CODEWITHSUNDEEP
cpointers
PfannkuchenXD
PfannkuchenXD

Reputation: 151

Writing into RAM Areas I am not addressing

I currently have a problem with my C code concerning RAM writes that appear to happen (I checked by watching a member of a structure via gdb) even though I did not actively intend to write to that area.

My structs are declared as followed:

typedef struct tcb {
    unsigned int tid;
    thread_state_t state;
    pthread_t thread;
    sem_t sema;
    unsigned long interval;
    unsigned long rtime;
} tcb_t ;

typedef struct node {
    tcb_t *tcb;
} node;

typedef struct entry entry; 

typedef struct entry {
    entry *next;
    entry *prev;
    node node;
} entry;

These definitions are provided and I can not change them. I now initialize an array of entry with this code:

entry = malloc((num_threads) * sizeof(entry));

for (i = 0; i < num_threads; i++)
{
    entry[i].next = NULL;
    entry[i].prev = NULL;
    entry[i].node.tcb = malloc(sizeof(tcb_t));
    entry[i].node.tcb->tid = i;
    entry[i].node.tcb->state = THREAD_RUNNABLE; //initialize value to show the scheduler this thread works for the first time
    entry[i].node.tcb->interval = interval;

}

The other fields are not meant to initialized at this point. However once I run my code with let's say num_threads = 6, the value for entry[0].node.tcb->tid lost its value 0 but holds something strange (looks like a pointer to me).

When setting a watchpoint in gdb for this field it notifies me at the line entry[i].node.tcb = malloc(sizeof(tcb_t)); that my value was overwritten (at this point i holds the value 2). Why is this?

Upvotes: 2

Views: 78

Answers (1)

Weather Vane
Weather Vane

Reputation: 34585

You cannot have the line 

entry = malloc((num_threads) * sizeof(entry));

as written - it does not compile. Since your program has lines such as

entry[i].next = NULL;

I guess it is actually

entry *entry = malloc((num_threads) * sizeof(entry));

So now what is sizeof(entry)? My little test program tells me it is 4, the size of the pointer, not the struct.

So please use unique identifiers.

Upvotes: 4

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