Using Malloc in C: Writing to a dynamic memory buffer

Question

I have the following structure in C

struct _MY_LIST                  
{
    short sRecNum;
    short sConfirm;
    short sFCount;
}my_list;

How do I use malloc to allocate memory for this structure as well to write this structure to dynamic memory?

Answer 1

You've defined a structure and a variable composed of the structure, but you need to define a pointer to that structure instead.

Pointers are a difficult topic to master and what I'm about to post will give you a sharp knife to play with -- but you could end up cutting yourself with it if you don't tread lightly! Learning them will take far more than a single SO answer could provide, but at least be sure to read the comments I sprinkled into this code snippet.

struct _MY_LIST                  
{
    short sRecNum;
    short sConfirm;
    short sFCount;
} *my_list_pointer; /* the asterisk says this is a pointer */


/* dynamically allocate the structure */
my_list_pointer = malloc(sizeof(*my_list_pointer));


/* required error checking! */
if (my_list_pointer == NULL) {
    /* do whatever you need, but do _not_ dereference my_list_pointer */
    exit(-1);
}


/* write to the structure */
my_list_pointer->sRecNum = 50;


/* read from the structure */
short the_record_number = my_list_pointer->sRecNum;


/* when finished with the allocation, you must release it */
free(my_list_pointer);

/* now, you must NOT dereference my_list_pointer anymore unless you malloc it again! */

Answer 2

#include <stdlib.h>

struct _MY_LIST                  
{
    short sRecNum;
    short sConfirm;
    short sFCount;
}my_list;


void main()
{   
    my_list * list;

    list = (my_list*) malloc(sizeof(my_list))

    my_list->sRecNum = 1;
    my_list->sConfirm = 2;
    my_list->sFCount = 3;

    free(list);

}

Never forget to free the pointer. I you can avoid it do not use malloc and free in ansi-c.

Here is an alternative if that is possible in your source.

struct _MY_LIST                  
{
    short sRecNum;
    short sConfirm;
    short sFCount;
}my_list;

void uselist(my_list * list);

void main()
{   
    my_list list;

    uselist(&list);
}

void uselist(my_list * list)
{

    list->sRecNum = 1;
    list->sConfirm = 2;
    list->sFCount = 3;

}

Answer 3

First of all, don't use identifiers starting with an underscore because they may clash with the identifiers reserved for the implementation of C , as mentioned by Jens in the comment.

struct myList {                
    short sRecNum;
    short sConfirm;
    short sFCount;
};

struct myList foo;

The above defines a structure of type struct myList . Now you can define variables of this type just like you would with any other type as type identifier; . You can combine the two and define a structure and create an instance of it in the same statement as

struct myList {                
    short sRecNum;
    short sConfirm;
    short sFCount;
} foo;

To dynamically allocate memory for a variable, use malloc declared in the header stdlib.h .

void *malloc(size_t size);

Here, size is the number of bytes to allocate and size_t is a type ( typedef , hence _t ) to represent the size of objects on the machine. It returns a pointer of type void * which is a generic type for pointers. A void * is assignment compatible to any pointer type, therefore you should not cast the result of malloc . malloc can fail to allocate memory when there isn't enough of it available. You need to check for it whenever you call malloc .

Use the sizeof operator to find the size of a type or a variable in bytes. Note that you don't need to use parentheses when the operand of sizeof is a variable.

struct myList *bar = malloc(sizeof *bar);

if(bar == NULL) {
    // memory allocation failed.
    // handle it
}
else {
    // assign values to the structure members

    bar->sRecNum = 1;
    bar->sConfirm = 2;
    bar->sFCount = 3;

    // do stuff with bar
    // after you are done free the memory allocated by malloc
    // it is a good practice to set bar to NULL after freeing it

    free(bar);
    bar = NULL;
}