Initializing a pointer to a structure

Question

another linked question is Segmentation fault while using strcpy()?

I have a structure:

struct thread_data{    
    char *incall[10];
    int syscall         


        

Answer 1

i think malloc(sizeof(struct thread_data)); should work, does it not?

Answer 2

The corresponding struct initialiser can look like this:

struct thread_data a = {
  .incall = {"a", "b", "c", "d", "e"},
  .arg_no = 5,
  .client_socket = 3
};

Then you can assign the address of this to a pointer:

struct thread_data *b = &a;

Answer 3

It depends if you need your variable to be temporary or not:

struct thread_data data; // allocated on the stack
// initialize your data.* field by field.

struct thread_data* data = malloc(sizeof (struct thread_data)); // allocated on the heap
// initialize your data->* field by field.

In both cases, you have to allocate your structure first to be able to access its fields.

Answer 4

Here's the answer to the question I think you're asking:

/**
 * Allocate the struct.
 */
struct thread_data *td = malloc(sizeof *td);

/**
 * Compute the number of elements in td->incall (assuming you don't
 * want to just hardcode 10 in the following loop)
 */
size_t elements = sizeof td->incall / sizeof td->incall[0];

/**
 * Allocate each member of the incall array
 */
for (i = 0; i < elements; i++)
{
  td->incall[i] = malloc(HOWEVER_BIG_THIS_NEEDS_TO_BE);
}

Now you can assign strings to td->incall like so:

strcpy(td->incall[0], "First string");
strcpy(td->incall[1], "Second string");

Ideally, you want to check the result of each malloc to make sure it was successful before moving on to the next thing.

Answer 5

You can write something just like this:

#define ARRAY_DIMENSION(a) (sizeof(a)/sizeof((a)[0]))

void init_func(void)
{
    struct thread_data arg_to_thread;
    int i;
    char buffer[100];

    buffer[0] = '\0';

    for ( i = 0; i < ARRAY_DIMENSION(arg_to_thread.incall); i ++ )
    {
         /* Do something to properly fill in 'buffer' */

         arg_to_thread.incall[i] = strdup(buffer);
    } 
}

Answer 6

Here is another possibility. It is unclear to me what you want the values initialized to, so this just grabs a number out of the air, which is almost certainly wrong.

struct thread_data *td;
int i;
// allocate memory for the structure
td = malloc( sizeof( struct thread_data ));
// then allocate/initialize the char* pointers.  It isn't clear
// what you want in them ... pointers to existing data?  Pre-allocated
// buffer?  This just allocates a fixed size buffer,
for ( i = 0; i < sizeof( td->incall ) / sizeof( td->incall[0] ); i++ )
   td->incall[i] = malloc( 42 );