memory allocation in C for pointers

Question

I'm trying to build a structure called PROCESS in C, this struct should contain the ID(id) and waiting time (wt) of the process.

typedef struct PROC{
    int id;
    int wt;
}PROCESS;

PROCESS *pt = NULL;

Now I want to make more then one instance of this struct like an array. what I want to do is something like this':

PROCESS pt[10];
pt[0].id = 5;
pt[1].id = 7;

But I want to do it using dynamic memory allocation:

pt = calloc(2,sizeof(PROCESS));

pt[0]->id = 5;

What is my mistake?

If it's really [tag:c] you don't need to cast to `struct _PROCESS`. — Iharob Al Asimi, Mar 02 '18 at 22:10
Should we chalk that off as a typo or is this something you didn't know about? — R Sahu, Mar 02 '18 at 22:12
Another issue: Use of "reserved identifiers", do some research on that term. — Ulrich Eckhardt, Mar 02 '18 at 22:12
@IharobAlAsimi Edited. Thanks for your comment, but that is not what I'm asking — Matt Belle, Mar 02 '18 at 22:14
@RSahu I can do that, but I have no idea of how log the array would be.It can be really long or very small — Matt Belle, Mar 02 '18 at 22:16
Whether you use `ptr[0]->id` or `ptr[0].id` has nothing to do with the length of the array. — R Sahu, Mar 02 '18 at 22:18
@MattBelle I didn't say that it was an answer. That's why it's a comment, [this is an answer](https://stackoverflow.com/a/49078203/1983495), and a good one. — Iharob Al Asimi, Mar 02 '18 at 22:24

Pablo · Accepted Answer · 2018-03-02T22:48:45.240

pt is a pointer to PROCESS, pt[0] is the first PROCESS object pointed to by pt. The -> operator to access members of a struct must be used with pointers only, otherwise use .

pt[0].id = 5;

would be correct.¹

An since you say you are doing C, you don't need to cast malloc or calloc.

PROCESS *pt = calloc(2, sizeof *pt);
if(pt == NULL)
{
    // errror handling
    // do not continue
}

pt[0].id = 5;
pt[1].id = 7;

Also don't forget to check the return value of calloc and don't forget to free the memory later with free(pt);.

Fotenotes

¹Note that this would be equivalent to

pt->id = 5;

but if you want to set id of the second element, you would need to do

(pt+1)->id = 7;

but I think it's more readable to do

pt[1].id = 7;

Anders Cedronius · Answer 2 · 2018-03-02T22:33:54.777

0

typedef struct process{
    int id;
    int wt;
}processes;

I would allocate like this->

int numberOfDynamicStructs=2;

processes* myProcesses= calloc(numberOfDynamicStructs,sizeof(processes));

Write->

myProcesses[0].id=1;
myProcesses[1].id=2;

Read->

printf("%d %d",myProcesses[0].id,myProcesses[1].id);

Free when done..

/A

edited Mar 02 '18 at 22:33

answered Mar 02 '18 at 22:26

Anders Cedronius

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memory allocation in C for pointers

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