Cannot manipulate the array of struct cause segmentation fault

Question

Hi i have the user that get randomly a number, make a structure called transaction and add it to a list. The problem is that give me back segmentation fault but i don't know where is the problem. This is a big problem cause this program is launched by another like his son, if the segmentation will not be solved the father cannot do his duty.

main:

int main(int argc, char const *argv[])
{
    id = atoi(argv[1]);
    int shmid = shmget(SH_KEY, SO_NODES_NUM * sizeof(pid_t), IPC_EXCL | 0666);
    int *nodeList = (int *)shmat(shmid, NULL, 0);

    initTransactionPool();
    struct Transaction prova1;

    while (1)
    {
        int r1 = rand() % totalBudget + 2;
        /*getBudget();
        setBudget();*/
        if (totalBudget<2)
        {
            break;
        }
        
        prova1.money = r1;
        prova1.receiver = 12;
        prova1.sender = 1;
        prova1.reward = 1;
        prova1.timestamp = "ciao";
        addTransaction(prova1);
        totalBudget-=r1;
        printf("\n il budget e' %d",totalBudget);
    }
    freeTransactionPool();
    return 0;
}

other function:

void initTransactionPool()
{
    transactionPool = (struct Transaction *)malloc(SO_TP_SIZE * sizeof(struct Transaction *));
}

void freeTransactionPool()
{
    free(transactionPool);
}

int addTransaction(struct Transaction transaction)
{
    if (sizePool >= SO_TP_SIZE - 1)
        return -1;
    ++sizePool;
    transactionPool[sizePool] = transaction;
    return 0;
}

the data: transactionPool

is saved in a ".h" like this : extern struct Transaction* transactionPool;

and in a ":c" like this: struct Transaction* transactionPool;

transactionPool = (struct Transaction *)malloc(SO_TP_SIZE * sizeof(struct Transaction *)); this smells because it looks like you are allocating only space for a pointer instead of the structure. Also note that casting results of malloc() family is considered as a bad practice. — MikeCAT
– MikeCAT, Commented Jul 21, 2022 at 16:32
transactionPool is a pointer like this saved in a labrary "user_manager.h" : struct Transaction *transactionPool; — Matteo Pagliarello
– Matteo Pagliarello, Commented Jul 21, 2022 at 16:36
It's not about the cast but about the size. You need sizeof(struct Transaction), not a pointer — Eugene Sh.
– Eugene Sh., Commented Jul 21, 2022 at 16:39

ryyker · Accepted Answer · 2022-07-21 19:13:41Z

1

"I removed the cast but the segmentation fault persit "

The seg fault is not being caused by the cast, it is occurring because in this call:

void initTransactionPool()
{
    transactionPool = (struct Transaction *)malloc(SO_TP_SIZE * sizeof(struct Transaction *));
}

transactionPool is allocated only the number of bytes that matches the size of a pointer on your system. (typically 4 on 32bit system or 8 for 64bit system) So, if an attempt is made to write anything to this allocated area of memory that is larger than 4 (or 8) bytes, it will optimally result in a seg-fault, clearly indicating to you that there is a problem in the code. Alternatively it could appear to do nothing, while in reality your code is brushing up against undefined behavior. (where anything can happen, even nothing for awhile, then for no apparent reason crash.)

Change it to:

struct Transaction* initTransactionPool(void)
{
    transactionPool = malloc(SO_TP_SIZE * sizeof(struct Transaction));
    return transactionPool ;//caller must check for NULL before using 
}

or

struct Transaction* initTransactionPool(void)
{
    transactionPool = malloc(SO_TP_SIZE * sizeof(*transactionPool));
    return transactionPool;//caller must check for NULL before using 
}

Note, added the return to allow caller to check for NULL upon return, eg:

//in calling function:
if(!initTransactionPool())
{
    //handle error
}
//continue.

About the cast...
It is removed here because although it is required if using C++, it is considered bad practice in C. Having the cast when using C is not a guaranteed problem, but can be for the reasons cited in the link.

There are other problems with this code as well, including the function addTransaction. If you get stuck with those, post another question with a mcve.

edited Jul 21, 2022 at 19:13

answered Jul 21, 2022 at 16:47

ryyker

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8 Comments

Matteo Pagliarello Over a year ago

I did the changes but still get segmenation fault when i start it. Even do i put a printf in the start of the code exit with segmentation fault and doesn't print anything

ryyker Over a year ago

@MatteoPagliarello - Have you run your program in debug, with break points in the spots where you suspect the problem occurs? Stepping through the code one line at a time will likely show you where exactly in your code the problem occurs, and from there the reason for the problem can likely be determined. (for example debugging in gcc).

ryyker Over a year ago

@MatteoPagliarello - I would suggest temporarily removing user input and hard code the value at top of main function. ( i.e. id = 10; ) Then also temporarily remove shm...() functions. If the seg-fault goes away, then they are the culprit. What are your permissions?

Matteo Pagliarello Over a year ago

I tried to run debug but it says that failed at start and exit with value -1

Matteo Pagliarello Over a year ago

Oh yes maybe the atoi could be a problem

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