: %d, %d + %d = %d\n", (*step)++, val1, val2, sum_add);
        *result_bak++ = sum_add;
    }
}
 
--------------------------------------------
 
$ gcc test.c -lrt
 
$ ./a.out 
C step: 0, 1 + 2 = 3
C step: 1, 3 + 4 = 7
C step: 2, 5 + 6 = 11
C step: 3, 7 + 8 = 15
P step: 4, 3 + 7 = 10
P step: 5, 11 + 15 = 26
P step: 6, 10 + 26 = 36
FINAL RESULT: 36