Dynamically allocating an array of structs in Main and then having a function assign to it

Question

I'm building a server for a project and I need to store a bunch of values in a ordered way. I've been searching for hours and I haven't figured out how.

I built a struct as follows:

struct WHITEBOARD{
    int line;
    char type;
    int bytes;
    char string[1024];  
} *Server;

Then in my main function I want to dynamically allocate memory to create an array of structs WHITEBOARD to a size of [argv[1]] (eventually). I would like to use calloc and in my research I have found the following:

void main()
{
    struct whiteboard (*Server) = (struct whiteboard*) calloc(10, sizeof(*Server));
    (*Server).line = 2;
    printf("Test: %d\n",(*Server).line);
}

This works but I can't seem to find out how to turn Server into an array of structs so that I can reference (*Server)[1].line and assign to this heap bound variable from a function. Which I intend to do as follows.

char* doThing(struct whiteboard Server)
{
    (*Server)[1].line = 4;
    return;
}

And be able to printf the newly bound variable from main.

This might be a dumb question, but any help would be awesome. Thanks!

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, Mar 14 '17 at 06:47
Sorry, the structure and main() should compile together. The function however makes no sense. It's kind of a pseudocode for what I'd like to do. — Gregory Gould, Mar 14 '17 at 06:56

David Ranieri · Accepted Answer · 2017-03-14T09:10:33.170

struct WHITEBOARD{
    int line;
    char type;
    int bytes;
    char string[1024];  
} *Server;

You have a variable (a pointer to struct WHITEBOARD) called Server at global scope, thus, you don't need to redeclare-it inside main nor inside your function parameters, also note that you are misusing the dereference operator (*), to access the element 1 in (*Server)[1].line = 4; just use Server[1].line = 4;

void doThing(void) /* Changed, you promise to return a pointer to char but you return nothing */
{
    Server[1].line = 4;
}

int main(void) /* void main is not a valid signature */
{
    Server = calloc(10, sizeof(*Server)); /* Don't cast calloc */

    Server[1].line = 2;
    doThing();
    printf("Test: %d\n", Server[1].line);
    free(Server);
}

Lundin · Answer 2 · 2017-03-14T12:09:24.573

Simply get rid of all the obscure syntax you have invented, don't "guess the syntax" when you are not sure of how to do something.

Keep struct declaration and variable declarations separate.
Don't use global variables.
Don't use parenthesis when there is no obvious need for it.
Don't dereference a pointer with both * and [] operators in the same expression.
Don't cast the result of calloc.
Don't write functions that return a type and then return nothing.
The signature of main() on a hosted system is int main (void).
Always free() allocated memory.

Example:

#include <stdio.h>
#include <stdlib.h>

typedef struct 
{
  int line;
  char type;
  int bytes;
  char string[1024];  
} whiteboard_t;


void do_thing (whiteboard_t* server)
{
  server[1].line = 4;
}

int main (void) 
{
  int n = 10;
  whiteboard_t* server = calloc(n, sizeof(whiteboard_t));

  server[0].line = 2;
  printf("Test: %d\n",server[0].line);

  do_thing(server);
  printf("Test: %d\n",server[1].line);

  free(server);
  return 0;
}

The `_t` type suffix is reserved by POSIX and it seems that OP wants an array of `10` elements, not an array of 10 * 10 elements, use `calloc(n, sizeof(whiteboard_t));` instead of `calloc(n, sizeof(whiteboard_t[n]));` — David Ranieri, Mar 14 '17 at 08:19
@KeineLust The question is about C not POSIX. That POSIX bans `_t` postfix is simply a design flaw in the POSIX standard, which I couldn't care less about. The C standard actively encourages the use of `_t` for typedefs. File a defect report to POSIX if you care. The typo has been fixed, thanks. — Lundin, Mar 14 '17 at 12:12

Dynamically allocating an array of structs in Main and then having a function assign to it

2 Answers2