Array of pointers to a struct

Question

typedef struct Oft{
   int   mode;
   int   refCount;
} OFT;

typedef struct proc{
   struct proc *next;
   int    uss, usp;
   int    pid;                // add pid for identify the proc
   int    status;             // status = FREE|READY|RUNNING|SLEEP|ZOMBIE    
   int    ppid;               // parent pid
   OFT    *fd[10];    
   int    kstack[1024];      // per proc stack area
}PROC;

How would I initialize and use my *fd[10] variable?

assume that I have

PROC *p;

Is this the correct way to initialize its *fd[10]?

for(i = 0; i < 10; i++)
{
    p->fd[i] = (OFT *) malloc (sizeof(OFT));
    (*p->fd)[0] = Some OFT object?   
}

why is p->fd[i] always equal to null when I do the following checking?

if (p->fd[i] == NULL)
    //Do sometiong

You must initialize p first before you initialize its members. Also check the return value of malloc. — user1508519
– user1508519, Commented Mar 2, 2014 at 10:17
Assume that I initialized it. I don't want to post more code that necessary — user3370394
– user3370394, Commented Mar 2, 2014 at 18:43

Guntram Blohm · Accepted Answer · 2014-03-02 10:35:10Z

2

p->fd[i] = malloc(sizeof(OFT));

is correct, as long as you've provided space for p to point to, by using malloc() or by making it point to some PROC variable. Note you don't need to cast, since malloc() returns a void * which is cast to any type of pointer automatically. You should, however, check if malloc() returns NULL.

(*p->fd)[0]

is wrong and will result in a compiler error. If you have an OFT object named myoft; you can assign it just like you did with malloc; of course, since fd[x] is a pointer, you want to have the address of the object:

p->fd[x] = &myoft;

If you want to access one of the components of one of your OFTs, you could, for example, use

p->fd[x]->refCount++;

I can't answer the last question, why if (p->fd[i] == NULL) is always true, unless you provide a complete example of your code.

answered Mar 2, 2014 at 10:35

Guntram Blohm

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2 Comments

user3370394 Over a year ago

if I do p->fd[x] = &myoft in a function and exit that function and check one of its components after, it returns values different from the ones that I assigned! Any idea?

user3370394 Over a year ago

Oh I just realized that the return value of malloc is NULL. I did check p->fd[i] after malloc and it's still null!

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