C++ 共享内存未授予父进程权限

我的C代码中有关于共享内存的问题.

我有一个C代码，它不能像我预期的那样工作:

#include "sys/ipc.h"
#include "sys/shm.h"

#include "unistd.h"
#include "sys/types.h"
#include "sys/wait.h"

#include "stdio.h"
#include "stdlib.h"

int main(){
    key_t key = ftok("shmfile.txt", 65);
    int shmid = shmget(key, sizeof(int), IPC_CREAT);
    if (fork() != 0){
        int * ptr = (int *) shmat(shmid, 0, 0); // This line returns -1
                            // This only happened
                            //  to parrent
        printf("shmid parrent = %d\n", shmid);
        perror("Error ");
        *ptr = 6;
        shmdt(ptr);
        wait(NULL);
        shmctl(shmid, IPC_RMID, NULL);
    }
    else{
        printf("shmid child = %d\n", shmid);
        int * ptr = (int *) shmat(shmid, 0, 0); // Works as expected
        printf("ptr address = %p\n", ptr);      
        shmdt(ptr);
        printf("Now child can exit normally");
    }
    return 0;
}

结果如下(Gdb):

shmid parrent = 45
Error : Permission denied
shmid child = 45
ptr address = 0xffffffffffffffff
Now child can exit normally
Program received signal SIGSEGV, Segmentation fault.
0x00005555555552d0 in main () at main_2.c:20
20          *ptr = 6;

我希望父进程和子进程都能正常工作.只有子进程正常退出.我不确定父进程有什么问题.非常感谢!

更新1:父进程和子进程中的PTR始终为(void *) -1.我不知道如何让shmat()不返回那个值.

更新2:文件"shmfile.txt"与main可执行文件位于同一位置.