共享 memory 上的分段錯誤

Question

我在使用共享 memory 時遇到了一些困難，當我嘗試查看變量內部的內容時出現分段錯誤

printf("|%d|", mappa->map[i][j]->taxi);

但不是當我寫它的時候

mappa->map[i][j]->taxi = rand() % 10;

我的第一個猜測是：好吧，我沒有分配足夠的空間。 但是，在“出租車”上寫字時，指令也不會失敗嗎？

PS我需要在一個結構內有“出租車”，因為稍后我將在該結構中有更多元素

#define KEY 6666

typedef struct cell {
    int taxi;
} cell;
typedef cell *cella;

typedef struct city
{
    cella map[5][5];
} city;

typedef city *citta;


int main(int argc, char const *argv[])
{
    citta mappa;
    cella single_cell;
    int id_mappa;
    int id_cell;
    id_mappa = shmget(KEY, sizeof(*mappa), IPC_CREAT | 0666);
    mappa = shmat(id_mappa, NULL, 0);
    srand(time(NULL));
    for (int i = 0; i < 5; i++)
    {
        for (int j = 0; j < 5; j++)
        {
            printf("ciao\n");
            id_cell = shmget(KEY + i + j, sizeof(*single_cell), IPC_CREAT | 0666);
            mappa->map[i][j] = shmat(id_cell, NULL, 0);
            mappa->map[i][j]->taxi = rand() % 10;
            printf("|%d|", mappa->map[i][j]->taxi);
        }
        printf("\n");
    }
    printf("-------------\n");
}

Answer 1

做的時候：

id_cell = shmget(KEY + i + j, sizeof(*single_cell), IPC_CREAT | 0666);

您多次使用相同的鍵，在循環的第一輪i和j是 0 所以使用的鍵是KEY + 0 + 0是已經用於mappa的KEY然后在循環的第一輪mappa->map[i][j] = shmat(id_cell, NULL, 0); 是'喜歡'做mappa->map[i][j] = mappa然后mappa->map[i][j]->taxi = rand() % 10; 沒有預期的行為，例如當您嘗試在下一行取消引用它以打印mappa->map[i][j]->taxi的值時，使用不是有效指針的隨機值設置mappa->map[i][j] mappa->map[i][j]->taxi 。

例如：

id_cell = shmget(KEY + i + j*10 + 1, sizeof(*single_cell), IPC_CREAT | 0666);

每次都有一個不同的鍵（當然，您可以將j乘以 5，或者更簡單地使用每次調用shmget等時遞增的 var）

after that :#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <time.h>
#include <stdlib.h>

#define KEY 6666

typedef struct cell {
    int taxi;
} cell;
typedef cell *cella;

typedef struct city
{
    cella map[5][5];
} city;

typedef city *citta;


int main(int argc, char const *argv[])
{
    citta mappa;
    cella single_cell;
    int id_mappa;
    int id_cell;
    id_mappa = shmget(KEY, sizeof(*mappa), IPC_CREAT | 0666);
    mappa = shmat(id_mappa, NULL, 0);
    srand(time(NULL));
    for (int i = 0; i < 5; i++)
    {
        for (int j = 0; j < 5; j++)
        {
            printf("ciao\n");
            id_cell = shmget(KEY + i + j*10 + 1, sizeof(*single_cell), IPC_CREAT | 0666);
            mappa->map[i][j] = shmat(id_cell, NULL, 0);
            mappa->map[i][j]->taxi = rand() % 10;
            printf("|%d|", mappa->map[i][j]->taxi);
        }
        printf("\n");
    }
    printf("-------------\n");
}

編譯和執行：

pi@raspberrypi:/tmp $ gcc -Wall c.c
pi@raspberrypi:/tmp $ ./a.out
ciao
|3|ciao
|5|ciao
|0|ciao
|9|ciao
|5|
ciao
|0|ciao
|7|ciao
|8|ciao
|3|ciao
|1|
ciao
|3|ciao
|2|ciao
|7|ciao
|0|ciao
|1|
ciao
|4|ciao
|4|ciao
|5|ciao
|2|ciao
|0|
ciao
|8|ciao
|2|ciao
|6|ciao
|4|ciao
|1|
-------------
pi@raspberrypi:/tmp $ 

和

pi@raspberrypi:/tmp $ valgrind ./a.out
==8741== Memcheck, a memory error detector
==8741== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==8741== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==8741== Command: ./a.out
==8741== 
ciao
|3|ciao
|2|ciao
|7|ciao
|6|ciao
|3|
ciao
|5|ciao
|8|ciao
|1|ciao
|9|ciao
|2|
ciao
|3|ciao
|7|ciao
|9|ciao
|0|ciao
|3|
ciao
|9|ciao
|1|ciao
|3|ciao
|9|ciao
|5|
ciao
|8|ciao
|6|ciao
|1|ciao
|3|ciao
|1|
-------------
==8741== 
==8741== HEAP SUMMARY:
==8741==     in use at exit: 0 bytes in 0 blocks
==8741==   total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==8741== 
==8741== All heap blocks were freed -- no leaks are possible
==8741== 
==8741== For lists of detected and suppressed errors, rerun with: -s
==8741== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
pi@raspberrypi:/tmp $ 

---

除此之外，通過typedef隱藏指針（例如typedef cell *cella; ）是一個非常糟糕的主意，會產生很多問題並降低代碼的可讀性，我強烈建議您讓*可見。